Nakshatras 21-12-15

http://shyamer.in/interesting/astra/earth/ Here, I have tried to explain our Planet system in a Simple language, Technically. It is a collection from various sources. Hope, you like these and can understand the Planet system around us.

Solar System Age Location No. of planets No. of known dwarf planets Orbital speed Orbital period

The

overall structure of the charted regions of the Solar System consists of the Sun, four relatively small innerplanets (Mercury, Venus,

4.568 billion years Local Interstellar Cloud, Local Bubble, Orion– Cygnus Arm, Milky Way 8 Mercury, Venus, Earth, Mars,Jupiter, Saturn, Uranus, Neptune 5 (IAU) Ceres, Pluto, Haumea, Makemake, Eris, hundreds of other possibilities 220 km/s 225–250 Myr

Earth, Mars) surrounded by a belt of rocky asteroids, and four gas giants (Jupiter, Saturn, Uranus, Neptune) surrounded by the Kuiper belt of icy objects....................Most of the planets in the Solar System possess secondary systems of their own, being orbited by planetary objects called natural satellites, or moons (two of which are larger than the planet Mercury), or, in the case of the four gas giants, by planetary rings; thin bands of tiny particles that orbit them in unison. Most of the largest natural satellites are in synchronous rotation, with one face permanently turned toward their parent...............Kepler's laws of planetary motion describe the orbits of objects about the Sun. Following Kepler's laws, each object travels along an ellipse with the Sun at one focus. Objects closer to the Sun (with smaller semi-major axes) travel more quickly because they are more affected by the Sun's gravity. On an elliptical orbit, a body's distance from the Sun varies over the course of its year. A body's closest approach to the Sun is called its perihelion, while its most distant point from the Sun is called its aphelion...........In modern times, Earth's perihelion occurs around January 3, and the aphelion around July 4............ First let us get a contemporary picture of the location of our solar system in the Milkiway galaxy..........A Galaxy is a huge group of stars, dust, gas, and other celestial bodies bound together by gravitational forces. There are spiral, elliptical, and irregularly shaped galaxies. Galaxies contain anywhere from 100,000 to 3,000,000,000,000 stars.............There are three major types of galaxies: spiral (with arms), elliptical (no arm), and irregular (without rotational symmetry). Galaxies radiate a continuous spectrum of energy. Some radiate radio waves, X rays, and infrared, and ultraviolet (UV) radiation.................The Earth, Sun and the rest of our solar system are a tiny part of the Milky Way Galaxy, a spiral galaxy.................The Milky way Galaxy is just one galaxy in a group of galaxies called the ‘Local Group’. Within the Local Group, the Milky Way Galaxy is moving about 300 km/sec (towards the Constellation Virgo)...............The Solar System is located in the Milky Way galaxy, a barred spiral galaxy with a diameter of about 100,000 light-years containing about 200 billion stars. Milkiway is estimated to be about 50000 light years in its diameter. The Sun resides in one of the Milky Way's outer spiral arms, known as the Orion–Cygnus Arm or Local Spur. The Sun lies between 25,000 and 28,000 light years from the Galactic Centre..............The Milkiway consists of a large number of stars and other matter. The stars visible to our naked eye are generally within a few hundred light years around us. Some giant stars are located nearly 1000 light years are also visible to naked eye. Thus most of the 10000 stars visible to us are very near our solar system....................In the illustration below, the center of Milkiway Galaxy is in the direction of Moola nakshatra. Thus bulk of the Milkiway matter is concentrated, as seen from earth, near stars Jyeshtha, Moola, Poorva/Uttarashadha...........The speed of Sun within the galaxy is about 220 kilometres per second (140 mi/s), so that it completes one revolution every 225–250 million years. This revolution is known as the Solar System's galactic year. The solar apex, the direction of the Sun's path through interstellar space, is near the constellation Hercules in the direction of the current location of the bright star Vega.

Gravitational Forces on the Planets......The planet with the strongest gravitational

attraction at its surface is Jupiter. Although Saturn, Uranus, and Neptune are also

very massive planets, their gravitational forces are about the same as Earth. This is because the gravitational force a planet exerts upon an object at the planet's surface is proportional to its mass and to the inverse of the planet's radius squared............A Day on Each of the Planets.........A day is the length of time that it takes a planet to rotate on its axis (360°). A day on Earth takes almost 24 hours.................The planet with the longest day is Venus; a day on Venus takes 243 Earth days. (A day on Venus is longer than its year; a year on Venus takes only 224.7 Earth days). ..............The planet with the shortest day is Jupiter; a day on Jupiter only takes 9.8 Earth hours! When you observe Jupiter from Earth, you can see some of its features change.......The Average Orbital Speed of the Planets........... As the planets orbit the Sun, they travel at different speeds. Each planet speeds up when it is nearer the Sun and travels more slowly when it is far from the Sun (this is Kepler's Second Law of Planetary Motion).............PLANETS........Mercury.........Mercury (0.4 AU from the Sun) is the closest planet to the Sun and the smallest planet in the Solar System (0.055 Earth masses). Mercury has no natural satellites.........Venus..........Venus (0.7 AU from the Sun) is close in size to Earth (0.815 Earth masses) and, like Earth, has a thick silicate mantle around an iron core, a substantial atmosphere, and evidence of internal geological activity. However, it is much drier than Earth, and its atmosphere is ninety times as dense. Venus has nonatural satellites. It is the hottest planet, with surface temperatures over 400°C...........Venus has no magnetic field that would prevent depletion of its substantial atmosphere, which suggests that its atmosphere is frequently replenished by volcanic eruptions........Mars...Mars (1.5 AU from the Sun) is smaller than Earth and Venus (0.107 Earth masses). It possesses an atmosphere of mostly carbon dioxide Its red colour comes from iron oxide (rust) in its soil. Mars has two tiny natural satellites (Deimos and Phobos)............Jupiter........Jupiter (5.2 AU), at 318 Earth masses, is 2.5 times the mass of all the other planets put together. It is composed largely of hydrogen and helium. Jupiter has 67 known satellites..........Saturn...........Saturn (9.5 AU), distinguished by its extensive ring system, has several similarities to Jupiter, such as its atmospheric composition and magnetosphere. Saturn has 62 confirmed satellites; two of which, Titan and Enceladus, show signs of geological activity, though they are largely made of ice. Titan, the second-largest moon in the Solar System, is larger than Mercury and the only satellite in the Solar System with a substantial atmosphere.........Uranus.......Uranus (19.2 AU), at 14 Earth masses, is the lightest of the outer planets. Uniquely among the planets, it orbits the Sun on its side; its axial tilt is over ninety degrees to the ecliptic. It has a much colder core than the other gas giants and radiates very little heat into space. Uranus has 27 known satellites.......Neptune...........Neptune (30 AU), though slightly smaller than Uranus, is more massive (equivalent to 17 Earths) and therefore more dense. It radiates more internal heat, but not as much as Jupiter or Saturn. Neptune has 14 known satellites.............Earth orbits the Sun at an average distance of about 150 million kilometers every 365.2564 mean solar days, or one sidereal year. From Earth, this gives an apparent movement of the Sun eastward with respect to the stars at a rate of about 1°/day, which is one apparent Sun or Moon diameter every 12 hours. Due to

this motion, on average it takes 24 hours—a solar day—for Earth to complete a full rotation about its axis so that the Sun returns to the meridian............Ecliptic (krāntivrtta)....................(The name "ecliptic" is derived from being the region where eclipses occur). The path the Sun appears to make amongst the stars is known as the ecliptic. Just like the Celestial Equator, it would make a large circle on the Celestial Sphere. In fact the ecliptic is a big circle that is tilted 23.5º relative to the circle made by the Celestial equator. This is shown in Figure

Earth’s axial tilt (or obliquity) and its relation to the rotation to axis and plane of orbit

The apparent motion of the Sun amongst the stars is due to the motion of the Earth around the Sun and our changing viewpoint. The stars that we would see behind the Sun in January would be different from the stars we would see behind the Sun in February, March, and every other month, since we are changing the location from which we view the Sun.

If

you were to map

out the path of the Sun relative to the stars, you would see it as a curved line on the Celestial Sphere. Take a look at Figure 1 to see the path relative to the Celestial Equator. This image is of a flattened out Celestial Sphere, and the dates mark the locations of the Sun relative to the stars over the course of the year

Figure

- The path of the Sun, the ecliptic, shown relative to the background stars and the Celestial Equator (dec=0).............Ecliptic and Earth..........Earth orbits around the Sun with Earth’s equator inclined at 7.155° to Sun’s equator..............Ecliptic and planets.............Most planets go in orbits around the sun which are almost in the same plane as the Earth's orbital plane, differing by a few degrees at most. As such they always appear close to the ecliptic when seen in the sky. Mercury with an orbital inclination of 7° is an exception. Pluto, at 17°, was previously the exception until it was reclassified a dwarf planet

Inclination

Terrestrials

Gas giants

Name

Inclination to ecliptic

Inclination to Sun's equator

Mercury

7.01

3.38

Venus

3.39

3.86

Mars

1.85

5.65

Jupiter

1.31

6.09

Saturn

2.49

5.51

Uranus

0.77

6.48

Neptune

1.77

6.43

Interplanetary medium........The vast majority of the volume of the Solar System

consists of a near-vacuum known as the interplanetary medium. However, along with light, the Sun radiates a continuous stream of charged particles (a plasma) known as the solar wind. This stream of particles spreads outwards at roughly 1.5 million kilometres (932 thousand miles) per hour............Earth's magnetic field stops its atmosphere from being stripped away by the solar wind. Venus and Mars do not have magnetic fields, and as a result, the solar wind causes their atmospheres to gradually bleed away into space..........CELESTIAL SPHERE........ The celestial sphere is an IMAGINARY ROTATING sphere of gigantic radius, concentric and coaxial with the Earth. All objects in the sky can be thought of as lying upon the sphere. Projected from their corresponding geographic equivalents are the celestial equator and the celestial poles. The celestial sphere projection is a very practical tool for positional astronomy.............The celestial sphere is divided by projecting the equator into space. This divides the sphere into the north celestial hemisphere and the south celestial hemisphere. The directions toward various objects in the sky can be quantified by constructing a celestial coordinate system...........Stars will rise in the east, culminate on the north-south line (meridian) and set in the west. On the next night a particular star will rise again, but with our normal clocks running a 24 hour 0 minutes cycle, it will do so 4 minutes earlier.

The Earth in its orbit around the Sun causes the Sun to appear on the celestial sphere moving over the ecliptic (red), which is tilted on the equator (blue)...............SKYMAP.........Sky maps are designed so that when held over your head and aligned with north, the stars in the sky match the stars on the map. On the sky map the Horizon is the the circular edge where the stars stop, and Zenith (the point directly overhead) is at the center of the sky map. The line in the sky from the north point on the horizon, through zenith, and on to the south point on the horizon is called the Meridian.

Notice that East and West in the sky (and hence on the sky map) are REVERSED compared to how they are usually presented on Earth maps. Earth maps are thought of as looking down on the globe, and Sky Maps are designed to be used looking up at the sky. This reversal in direction-of-view results in the reversal of east and west. Notice that if you are looking at the sky facing south, east will be on your left exactly as it is displayed on the Sky Map.............As the Earth rotates from west to east around its axis once every 23 hours 56 minutes, the Celestial Sphere and all objects on it appear to rotate from East to West around the celestial poles in the same time. This is the DIURNAL MOTION...........The path that a star takes over a day is a diurnal circle. Diurnal circles are parallel to the celestial equator, and (except for the celestial equator) are small circles.........Notice that the diurnal circles near the celestial pole never hit the horizon. The stars near the pole don't set...they just circle the pole once per day. Stars near the pole that don't set are called ‘Circumpolar stars’.

The Celestial Co-Ordinate System

The equatorial coordinate system is centered at Earth's center, but fixed relative to distant stars and galaxies. The co-ordinates are based on the location of stars relative to Earth's equator if it were projected out to an infinite distance. The equatorial describes the sky as seen from the solar system. The ecliptic system describes the planets' orbital movement around the sun, and centers on the ‘barycenter’ of the solar system (i.e. very close to the sun). The fundamental plane is the plane of the Earth's orbit, called the Ecliptic Plane. The system is primarily used for computing the positions of planets and other solar system bodies, as well as defining their orbital elements. Celestial Equator

Earth's equator projected out into the celestial sphere is called the Celestial Equator. The Celestial Equator, tilted at 23.5° to the Ecliptic, currently intersects the Ecliptic at 5° sidereal Pisces, which is called the vernal point (VP) (March) and 5° sidereal Virgo (@2000 A.D.), which is called the Anit-VP (September).

North Celestial Pole (NCP) & South Celestial Pole (SCP)

The whole celestial sphere seems to rotate about an axis through its center. We see this axis as two centers of rotation (and hence fixed points) in the sky: the north celestial pole (NCP) and the south celestial pole (SCP). (The south celestial pole is diametrically opposite the NCP, and hence visible only in the southern hemisphere of the Earth.) Of course really the sky is not rotating; rather the Earth is rotating. The

apparent rotation centers in the sky are just the projection of the Earth's rotation axis into the sky. Exactly between these two centers of rotation we sketch the imaginary line of the celestial sphere's equator. The celestial equator is just the projection of the Earth's equator into the sky. Earth's North and South Celestial Poles, which are Earth's poles projected into the heavens, are labelled NCP and SCP. They move very slowly through the heavens as well due to Earth's precession. Earth's NCP currently lies in Ursa Minor, near the star Polaris, and enters the ecliptic at 5° sidereal Gemini (2000 AD). Earth's SCP lies in Octans and enters the ecliptic at 5° sidereal Sag (2000 AD) In the northern hemisphere, the NCP is relatively easy to find in the sky, as it is near (within 1°) a relatively bright (magnitude=2) star: Polaris. Polaris is at the far end of the handle of the "Little Dipper" in the constellation Ursa Minor. To make things even easier, the "Big Dipper" (part of the constellation Ursa Major) can be used to find the "Little Dipper". The two stars that form the lip of the bowl of the "Big Dipper" -known as the "Pointers" -- point the way to Polaris: If you follow the line (green in the below figures) made by connecting the two Pointer stars, you will run into the "Little Dipper" and its brightest star Polaris. (Polaris is about 30°, or two handspans, away from the top pointer star.) Since the stars have constant relative positions, this method works any time you can see the Pointers. Of course, once you've found the NCP, you've also found the direction true north. Here are a couple of examples:

ORBIT OF SUN

While each day the Sun (and the celestial sphere) circles our sky moving from east to west, the Sun is also moving relative to the stars. The Sun's motion through the constellations is much slower and in a direction opposite to the rotation of the celestial sphere: The Sun moves about 1° east per day on the celestial sphere. Over a year the Sun completes a great circle on the celestial sphere. The path of the Sun on the celestial sphere is called theecliptic. The ecliptic is inclined about 23.5° compared to the celestial equator. The moments, two each year, when the Sun moves between hemispheres are calledequinoxes; one happens around March 21, the other around September 21. The spring equinox (when the Sun moves from the southern hemisphere to the northern hemisphere) is called the vernal equinox

This is what the ecliptic looks like on the sky map. Note that celestial equator is labeled, from the vernal equinox going east, 1h, 2h, 3h... this is the Right Ascension (described below). The eastward ecliptic is labeled 15°, 30°, 45° ... this is "celestial longitude" often denoted by the Greek letter lambda: λ. The Sun slowly moves east on the ecliptic, increasing its celestial longitude by about 1° per day. Notice that the Sun's motion on celestial sphere in the opposite direction that the celestial sphere rotates. Thus on the sky map, if we stopped the sphere's rotation while the Sun was on the surface facing you, the Sun would be slowly creeping to your left: eastward. Since the creep of the Sun along the ecliptic (one rotation per year) is so much slower than the daily rotation of the whole celestial sphere (one rotation per day), the Sun is seen to move from east to west in our sky.

CELESTIAL CO-ORDINATES MEASUREMENT

To denote the positions of objects in the sky, astronomers use a system based on the celestial sphere -- two measurements – RIGHT ASCENSION and DECLINATION. Right ascension (abbreviated RA) is similar to longitude and is measured in hours, minutes and seconds eastward along the celestial equatorwith VERANL EQUINOX as the ORIGIN. The distance around the celestial equator is equal to 24 hours.The right ascension of the vernal equinox is 0h 0m 0s. 1 hr=15° ; 1 min=15’ ; 1 sec=15” Declination is similar to latitude and is measured in degrees, arcminutes and arcseconds, north or south of the celestial equator. Positive values for declination correspond to positions north of the equator, while negative values refer to positions south of the equator. The declination of the north celestial pole is 90° 0' 0" and the south celestial pole's declination is -90° 0' 0". Declination at the equator is 0° 0' 0". Right ascension and declination are like longitude and latitude on the surface of the Earth except that they are measured with respect to the celestial spherewith the vernal equinox as the origin.

The advantage of the equatorial coordinate system is that it expresses the position of a star or galaxy in a way that is independent of the observer's position on Earth. However, the right ascension and declination of a given object change slowly over time, mainly due to a phenomenon called PRECESSION. This happens because both the ecliptic and the equator are slowly moving, as a result of tidal forces from the Sun, Moon and planets. The main effect is from the Moon and (to a lesser extent) the Sun, which makes the celestial pole orbit around the ecliptic pole once every 26,000 years. So along with the RA and Dec of an object, you will usually see the date, expressed in years, when those coordinates were approximately valid. This date, or "epoch", defines the precessing equator and equinox used to construct the star catalog. Common examples are B1950.0 and J2000.0, where the B and J stand for slightly different sorts of year. ECLIPTIC CO-ORDINATE SYSTEM

The Ecliptic Co-ordinate System is a celestial co-ordinate system commonly used for representing the positions and orbits of Solar System objects. Because most planets (except Mercury), and many small solar system bodies have orbits with small inclinations to the ecliptic, it is convenient to use it as the fundamental plane. Its primary direction is towards the vernal equinox, and it has a right-handed convention. Ecliptic longitude or celestial longitude (symbol: geocentric ) measures the angular distance of an object along the ecliptic from the primary direction. Like right ascension in the equatorial coordinate system, the primary direction (0° ecliptic longitude) points from the Earth towards the Sun at the vernal equinox of the Northern Hemisphere. Because it is a right-handed system, ecliptic longitude is measured positive eastwards in the fundamental plane (the ecliptic) from 0° to 360°. Ecliptic latitude or celestial latitude (symbol:geocentric ), measures the angular distance of an object from the ecliptic towards the north (positive) or south (negative) ecliptic pole. For example, the north ecliptic pole has a celestial latitude of +90°.

Earth-centered Ecliptic coordinates as seen from outside the celestial sphere. Ecliptic longitude (red) is measured along the ecliptic from the vernal equinox. Ecliptic latitude (yellow) is measured perpendicular to the ecliptic.

ECLIPTIC POLES

les are two points in the heavens that lie exactly perpendicular to the ecliptic TheEcliptic Poles are two points in the heavens that lie exactly perpendicular to the ecliptic plane. They are the "North Ecliptic Pole" (NEP) and "South Ecliptic Pole" (SEP). Because the Star Chart is a rectangular map of the celestial sphere, these points are stretched out to create the entire top and entire bottom of the chart. Planets move west to east in the heavens--right to left in the Star Chart. Most planets lie close to the plane of the ecliptic, but many of the asteroids lie north and south of the ecliptic because their orbits around the sun are tilted with respect to Earth's orbital plane (the ecliptic).

The Galactic Equator

The Galactic Equator, a great circle in our celestial sphere shown as the violet line in the map below, is defined by the spinning disc of our galaxy. The Galactic Equator is tilted at a 60° angle to the ecliptic and intersects the ecliptic at 5° sidereal Sagittarius, the galactic equatorial node (GEN), also called the "Gate of God," and 5° sidereal Gemini, the anti-GEN, called the "Gate of Man." The constellations which lie along this great circle are the "Galactic Constellations." The Center of our Galaxy lies along the galactic equator just south of the Gate of God. Galactic Center (GC) enters the ecliptic at about 2° sidereal Sag. The North and South Galactic poles, which are perpendicular to the galactic plane (galactic equator) are labeled NGP and SGP. The NGP lies in the constellation Coma Berenices, which lies 30° north of the ecliptic, above the head of the Virgin. The NGP enters the ecliptic at 5° sidereal Virgo. The SGP lies in the constellation Sculptor, 30° south of the ecliptic, and enters the ecliptic at 5° sidereal Pisces.

NOTE:

1) Drawin g shows the ecliptic plane passing through the

galactic centre --WRONG – Actually the galactic center is nearly 6° south of the ecliptic. 2)

Similarly, the drawing shows the celestial equator and the NCP being centred on the GC -- WRONG -- The celestial equator is centered on the Earth, not the galactic center. The north celestial pole lies about 27° away from the galactic equator.

Effects of precession

The Earth's axis rotates slowly westward about the poles of the ecliptic, completing one circuit in about 26,000 years.This effect, known as PRECESSION, causes the coordinates of stationary celestial objects to change continuously, ifrather slowly. The Celestial Equator and the vernal points move westward (to the right) through the ecliptic at a rate of about 1° per 72 years with respect to the Galactic Equator, to the sidereal signs, and to the stars--all which remain fixed in the heavens--due to Earth's 25,000-year precessional cycle. Precessional movement is east to west in the heavens, the opposite direction of planetary movement. Therefore, equatorial coordinates (including right ascension) are inherently relative to the year of their observation, and astronomers specify them with reference to a particular year, known as an epoch. Coordinates fromdifferent epochs must be mathematically rotated to match each other, or to match a standard epoch. The currently used standard epoch is J2000.0, which is January 1, 2000 at 12:00 TT. The prefix "J" indicates that it isa Julian epoch. Prior to J2000.0, astronomers used the successive Besselian Epochs B1875.0, B1900.0, andB1950.0.

................................................................................................... Nakshatras ......Definition:  

Naksha- means to approach and Tra means to guard. The Sanskrit word Nakshatra is the term for lunar mansion in Hindu Astrology and means “that which does not decay.” A Nakshatra is one of 27 (sometimes also 28) sectors, identified by the prominent star(s) in them, that the Moon passes through during its monthly cycle, along the ecliptic.The 27 Nakshatras represent consciousness and each Nakshatra represents a particular

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quality of consciousness. Each of the nakshatras is governed as 'lord' by one of the nine ‘Graha’ in the following sequence: Ketu (South Lunar Node), Shukra (Venus), Ravi or Surya (Sun), Chandra (Moon), Mangala (Mars), Rahu (North Lunar Node), Guru or Brihaspati(Jupiter), Shani (Saturn) and Budha (Mercury). This cycle repeats itself three times to cover all 27 nakshatras. Bharateeya Jyotishya shaastra’ states that each Nakshatra name corresponds to a group of stars called ‘star mansions’ or ‘Asterisms’. The concept is that moon visits these mansions in his trajectory around earth. Each nakshatra represents a division of the ecliptic (of 13 degree 20 minutes) similar to the zodiac signs. The mansion associated with a given date corresponds to the constellation which the Moon is passing through at that time. Each nakshatra is further subdivided into quarters – ‘PADAS’, each of 3 degree and 20 minutes. NOTE: 4 * 3° 20’ = 13° 20’.

POSITION: · The starting point for the nakshatras is the point on the ecliptic directly opposite to the star Spica called Chitrā in Sanskrit. It is called Meshādi or the "start of Aries", this is when the equinox — where the ecliptic meets the equator — was in Aries (today it is in Pisces, 28 degrees before Aries starts. Hence, all our Rashis as per Western Stars shall be one before, Ex: Taurus Rashi shall be shifted to Aries – only as per Western Stars, Lunar stars remain same ). The difference between Meṣhādi and the present equinox is known as Ayanāśa denoting by how much of a fraction of degrees & minutes the ecliptic has progressed from its fixed (sidereal) position. Given the 25,800 year cycle for the ‘precession’ of the equinoxes, the equinox was directly opposite Spica in 285 CE. · The ecliptic is divided into each of the nakshatras eastwards starting from this point. · The number of nakshatras reflects the number of days in a sidereal month (modern value: 27.32 days), that the width of a nakshatra is traversed by the moon in about one day. · However, as per the present association of stars with these Nakhatras, several of them are as far as 25º away from the ecliptic where as Moon travels only about 5º on either side of the ecliptic. · The Nakshatras were designed to keep track of the moon’s path in the night sky. · They were also probably used for time keeping over days. LUNAR MONTH...........During the traversal of Moon around the Earth, the Moon is close to some of the fixed stars. Twenty-seven groups of stars that fall on the path of the Moon are identified. In 27.3 days, that is, Moon’s one sidereal revolution, Moon travels through 27 stars that were said to form the 27 Nakshatras...........Hence, on an average Moon travels one Nakshatra every day. The star, which is Closest to the Moon on its path, is called Moon’s Nakshatra. Note that the Sidereal period of the Moon (from Full/New moon to the next full/new moon is 29 days and the Lunar month is defined by this period. Hence in one Sidereal (normal) month, the moon will travel 27 Nakshatras and repeat two more.

NAMES AND POSITION OF NAKSHATRAS No. Name Location (Sidereal Longitude) Ruler 1 Aśvinī (अशशशनन) 0 – 13°20' Aries Ketu 2 Bharaṇī (भरणन) 13°20' – 26°40' Aries (13°20' – 26°40') Venus 3 Kṛttikā (कक शतकक) 26°40' Aries – 10°00' Taurus (26°40' – 40°00' ) Sun 4 Rohiṇī (ररशहणन) 10°00' – 23°20' Taurus (40°00' – 53°20') Moon 5 Mṛgaśiras (मकगशशरक) 23°20' Taurus – 6°40' Gemini (53°20' – 66°40') Mars 6 Ārdrā (आरकर) 6°40' – 20°00' Gemini (66°40' – 80° 00') Rahu 7 Punarvasu (पपनशरसप) 20°00' Gemini – 3°20' Cancer (80° – 93°20') Jupiter 8 Puṣya (पपषय) 3°20' – 16°40' Cancer (93°20' – 106°40') Saturn 9 Āśleṣā (आशललषक) 16°40' Cancer – 0°00' Leo (106°40' – 120°) Mercury 10 Maghā (मघक) 0°00' – 13°20' Leo (120° – 133°20') Ketu 11 Pūrva Phalgunī (पपशरफलगपनन) 13°20' – 26°40' Leo (133°20' – 146°40') Venus 12 Uttara Phalgunī (उतरफलगपनन) 26°40' Leo – 10°00' Virgo (146°40' – 160°) Sun 13 Hasta (हसत) 10°00' – 23°20' Virgo (160° – 173°20') Moon 14 Citrā (शचतक) 23°20' Virgo – 6°40' Libra (173°20' – 186°40') Mars 15 Svātī (सशकतन) 6°40' – 20°00 Libra (186°40' – 200°) Rahu 16 Viśākhā (शशशकखक) 20°00' Libra – 3°20' Scorpio (200° – 213°20') Jupiter 17 Anurādhā (अनपरकधक) 3°20' – 16°40' Scorpio (213°20' – 226°40') Saturn 18 Jyeṣṭha (जयलषक) 16°40' Scorpio – 0°00' Sagittarius (226°40' – 240°) Mercury 19 Mūla (मपल) 0°00' – 13°20' Sagittarius (240° – 253°20') Ketu 20 Pūrva Āṣāḍha (पपशकरषकढक) 13°20' – 26°40' Sagittarius (253°20' – 266°40') Venus 21 Uttara Āṣāḍha (उतरकषकढक) 26°40' Sagittarius – 10°00' Capricorn (266°40' – 280° ) Sun 22 Śravaṇa (शशण) 10°00' – 23°20' Capricorn (280° – 293°20') Moon 23 Dhaniṣṭha (धशनष) 23°20' Capricorn – 6°40' Aquarius (293°20' – 306°40' ) Mars 24 Śatabhiṣaj (शतशभषक) 6°40' – 20°00' Aquarius (306°40' – 320°) Rahu 25 Pūrva Bhādrapadā (पपशरभकरपदक) 20°00' Aquarius – 3°20' Pisces (320° – 333°20')Jupiter 26 Uttara Bhādrapadā (उतरभकरपदक) 3°20' – 16°40' Pisces (333°20' – 346°40') Saturn 27 Revatī (रलशतन) 16°40' – 30°00' Pisces (346°40' – 360°) Mercury This illustration shows the sequence of the 27 equal segments of the Nakshatras, each consisting of 13 degrees 20 minutes, with their Sanskrit names and numbers. As seen from the earth, the moon passes through this circle of the 27 Nakshatras in about 27 days. Thus it takes the moon about one day to pass through one Nakshatra. The moon is shown here moving through Pushya Nakshatra in Karka Rashi. This example also shows the sun moving through Ashvini Nakshatra in Mesha Rashi. Each of the other planets are also moving through the 27 Nakshatras. Ecliptic and stars The ecliptic serves as the center of a region called the zodiac which constitutes a band of 9° on either side. Traditionally, this region is divided into 12 signs of 30° longitude each. By tradition, these signs are named after 12 of the 13 constellationsstraddling the ecliptic. The ecliptic is divided into 27 Nakṣhatras, which are variously called lunar houses or asterisms.

These reflect the moon's cycle against the fixed stars, 27 days and 73 hours, the fractional part being compensated by an intercalary 28th nakṣhatra titledAbhijit. Nakṣatra's computation appears to have been well known at the time of the Ṛigveda (2nd–1st millennium BCE). The ecliptic is divided into the nakhṣhatras eastwards starting from a reference point which is traditionally a point on the ecliptic directly opposite the star Spica called Citrā in Sanskrit. (Other slightly different definitions exist.) It is called Meṣādi - "start of Aries"; this is when the equinox — where the ecliptic meets the equator — was in Aries (today it is in Pisces, 28 degrees before Aries starts). The difference between Meṣādi and the present equinox is known as Ayanāṃśa - denoting by how much of a fraction of degrees & minutes the ecliptic has progressed from its fixed (sidereal) position. Given the 25,800 year cycle for the precession of the equinoxes, the equinox was directly opposite Spica in 285 CE, around the date of the Sūrya Siddhānta. Precession The position of the vernal equinox is not fixed among the stars but due to the lunisolar precession slowly shifting westwards over the ecliptic with a speed of 1° per 72 years. Said otherwise the stars shift eastwards (increase their longitude) measured with respect to the equinoxes. Using the current official constellation boundaries—and taking into account the variable precession speed and the rotation of the ecliptic—the equinoxes shift through the constellations in the Astronomical Julian calendar years (in which the year 0 = 1 B.C.E., -1 = 2 B.C.E., etc.) as follows: The March equinox passed from Taurus into Aries in year -1865, passed into Pisces in year -1967, will pass into Aquarius in year 2597, will pass into Capricorn in year 4312. The June solstice passed from Leo into Cancer in year -1458, passed into Taurus in December year 1989, will pass into Aries in year 4609. The September equinox passed from Libra into Virgo in year -1729, will pass into Leo in year 2439. The December solstice passed from Capricorn into Sagittarius in year -1130, will pass into Ophiuchus in year 2269, and will pass into Scorpius in year 3597. Figure 13 shows Sun location on March 21, 2400 BC . The Sun is on equator and is pointing to Krittika nakshatra.

Figure 16 shows the position of Sun on Spring Equinox day of 400BC (March 21).

Compare this to Figure 14 which shows Sun 4400 years later, on March 21,2000 at same equator but Sun is on Poorva Bhadrapada. Thus Sun has Precessed by about 57 degrees in 4400 years

Since each sign of the zodiac is

composed of 30 degrees, each astrological age might be thought to last about 72 (years) × 30 (degrees) = about 2160 years. This means the Sun crosses the equator at the vernal equinox moving backwards against the fixed stars from one year to the next at the rate of one degree in seventy-two years, one constellation (on average) in about 2160 years, and the whole twelve signs in about 25,920 years, sometimes called a Platonic Year. However the length of the ages are decreasing with time as the rate of precession is increasing. Therefore no two ages are of equal length. ****

MOON Characteristics :: Diameter

3,474.8 km

Mass

7.349×1022 kg

Semi-major axis

384,400 km

Orbital period

27 d 7 h 43.7 m

Mean inclination of orbit to ecliptic Mean obliquity

5.14° (4.99 – 5.30) 6.58°

Mean inclination of lunar equator to ecliptic 1.543° IMPORTANT NOTE :: The Earth–Moon plane is tilted up to ±5.14 degrees against the Earth–Sun plane. Without this tilt, there would be an eclipse every two weeks, alternating between lunar eclipses and solar eclipses.

ORBIT :: The Earth and Moon orbit about their barycentre (common centre of mass), which lies about 4600 km from Earth's centre (about three quarters of the Earth's radius). The Moon differs from most satellites of other planets in that its orbit is close to the plane of the ecliptic, and not to the Earth's equatorial plane. The lunar orbit plane is inclined to the ecliptic by about 5.1°, whereas the Moon's spin axis is inclined by only 1.5°. On average, the Moon is at a distance of about 385000 km from the centre of the Earth, which corresponds to about 60 Earth radii. With a mean orbital velocity of 1,023 m/s, the Moon moves relative to the stars each hour by an amount roughly equal to its angular diameter, or by about 0.5°. The Moon differs from most satellites of other planets in that its orbit is close to the plane of the ecliptic, and not to the Earth's equatorial plane. The lunar orbit plane is inclined to the ecliptic by about 5.1°, whereas the Moon's spin axis is inclined by only 1.5°. The orbit of the Moon is distinctly elliptical.

The Earth's Moon is the fifth largest in the whole solar system, and is bigger than the planet Pluto. The Moon has a nearly circular orbit (e=0.05) which is tilted about 5° to the plane of the Earth's orbit. Its average distance from the Earth is 384,400 km. The combination of the Moon's size and its distance from the Earth causes the Moon to appear the same size in the sky as the Sun, which is one reason we can have total solar eclipses.

Inclination :: The mean inclination of the lunar orbit to the ecliptic plane is 5.145°. The rotation axis of the Moon is also not perpendicular to its orbital plane, so the lunar equator is not in the plane of its orbit, but is inclined to it by a constant value of 6.688° (this is the obliquity). One might be tempted to think that as a result of the precession of the Moon's orbital plane, the angle between the lunar equator and the ecliptic would vary between the sum (11.833°) and difference (1.543°) of these two angles. However, as was discovered by Jacques Cassini in 1721, the rotation axis of the Moon precesses with the same rate as its orbital plane, but is 180° out of phase (see Cassini's Laws). Thus, although the rotation axis of the Moon is not fixed with respect to the stars, the angle between the ecliptic and the lunar equator is always 1.543°. Ecliptic and Moon The orbit of the Moon is inclined by about 5° on the ecliptic. Its nodal line is not fixed either, but regresses (moves towards the west) over a full circle every 18.6 years. This is the cause of nutation and lunar standstill. The moon crosses the ecliptic about twice per month. If this happens during new moon a solar eclipse occurs, during full moon a lunar eclipse. This was the way the ancients could trace the ecliptic along the sky; they marked the places where eclipses could occur.

The Moon's orbital period is 27.322 days. Because of this motion, the Moon appears to move about 13° against the stars each day, or about half of a degree per hour. If you watch the Moon over the course of several hours one night, you will notice that its position among the stars will change by a few degrees. The changing position of the Moon with respect to the Sun leads to lunar phases. Have you ever heard the term the 'far-side' of the Moon? Because of the effect on the Moon of tidal forces due to the Earth, the same side of the moon always faces the Earth. The rotation period and the orbital period of the Moon are the same. Therefore, Earth-bound observers can never see the 'far-side' of the Moon. Tidal forces cause many of the moons of our solar system to have this type of orbit. Revolution in Orbit The Moon appears to move completely around the celestial sphere once in about 27.3 days as observed from the Earth. This is called a ‘sidereal month’, and reflects the corresponding orbital period of 27.3 days The moon takes 29.5 days to return to the same point on the celestial sphere as referenced to the Sun because of the motion of the Earth around the Sun; this is called a ‘synodic month’ (Lunar phases as observed

from the Earth are correlated with the synodic month).. Since the Moon must move Eastward among the constellations enough to go completely around the sky (360 degrees) in 27.3 days, it must move Eastward by 13.2 degrees each day (in contrast, remember that the Sun only appears to move Eastward by about 1 degree per day). Thus, with respect to the background constellations the Moon will be about 13.2 degrees further East each day. Since the celestial sphere appears to turn 1 degree about every 4 minutes, the Moon crosses our celestial meridian about 13.2 x 4 = 52.8 minutes later each day. The Moon has a rotational period of 27.3 days that (except for small fluctuations) exactly coincides with its (sidereal) period for revolution about the Earth. As we will see later, this is no coincidence; it is a consequence of tidal coupling between the Earth and Moon. Because of this tidal locking of the periods for revolution and rotation, the Moon always keeps essentially the same face turned toward the Earth. The Quarter Moons occur when the Sun and the Moon are 90º degrees apart in the sky as viewed from the Earth. The New and Full phases occur during times when the Earth, Moon and Sun are in a straight line. How long does it take the Moon to orbit once around the Earth? It takes about 27.3 days. Why not 29.5 days (the time for the phase cycle)? Again, it has to do with the fact that the Earth is moving around the Sun. Take a look at Figure 1. It shows the variation from one Full moon to the next. Remember, the Moon has to be in a straight line with the Earth and Sun for it to be Full. It starts out lined up with the Sun, but after 27.3 days, the Moon will have made one complete orbit of the Earth (again be located to the left of the Earth). At this time is it Full? No, because it is not in a perfect line with the Sun. You have to wait about 2 more days for it to again be aligned with the Sun and for it to be Full again.

Figure 1. The Moon makes one complete orbit of the Earth in 27.3 days, but it will not be again Full until a total of 29.5 days has passed.

One orbit of the Moon takes 27.3 Days. This would be the Moon's Sidereal Period since it is the time for the Moon to be back in the same location relative to the stars, and this is also the time for one orbit. How long does it take for one rotation (spin) on its axis? Does the Moon actually spin on its axis? If you said "no," then you're wrong. The Moon does spin on its axis, but it does it in 27.3 days. That's the same amount of time for one orbit - what does that mean? It means that one side of the Moon always faces the Earth - that the Moon has one side tidally locked with the Earth Cycle lengths :: However, regardless of the culture, all lunar months approximate the mean length of the synodic month, or how long it takes on average to pass through each phase (new, half, full moon) and back again. It takes 29.53059 days (29 days, 12 hours, 44 minutes and 3 seconds). The moon completes its orbit around the earth in 27.3 days (the sidereal month), but due to the Earth's motion around the sun it has not finished a full (synodic) cycle until it reaches the point in its orbit where the sun is in the same position. A synodic month is the most familiar lunar cycle, defined as the time interval between two consecutive occurrences of a particular phase (such as new moon or full moon) as seen by an observer on Earth. The mean length of the synodic month is 29.53059 days (29 days, 12 hours, 44 minutes, 2.8 seconds). Due to the eccentric orbit of the lunar orbit around Earth (and to a lesser degree, the Earth’s elliptical orbit around the Sun), the length of a synodic month can vary by up to seven hours. 2. The draconic month or nodal month is the period in which the Moon returns to the same node of its orbit; the nodes are the two points where the Moon's orbit crosses the plane of the Earth's orbit. Its duration is about 27.21222 days on average. 3. The tropical month is the average time for the Moon to pass twice through the same equinox point of the sky. It is 27.32158 days, very slightly shorter than the sidereal month (27.32166) days, because of precession of the equinoxes. Unlike the sidereal month, it can be measured precisely. 4. The sidereal month is defined as the Moon's orbital period in a non-rotating frame of reference (which on average is equal to its rotation period in the same frame). It is about 27.32166 days (27 days, 7 hours, 43 minutes,11.6 seconds). The exact duration of the orbital period cannot be easily determined, because the 'non-rotating frame of reference' cannot be observed directly. However, it is approximately equal to the time it takes the Moon to pass twice a "fixed" star (different stars give different results because all have proper motions and are not really fixed in position). A synodic month is longer than a sidereal month because the Earth-Moon system is orbiting the Sun in the same direction as the Moon is orbiting the Earth. Therefore,

the Sun appears to move with respect to the stars, and it takes about 2.2 days longer for the Moon to return to the same apparent position with respect to the Sun. A draconic month is shorter than a sidereal month because the nodes move in the opposite direction as the Moon is orbiting the Earth, one revolution in 18 years. Therefore, the Moon returns to the same node slightly earlier than it returns to the same star.

5 Anomalistic month :: Like all orbits, the Moon's orbit is an ellipse rather than a circle. However, the orientation (as well as the shape) of this orbit is not fixed. In particular, the position of the extreme points (the line of the apsides: perigee and apogee), makes a full circle (lunar precession) in about nine years. It takes the Moon longer to return to the same apsis because it moved ahead during one revolution. This longer period is called the anomalistic month, and has an average length of 27.554551 days (27 d 13 h 18 min 33.2 s). The apparent diameter of the Moon varies with this period, and therefore this type has some relevance for the prediction of eclipses (see Saros), whose extent, duration, and appearance (whether total or annular) depend on the exact apparent diameter of the Moon. The apparent diameter of the full moon varies with the full moon cycle which is the beat period of the synodic and anomalistic month, and also the period after which the apsides point to the Sun again. Tithi :: The lunar day, called a tithi,-- (Tithi (Lunar Phase)-- stated a tithi is a measurement of 12 degrees of longitudinal separation between the sun and the moon. At new moon (amavasya) the sun and the moon are separated by zero degrees. We can say they overlap. As they begin to separate the first tithi begins when the sun and the moon have separated by 12 degrees. The second tithi begins when they are separated by 24 degrees. The third tithi begins when they have separated by 36 degrees. The digit of the moon is new clearly visible. And so it goes until the sun and moon have separated by 180 degrees. This tithi is called full moon, purnima. These first 15 tithis or phases of the moon make up the waxing phases of the moon which in Sanskrit this is called the sukla-paksa. This is the bright side of the lunar month. After purnima, full moon, the tithi begins again counting from one as the longitudanal separation between the sun and the moon decreases back to zero. This is called the waning phase of the moon or in Sanskrit, the krsna-paksa or dark side of the lunar month. At certain times of the month when the sun and moon can both be seen in the sky at the same time you can estimate the tithi by using the hand method to measure the longitudinal separation between the sun and the moon.

“Solar day begins at midnight whereas the lunar tithi can begin at anytime of the solar day. tithi can last between 19 to 26 hours due to the changing speed of the earth and moon in their obits. On average a tithi lasts for only 0.95 of a solar day.” Yoga (The Luni-solar Day) :: The yoga (luni-solar day) is the period during which the combined longitudinal motion of the sun and moon amounts to 13 degrees and 20 minutes. Like the naksatras there are 27 yogas. Recall that a tithi was 12 degrees of longitudinal separation between the sun and moon, the yoga is the combined longitudinal motion of the sun and the moon. Masa (Month) :: The Hindu year contains twelve lunar months named after the nakshatra in which the moon is full. The names of the Indian months originated from the names of the nakshatras where purnima (the full moon) always takes place. Of the twenty-seven nakshatras only twelve of them have full moons. Caitra (March - April) (citra-naksatra) Vaisakha (April - May) (visakha-naksatra) etc. The Year :: Another aspect of the lunar calendar is that its twelve months based on the lunar days (tithis) contain about 354 days. So just as every 4th year on the solar calendar must add an extra day to make up for the discrepancy in the earth's orbit around the sun, so every 30 months the lunar calendar must add an extra month. This leap-month (adika-masa) is generally inserted after the months of Asadha or Sravana and is called either a second Asadha or Sravana. Thus every second or third year contains 13 months. The Hindu calendar is therefore luni-solar, with a precise month and an approximate year. Path of Earth and Moon around Sun :: When viewed from the north celestial pole i.e., from the star Polaris, the Moon orbits the Earth counter-clockwise, the Earth orbits the Sun counter-clockwise, and the Moon and Earth rotate on their own axes counter-clockwise. The orbital velocity of the Moon about the Earth (1 km/s) is small compared to the orbital velocity of the Earth about the Sun (30 km/s). Precession :: There are two important precessional motions in the Orbit of the Moon.

The long axis (line of the apsides: perigee and apogee) of the moon's elliptical orbit precesses about once in just under 9 years. It is caused by the solar tide. This precession period is equal to the time that number of sidereal months counted exceeds the number of anomalistic months counted by exactly one. This happens after about 3233 days. This precession causes the full moon cycle to be over a month longer than a sidereal year. There are approximately two such lunar precession cycles in a saros cycle. This is to be distinguished from precession of the lunar nodes of the lunar orbit on the plane of the ecliptic. This is mainly caused by the oblation of the Earth; it is the period of the main nutation term in the orientation of the polar axis of the Earth. This nodal period is about twice as long as the apsidal precession period discussed above. After the nodal period, the number of draconic months counted exceed the number of sidereal months counted by exactly one: this happens after about 6793 days (18.6 years). Saros Cycle :: The Saros cycle is an eclipse cycle with a period of about 18 years 11 days 8 hours (approximately 6585⅓ days) that can be used to predict eclipses of the Sun and Moon. One Saros after an eclipse, the Sun, Earth, and Moon return to approximately the same relative geometry, and a nearly identical eclipse will occur. The Saros cycle of 18 years 11 days 8 hours is very useful for predicting the times at which nearly identical eclipses will occur, and is intimately related to three periodicities of the lunar orbit: the synodic month, the draconic month, and the anomalistic month. For an eclipse to occur, either the Moon must be located between the Earth and Sun (as for a solar eclipse) or the Earth must be located between the Sun and Moon (as for a lunar eclipse). This can happen only when the Moon is new or full, and repeat occurrences of these lunar phases are controlled by the Moon's synodic period, which is about 29.53 days. Most of the times during a full and new moon, however, the shadow of the Earth or Moon falls to the north or south of the other body. Thus, if an eclipse is to occur, the three bodies must also be nearly in a straight line. This condition occurs only when the Moon passes close to the ecliptic plane and is at one of its two nodes (the ascending or descending node). The period of time for two successive passes of the ecliptic plane at the same node is given by the draconic month, which is 27.21 days. Finally, if two eclipses are to have the same appearance and duration, then the distance between the Earth and Moon must be the same for both events. The time it takes the Moon to orbit the Earth once and return to the same distance is given by the anomalistic month, which has a period of 27.55 days.

EARTH Symbol

Orbital period

365.256363004 days -- 1.000017421 year

Average orbital speed

29.78 km/s -- 107,200 km/h

Orbit Inclination

7.155° to Sun's equator, 1.57869° to invariable plane

Aphelion 1.01671388 AU

152,098,232 km (94,509,460 mi),

Perihelion 0.98329134 AU

147,098,290 km (91,402,640 mi),

Equatorial rotation velocity

1,674.4 km/h (465.1 m/s)

Axial tilt

23°26'21".4119

EARTH ROTATION Earth's rotation is the rotation of the solid Earth around its own axis. The Earth rotates from the west towards the east. As viewed from the North Star or pole star Polaris, the Earth turns counter-clockwise. Earth gravitationally interacts with other objects in space, especially the Sun and the Moon. The Earth rotates once in about 24 hours with respect to the sun and once every 23 hours 56 minutes and 4 seconds with respect to the stars. Hence, during one orbit around the Sun, the Earth rotates about its own axis 366.26 times for one sidereal year, creating 365.26 solar days. Sidereal and Solar day Length of a Day - Solar versus Sidereal How long does it take the Earth to spin around exactly once?

If we time the motion of the Sun, we see that it takes almost exactly 24 hours for the Sun to get back to where it started from one day to the next. How long does it take a star to get back to the same place in the sky from one day to the next? Does it take 24 hours for one complete rotation? No it doesn't. It takes 23 hours and 56 minutes. If a star rises tonight at 8 P.M., it will rise at 7:56 the next night, then 7:52 the night after, and then 7:48 the next night. A week after the first rise time, it will rise 4 x 7 = 28 minutes earlier (7:32). In one week, a star will be rising about half an hour earlier - that's a pretty big difference, so don't ignore those four minutes. Remember, it is the spinning of the Earth that causes the observed motions of the Sun and the stars over the course of the day (or night) - but there are two different time spans here - which one corresponds to the rotation period of the Earth? It is the stars, not the Sun that determine the amount of time for one rotation of the Earth. While all clocks on the Earth are based on the 24 hour time scale of the Solar Day, it is the more subtle Sidereal Day (or "star" day) that tells us how fast the Earth is spinning. It takes the Earth 23 hours and 56 minutes to complete one rotation. A ‘nakshatra dina’ or a sidereal day is the time taken by any nakshatra (fixed star) to perform one complete revolution around the Earth (relative to an observer on Earth). The Latin word "sidereal" means "pertaining to the (fixed) stars" The duration of a sidereal day is thus the period of one complete rotation of Earth around its axis. The sidereal day differs slightly from the commonly used civil day comprising 24 hours. The latter is called ‘sāvana dina’ or solar day. The length of a solar day is the time taken by the Sun to go around the Earth once; for instance, the duration between two successive sunrises. The sidereal day is shorter than the solar day, as can be seen from following figure. At time1, the Sun and a certain distant star are both overhead. At time2, the planet has rotated 360° and the distant star is overhead again but the Sun is not (1→2 = one sidereal day). It is not until a little later, at time 3, that the Sun is overhead again (1→3 = one solar day).

Note: SIDEREAL DAY IS SHORTER THAN SOLAR DAY Axial tilt and seasons Due to the axial tilt of the Earth, the amount of sunlight reaching any given point on the surface varies over the course of the year. This causes seasonal change in climate, with summer in the northern hemisphere occurring when the North Pole is pointing toward the Sun, and winter taking place when the pole is pointed away. During the summer, the day lasts longer and the Sun climbs higher in the sky. In winter, the climate becomes generally cooler and the days shorter. In the southern hemisphere the situation is exactly reversed, with the South Pole oriented opposite the direction of the North Pole. EARTH ORBIT Earth orbits the Sun at an average distance of about 150 million kilometers every 365.2564 mean solar days, or one sidereal year. From Earth, this gives an apparent movement of the Sun Eastward with respect to the stars at a rate of about 1°/day. Due to this motion, on average it takes 24 hours —a solar day—for Earth to complete a full rotation about its axis so that the Sun returns to the meridian. The orbital speed of the Earth averages about 29.8 km/s (107,000 km/h), which is fast enough to travel a distance equal to the planet's diameter, about 12,742 km, in seven minutes, and the distance to the Moon- 384,000 km in about 3.5 hours. Ecliptic and equator The plane of the Earth's orbit is called ECLIPTIC when it is projected onto the imaginary “celestial sphere”. The Earth's axis of rotation is at a 23.5° to the plane of the Earth's orbit. As the rotation axis of the Earth is not perpendicular to its orbital plane, the equatorial plane is not parallel to the ecliptic plane, but makes an angle of about 23°26' which is known as the obliquity of the ecliptic. The celestial equator and the ecliptic are also at a 23.5° angle to each other. The intersections of the equatorial and ecliptic planes with the celestial dome are great circles known as the celestial equator and the ecliptic respectively.

The intersection line of the two planes results in two diametrically opposite intersection points, known as the EQUINOXES. The Equinox which the Sun passes from south to north is known as the vernal equinox or first point of Aries. Ecliptic longitude, usually indicated with the letter λ, is measured from this point on 0° to 360° towards the east. Ecliptic latitude, usually indicated with the letter β is measured +90° to the north or -90° to the south. The same intersection point also defines the origin of the equatorial coordinate system, named RIGHT ASCENSION measured from 0 to 24 hours also to the east and usually indicated with α or R.A., and DECLINATION, usually indicated with δ also measured +90° to the north or -90° to the south. Simple rotation formulas allow a conversion from α, δ to λ, β and back (see – ‘Celestial Sphere’) The ecliptic is the apparent path that the Sun traces out in the sky, as it appears to move in the sky in relation to the stars, this apparent path aligns with the planets throughout the course of the year. More accurately, it is the intersection of a spherical surface, the celestial sphere, with the ecliptic plane, which is the geometric plane containing the mean orbit of the Earth around the Sun. The name ecliptic is derived from being the place where eclipses occur. Solictice and Equinox

By astronomical convention, the four seasons are determined by the Solstices (summer & winter) — the point in the orbit of maximum axial tilt toward or away from the Sun and the Equinoxes (autumn & spring), when the direction of the tilt and the direction to the Sun are perpendicular. In the northern hemisphere, Winter Solstice occurs on about December 21, Summer Solstice is near June 21, Spring Equinox is around March 20 and Autumnal Equinox is about September 23. In the Southern hemisphere, the situation is reversed, with the Summer and Winter Solstices exchanged and the Spring and Autumnal Equinox dates switched. It is 94 days from the June solstice to the September equinox, but only 89 days from the December solstice to the March equinox. The seasons are not of equal length because of the variable speed the Earth has in its orbit around the Sun. The instances of the equinoxes are not fixed but fall about six hours later every year, amounting to one full day in four years, but then they are reset by the occurrence of a leap year. The Gregorian calendar is designed to follow the seasons as accurately as is practical. Within one year, the Sun is north of the equator for about 186.40 days, while it is 178.24 days south of the equator. The following diagram shows the relation between the line of solstice and the line of apsides of Earth's elliptical orbit. The orbital ellipse (with eccentricity exaggerated for effect) goes through each of the six Earth images, which are sequentially 1)the perihelion (periapsis—nearest point to the Sun) on anywhere from 2 January to 5 January 2)the point of March equinox on 20 or 21 March 3)the point of June solstice on 20 or 21 June 4)the aphelion (apoapsis—farthest point from the Sun) on anywhere from 4 July to 7 July 5)the September equinox on 22 or 23 September, and 6)the December solstice on 21 or 22 December. Note that the diagram shows an exaggerated representation of the shape of Earth's orbit. In reality, the actual path of Earth's orbit is not as eccentric as that portrayed in the diagram.

Earth’s Precession There are two possible definitions of a year as observed from Earth. A ‘Sidereal year’ is time taken for Sun to move from one star, and then come back to same star. This is full 360 degrees movement of Earth around the Sun (360 degrees). A ‘Solar year’ is the time taken by Sun in its passage from one equinoctical point back to same point (359.9864 degrees).

Solar year is shorter than Sidereal year by about 19 minutes and 50 seconds and is said to be caused by inertial effects. It is called ‘Earth's Precession’. The position of ‘Chitra’ (Spica), which is very near the ecliptic, was recorded by Hipparchus (circa 150 BC) on the autumnal Equinox day with reference to Sun. After 300 years, it was found that ‘Chitra‘ appeared to have moved about 3 degrees toward Sun. This is because Sun is moving one degree for 100 years in reference to stars. Today we know that Earth's Precession rate is about 1.36 degrees for 100 years. But the 359.9864 degrees Solar year is relevant because of repeating weather and seasons and hence Solar year is a natural year. That means every solar year, the Sun position drops back by about 0.0136 degrees with respect to stars. Also, it may be noted that the earth’s spin polar axis also shifts by same angle of 0.0136 degrees per year in a coning motion. The diagram below shows 27 nakshatra's in a circle of diameter of the order of few hundred light years with sun at the center and the effect of Earth's Precession on Sun/Star/Earth's position exactly at the time of Spring Equinox (March 21). In this diagram, the Sun is always on equator and the day time equals night. Diagram shows Earth's position from 2400 BC to 2000 AD. Every year, the Earth's seasons start occurring 0.0136 degrees (or roughly 0.0136 days) earlier. The diagram also shows twenty-seven Bharateeya nakshatra's in the infinite distance in the ecliptic plane. It should be noted that the Precession does not change the Equator position or the Earth's tilt of 23.5 degrees. It only changes the direction of polar axis.

Now let us go out of Earth to‘Celestial Sphere’