PRECESSION OF EQUINOXES

PRECESSION OF EQUINOXES / PRECESSION (SAMPĀT-CHALANAM)

Dictionary meaning: Precession is a comparatively slow gyration of the rotation axis of a spinning body about another line intersecting it so as to describe a cone.
• The Earth's axis rotates (precesses) just as a spinning top does. The period of precession is about 26,000 years.
• Therefore, the North Celestial Pole will not always be point towards the same starfield.

Precession is caused by the gravitational pull of the Sun and the Moon on the Earth. Motion of the equinoxes along the ecliptic (the plane of Earth’s orbit) caused by the cyclic precession of Earth’s axis of rotation.

Our rishis observed skies continuously for a long period and noticed that the longitude of all the stars was increasing by the same amount and there were no change in latitude. They concluded that ecliptic was not moving backward but the Vernal Equinox is moving backward this backward motion is called Precession of the equinoxes. In other words we can say Motion of the equinoxes along the ecliptic (the plane of Earth’s orbit) caused by the cyclic precession of Earth’s axis of rotation.

Hipparchus (Greek astronomer)about 125 b.c. also led to the conclusion that while the ecliptic is practically a fixed great circle on the celestial sphere with reference to the background of stars, yet the celestial equator moves so that the first point of Aries is carried backwards on the ecliptic. He did not know the cause of the phenomenon, but was able to measure it with a fair degree of accuracy.

The effect of precession is to make backwards move along the ecliptic at the rate of 50"2 a year, so that the longitudes of the stars increase by this amount each year while their latitudes remain unchanged. 

The earth revolves around the sun in a Elliptical orbit -- and at the same time the earth wobbles on its own axis Precession is caused by the gravitational pull of the Sun and the Moon on the Earth Because the protuberance (Earth is bulge out) at the equator is slightly nearer the sun and moon than are the other portions of the earth, the attraction there is greater, and the tendency of the pull of the sun and moon is to make the equator coincide with the ecliptic. This causes the wobbling motion of earth like a spinning top so that the axis of earth pointing in heavens to which it is directed (Celestial pole) describes a small circle round the Pole of Ecliptic (Kadamb). The North Pole of axis completes one revolution round the pole of ecliptic in 25800 years.

  
 

Ayanamsa : Sanskrit term Ayanamsa is the angle by which the sidereal ecliptic longitude of a celestial body is less than its tropical ecliptic longitude. Ayanamsa is around 24°03’54’’ (Lahiri) today. The ayanamsha describes the increasing gap between the tropical and sidereal zodiacs. The ayanamsa, changes continually through the Precession of the Equinoxes at the rate of approximately 50" a year. Suryasiddhaanta states that ayanamsha was zero in 499 AD (Mesha Samkranti). As earth’s precession changes every year, the difference between the tropical zodiac and the sidereal zodiac changes. This difference is called “ayanamsa” (sidereal difference) The angular distance between vedic first point and the vernal equinox is the ayanamsa. There are so many other definitions on Ayanmsa and so many values taken by different scholars at different times.  Lahiri - 285 AD ,B. V. Raman - 397 AD, Krishnamurthi - 291 AD, Yukteshwar - 499 AD

Nutation : The moon's orbit is inclined at more than 5° to the ecliptic, and intersects the ecliptic in two points known as the nodes. These nodes have a motion round the ecliptic, completing a revolution in less than 19 years, and during this time the inclination of the moon's orbit to the equator varies between 23£° ± 5°, that is, between 18£° and 2U°. Her effect on the earth's equatorial regions varies also owing to the different inclinations; hence, precession does not proceed at a uniform rate. The result is that the curve described by the axis of the earth is not exactly a circle but fluctuates slightly, the pole 'nodding', for which reason this phenomenon is called nutation (from the Latin nutare, to nod). Although (51) gives accurate co-ordinates of the stars, provided the interval is not too long, it takes no account of nutation effects. These must be taken into consideration in all cases where accuracy is required, and formulae for computing nutation, etc., will be given later when we come to deal with certain constants given in the A.E.

The Seasons on Earth

The Earth's axis is tilted from perpendicular to the plane of the ecliptic by 23.45°. This tilting is what gives us the four seasons of the year - spring, summer, autumn (fall) and winter.

Since the axis is tilted, different parts of the globe are oriented towards the Sun at different times of the year

The combined effect of the Earth's annual motion around the Sun and the tilt of its axis cause the seasons.

When the earth's axis points towards the sun, it is summer for that hemisphere. When the earth's axis points away, winter can be expected. Since the tilt of the axis is 23 1/2 degrees, the North Pole never points directly at the Sun, but on the summer solstice, it points as close as it can, and on the winter solstice as far as it can. Midway between these two times, in spring and autumn, the spin axis of the earth points 90 degrees away from the sun. This means that on this date, day and night have about the same length: 12 hours each, more or less.The length of time the Sun is above the horizon (daylight) changes. The path the Sun follows across the sky varies; the Sun's declination changes.

The noontime altitude of the Sun changes, causing the amount of concentration (area over which rays of sunlight are spread) of the sunlight to vary.

In summer, the Earth is tipped toward the Sun; in winter, away.

When it is summer in the northern hemisphere, it is winter in the southern.

The Sun's declination changes during the year too.