Axial Precession

Axial precession is an astronomical concept. It concerns the change of orientation of a heavenly body in its rotational axis. Precession is a slow rotation of the Earth’s axis around an axis perpendicular to the ecliptic plane. 

It is similar to the motion of a spinning toy with its top tilting little downwards from the vertical which moves in a circular axis. Earth faces the axial precession due to its non-spherical shape and the gravitational tidal forces. The precession of the equinoxes (lunisolar precession) is caused by the gravitational forces of the Sun and the Moon and to a lesser extent other bodies, on the Earth. It was first explained by Sir Isaac Newton. It causes seasonal effect; slow shift of the season backwards every year.

History

Greek astronomer, mathematician Hipparchus first estimated the precession of the Earth’s axis around 130 B.C. He noticed that the positions of the stars were shifted systematically from its initial position every year. As the separation of stars didn’t vary, this indicated that it was not the stars that were moving but rather the observing platform-Earth. Unfortunately, all of the writings of Hipparchus are lost, including his work on precession.

The first astronomer known to have continued Hipparchus’ work on precession is Ptolemy in the second century AD. He measured the longitudinal arc separating the Moon from the Sun. Also, he measured the arc from the Moon to the star. Using Hipparchus’ model to calculate the Sun’s longitude, he made corrections for the Moon’s motion and its parallax.

Over a century later, precession was explained by Sir Isaac Newton. Newton’s original precession equations did not work. And they were corrected by the group of scientists after certain decades.

What causes the Precession of Earth?

Earth faces two kinds of precession:

1) Lunisolar precession

Lunisolar precession is about 500 times greater than planetary precession which leads to the dominant component. The gravitational forces (tug) of the Moon and Sun on Earth’s equatorial bulge caused the Earth’s axis to move into an inertial space. The revolution of the Earth around the sun and the revolution of the Moon about the Earth causes the Earth to jiggle about its axis of rotation (earth tilts by 23.5° from the vertical). Hence, the axis of the Earth undergoes precession due to a combination of the Earth’s non-spherical shape and the gravitational tidal forces, applying torque. This type of precession is Lunisolar precession.

2) Planetary precession

Planetary precession is due to the small angle between the gravitational force of the other planets on Earth and its orbital plane. It is the perturbation of the Earth’s orbital plane by the attractions of the planets on the Earth’s Centre of mass. It causes the equinox to move eastwards along the celestial equator (i.e. in the opposite direction to lunisolar precession).

What are the effects of precession?

Its effect very less so takes lots of years to complete one rotation. The period of precession is about 25,772 years. Some major effects are :

  1. About 13,000 years later, i.e. after half time of rotation, winter will be in the months of June-July and people will celebrate Christmas on a hot summer day in the Northern Hemisphere.

2. While today the star Polaris lies approximately at the north celestial pole, this will change over time, and other stars will become the “north star.” 

3. Approximately 12,000 years from now, Earth’s axis of rotation line up with star Vega and after 23,000 years, with star Thuban (Alpha Draconis).

4. The length of the sidereal year is 31,558,149.993 secs, whereas the length of the tropical year is of 31,556,925.511 secs. The sidereal year is about 1,224.48 secs longer than the tropical year. So, every year we face the shift of seasons by 1,224.48 secs ≈ 20.4 min.

5. In every 72 years, the seasons shift by 1 day early and it takes about 71.6 years for the stars to move by 1° from the original position.

6. Hindu Nepali astrologers believe that the shortest day is 15 of Poush (December 30) and people celebrate this day as a special event.

But, scientists claim December 22 as the shortest day of the year. Hindu astrologers and modern scientific fact don’t match because they predicted Poush 15 (December 30) as shortest day 8×72 years earlier from the present.

References

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