Precession of the Equinoxes

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Precession of Earth

The precession of the earth is a type of slow gyration, that rotates in its rotational axis like a spinning type body about another line that intersects it so as to describe a cone structure.

The Precession of the Earth's Axis:

  • The Earth's axis rotates similar to a spinning top. The period taken by the precession is approximately 26,000 years.

  • Hence, the North Celestial Pole will not always be pointing towards the same starfield.

  • The precession of the earth is for the gravitational pull of the Sun and the Moon on the Earth.


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Hipparchus was the first scientist to discover the precession of the earth, and he estimated that the precession of the earth’s axis is around 130 BC. While the period of precession being 26,000 years. 

The Pole Star located in the northern hemisphere is called the Polaris this is of 3000 B.C. The north celestial pole coincides with the star called the Thuban star, this star is of the constellation of Draco. In 14,000 A.D. the Vega, in Lyra, will be the northern pole star.


Precession of the Equinoxes 


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Precession of the equinoxes means the motion caused by the equinoxes along the ecliptic, that is on the plane of the earth’s orbit. This is caused by the cyclic precession of the earth’s axis in its rotation.  

While compiling his star catalogue which completed in 129 BCE, the Greek astronomer, named Hipparchus noticed that the positions of the stars get shifted in a systematic way from the Babylonian (Chaldean) measures. This indicated that it was not the stars that were moving but it was the observing platform that is the earth that was moving. This motion of the body is called the precession and this consists of a cyclic type wobbling which happens in the orientation of Earth’s axis of rotation with a period of approximately 25,772 years. The motion of precession is the third-discovered motion of Earth, the other two preceding it are the daily rotation and the annual revolution. The precession of the earth is caused by the gravitational influence of the sun and by the moon which acts on the Earth’s equatorial bulge. While, to a lesser extent, the planets exert an influence on each other as well.

The projection of the sky in the Earth’s axis of rotation results in two points that are in the opposite directions, they are - the north and south celestial poles. This is for the precession the points trace out as circles on the sky. The north celestial pole generally points to within 1° of the arcs of the Polaris region. This will point closest to the Polaris of about 2100 CE. In the next 12,000 years, the north celestial pole will face about 5° from Vega. At present, the south celestial pole does not point towards the vicinity of any bright star.

This projection is of the sky of the Earth’s Equator. This projection is a great circle, which is called the celestial equator. Further, the celestial equator intersects another useful great circle, called the ecliptic. As the Earth orbits or revolves around the Sun, this constantly changes its own direction at an angle, from which the Sun is viewed to trace out to the ecliptic. The celestial equator is inclined at a 23.44° angle towards the ecliptic. The celestial equator and the ecliptic intersect at two points which are called the equinoxes (they are vernal and autumnal equinoxes). During the year, the earth orbits the Sun, and later it is seen crossing the Equator twice, in the month of March it moves from the Southern Hemisphere towards the Northern Hemisphere, and in the month of September, it moves in the opposite direction. The equinoxes move along the west, which is the ecliptic at the rate of 50.3 arcseconds in terms of annually as the celestial equator moves with the Earth’s precession.


Axial Precession 


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In astronomical science, the axial precession occurs because of the gravitational, slow, and continuous change in the orientation of an astronomical body's rotational axis. In particular, this is to be referred to as the gradual shift in the orientation of the Earth’s axis of rotation in a cycle which is about 26,000 years. This is quite similar to the precession of a spinning top, with its axis tracing out the structure of cones which are joined at their apices. The term is known as "precession" generally refers majorly to the largest part of the motion. While other changes in the alignment of Earth's axis is called nutation and polar motion which are quite smaller in magnitude.


Define Rotation in Astronomy


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A rotation is a type of circular movement of an object which is around the center (or at a point) of a rotation. While the geometric plane along which the rotation takes place is called the rotational plane, the imaginary line that extends from the centre and which is perpendicular to the rotation plane surface is defined as the rotation axis. 

In the field of astronomy, rotation is a commonly observed phenomenon. The stars, planets and all other similar bodies rotate around their own axis.  

This rotation is named the centrifugal acceleration which is in the reference frame of the earth that counteracts the effect of gravity and which is closer to the equator.  

Another result of the rotation of a planet is the phenomenon of precession. 

FAQs (Frequently Asked Questions)

1. What are Equinoxes?

Ans. The term ‘equinox’ is like the solstice which finds its origin in Latin with the roots aequus, this means “Equal” while nox means the “Night.” The astronomers define the equinox as to be the moment the Earth's Equator while it is on its axis and it passes the same plane of the Sun's equator, while its name reveals more of that is being experienced on the month of March and September.

2. What Do You Mean By Annual Revolution?

Ans. The second motion experienced by the earth, revolving around the sun while being in its orbit is known as the annual revolution. Annual Revolution takes 365 days (that is one year) to revolve around the sun. 

3. What Do You Mean By Celestial Poles?

Ans. The north and south celestial poles are actually the imaginary points that are in the sky.  Here the Earth's axis of rotation takes place, this is indefinitely extended and intersects with the celestial sphere. The north and the south celestial poles appear to be permanently directly overhead, this is to observe at the Earth's the North Pole and the South Pole, respectively.

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