Rotation and Revolution of the Earth are well known movements of the Earth which we study from our school days. Apart from these, there is another slow and continuous movement of the earth which is called ‘The precession of rotational axis of the Earth’. Earth revolves round the Sun and rotates around itself on an axis which is inclined about 23.40 away from the perpendicular of its orbital plane. Any movement of celestial body is always influenced by the gravitational force existing on the body because of its neighbouring massive objects in the space. So also, the planet Earth is strongly influenced by the gravity of the Sun and the Moon apart from less influencing gravitational forces by the other planets and debris of our solar system. This gravity of the Sun and Moon is the basic responsible factors for the movement of the Earth. Not only the mass of objects but also their distance and orientation with reference to the Earth also causes variations in gravitational force and there by variations in movements of the Earth. Further, our Earth is not in perfect spherical shape but it is bulged at the equator and flattened at poles due to centrifugal forces from its rotation. Rotation causes a distortion from its spherical shape due to envelope of ocean fluids and we see a bulge in the ocean envelopes around the earth. Sea level at the equator is 21 KM higher than the seal level at poles, the distances measured from the centre of the Earth. The gravitational pull of the Sun and Moon on the gyroscopic Earth bulge produces torque on its rotational axis causing it to drift periodically from East to West completing a turn in 25,800 years. This movement of Earth’s rotational axis is called the Precession.
This wobbling of the Earth is similar to the spinning of top or the gyroscope is shown below.
Effects of Earth’s Precession
The precession of the Earth’s axis has some important effects on the Earth’s seasons etc. which are discussed below.
Shifting of Pole Star:
Pole star is a visible star that is approximately pointed by the rotational axis of the Earth and can be viewed directly overhead from North Pole or South Pole. As the precession of the Earth axis continues, the location in the space that is pointed by the axis also changes and over period of time, the axis points towards another star which then becomes the pole star for that period. On the average, the axis will return pointing to the same star over a period of 25,800 years. This movement of the Earth is very subtle and approximately the change of axis is about 10 for every 72 years. Currently, Polaris is the pole star and it changes over a period of time as shown below.
Shifting of Equinoxes and Solstices:
Shifting of Equinoxes and Solstices is another phenomenon that is caused by the Earth’s precession. As the Earth revolves round the Sun, the position of occurrence of Equinoxes and Solstices will not be same every year because of the precession of the rotational axis. By the time, the Earth reaches the same point around the Sun in its orbit the rotational axis orientation would be slightly shifted due to precession. It is measured that the tropical year (period between equinoxes or solstices) is about 20 minutes shorter per year than the Sidereal year which is measured by the Sun’s apparent position relative to the stars. As the equinoxes and solstices are noting but the intersection points of ecliptic and celestial equator on the celestial sphere, as the Earth precession goes on , the intersection points are also shifted and hence the Equinoxes and Solstices. The shift in Equinoxes and Solstices changes the occurrence of seasons for the given position of the Earth in its orbit. Approximately, the Equinoxes and Solstices will be shifted by 1 day for every 70.6 years. The seasonal cycle is exactly repeated again after 25,800 years again. The change of seasons over period of thousands of years is shown below.
The precession movement of the Earth axis is very subtle and it is almost impossible to detect with the naked eye astronomical observations. But, a Greek astronomer, Hipparchus, observed this subtle movement of the Earth in 134 BC. He studied number of star maps and astronomical data available in his previous times and compared with his own maps indicating star positions in different times. He further observed the Sun-shading patterns of a vertical pole and found that over period of years the Equinoxes and Solstices are shifting their time of occurrence. After observing for many years in his life time, he could guess that the shift of the Equinoxes should not be there as long as the Earth’s orbit around the Sun is same, unless there is a deviation in the Earth’s axis of rotation. His observation is one of the marvellous astronomical achievements that could be possible in a person’s life time.
In the history, many astronomical calculations and even constructions of large structures like Pyramids etc. involved the position of pole star at that time. As the years passed by, pole star of their time changed to another star due to precession of the Earth axis. As we know the details about the periodicity of the Precession, we can guess the time of their constructions and calculations based on what pole star they observed and referred.
In one of the interesting story in Hindu mythology, Lord Vishnu appears and blesses his devotee, Dhruva, who he will be remained as constant star in the space and all the stars (Saptha Rishis) will revolve around him for 26,000 years! Hindu astronomers called the pole star as ‘Dhruva Tara’. Do they really know about the periodicity and the Precession of the Earth axis? If not, how Hindus referred this period exactly in their age old mythological story is a mystery.
On the first read itself , it is hard to imagine the complex movements of Earth unless it is demonstrated in a video. Hence , I feel it is essential to post some of the videos from YouTube on this topic for better understanding and to create enthusiasm.
Note : The precession of the Earth axis would not have been there if the Earth is not in oblate spheroid shape i.e.. bulged at its equatorial regions due to ocean envelope.