continued from Starlit Sky: 13. Great Bear
We have now come to the star that has been used by mankind since time immemorial to find directions, even during the times when compass was not invented: The Pole Star.
Pole star is called Dhruva in the Indian Astronomical System.
It is very easy to identify the pole star in the sky. Locate the Dubhe and Merak stars of the Great bear (look at the previous post to know more about Great Bear). Join the two stars with an imaginary straight line. Now extend this line about 5 times in the north direction. You will now reach a bright star with a magnitude of about 2. That is the pole star.
Pole star is so called because, of all the stars in the sky, it is the only one that does not seem to move like all the others and remains stationary in the Earth’s geographic north direction. It’s elevation can be used to determine the latitude too. That is why it was used to find directions by mankind when sailing the oceans.
Pole star is called by other names too, like, North star, Lodestar, and Guiding star.
Pole star is a part of the constellation Ursa Minor or the little bear or the little dipper. Polaris, as it is officially called, is the alpha Ursae Minoris star of the constellation. It is the brightest of the constellation. It is a yellow-white supergiant. The Beta Ursae Minoris, also called Kochab, and Gamma Ursae Minoris, also called Pherkad, are called the “Guardians of the Pole star”
Now to answer the question: Why is the pole star never moving, and all other stars seem to be going around pole star.
We all know that the Earth takes an year to make one complete rotation around the Sun. This is called revolution. But all the while, the Earth also turns around its own axis. This is called Rotation. One complete rotation is completed in 24 hours. The axis of the Earth is an imaginary line running from the north pole to the south pole through the centre of the Earth. This axis is tilted at an angle of 23 ½ degrees. This tilt causes the seasons to occur.
The axis of the Earth when extended in the north direction indefinitely, reaches the Polaris. Since the axis is fixed, the Polaris is fixed when viewed from Earth. All other stars move around the Polaris. That is why Kochab and Pherkad are called the Guardians of Pole star. They move around Polaris whereas Polaris is stationary.
Now for the question: Will the Pole star remain a pole star always?
The rotation of the Earth around its axis is like a spinning top. The Earth is not an exact sphere. It is bulged at the centre at the equator. The gravitational pull of the Sun and Moon on the Earth causes tidal motions. These tidal motions and the equatorial bulge makes the axis too to have a rotation around the vertical axis. This wobbling motion of the Earth is called precessional motion. The precessional motion of the Earth has a time period of 23,000 years. This motion cannot be observed by us in one lifetime (or for that matter in multiple life times). But what this means is that the tilt of the Earth too is not a constant and it changes over time.
That means, Polaris is the pole star at this point in time. Over centuries, when the Earth’s axis tilts away, it will no longer be the pole star. A star named Gamma Cephei is set to be the pole star around 3000 AD. Another star called Vega will become the pole star in 12,000 years from now. A star named Thuban was the pole star 4th to 2nd millennium BC
Isn’t that fascinating?
After all that heavy stuff, are you ready for a story?
This is a story that every child in India knows. It is the story of how the Dhruva star came into existence.
Once there was a king named Uttanapada. He had 2 wives, Suniti and Suruchi. Dhruva was the son of Suniti and Uttama was the son of Suruchi. Suruchi was the favourite queen on Uttanapada. She made sure that Suniti and her son were kept away from the king at all times. Uttanapada too was complacent with this arrangement.
One day five year old Dhruva wanted to sit on the lap of his father just like Uttama. He was pushed away by Suruchi, who told him that he had no privilege to sit there. Dhruva went crying to his mother. She told him that only God could not grant him that privilege and that she was helpless.
Young Dhruva decided to seek God and ask for this privilege. He set off for the forest to perform penance. Seeing his resolve, the divine sage Narada gave him a mantra “Om Namo Bhagavate Vasudevaaya” to chant with all devotion. Dhruva was immersed in the devotion of the Lord and he did tapasya (penance). When Lord Vishnu appeared before him, Dhruva was speechless. He was so immersed in his devotion that he forgot his original purpose. Pleased with his bhakti, the Lord granted him the Dhruva-pada, or the pole star position to be taken at the end of his earthly life to be immortalised forever.
Dhruva went back to his kingdom, grew up and ruled justly. At the end of his life, he became the pole star. The star that does not move and shows everyone their path.
to be continued ………….