The shape of the Earth has engaged the attention of several cultures down the ages. Ancient astronomical studies reveal calculations of the Earth’s diameter and circumference. In the 7th century, Brahmagupta gave an almost accurate measure of the circumference of the Earth. Greek scholars imagined the Earth as a sphere as perfect as the sun and the moon. The idea was popularized by Pythagoras in the 6th century BC, the finest achievement of ancient Greece was in 3rd century BC, when Eratosthenes (276-194 BC) of Egypt determined the circumference of the Earth with remarkable accuracy. He observed that the sun shone right down a well in Syene (Aswan) in mid-summer while at Alexandria, which was almost due north, it cast a shadow marking an angle of 7.2’ to the vertical or one-fiftieth of a circle.
He concluded that the Earth circumference should be 50 times the distance from Aswan to Alexandria. He calculated the distance between the two towns on the basis of a camel’s average speed. As a result, he came within just 1 percent of the correct value of the Earth’s circumference, viz. 39,852 km as against the present-day measurement of about 40,075 km.
India’s Aryabhatta (born in AD 476) was an intellectual rebel. He was 23 when he became famous in Kusumapura, which is now Patna. He boldly described the Earth as spherical, ‘circular in all directions’. About 1000 years before Copernicus, he declared in beautiful verse that the Earth rotated on its axis and went around the sun. Aryabhatta explained it in simple terms. He had stated in an aphorism
” Like a man in a boat moving forward sees only the static objects on either side of the river as moving backward, just so are the static stars seen by the people of Lanka (the equator) moving in the direction of the west ”.
Aryabhatta did not merely give ideas. He calculated, for example, the duration of a sidereal day (with regard to fixed stars in the sky). He pointed out that a Yuga consisted of 1,57,791,7500 days, which when divided by the number of the Earth’s revolutions during that period (1,58,223,7500), yields the day’s duration as 23 hours, 56 minutes and 4 seconds, which is amazingly close to the modern estimate of 23 hours, 56 minutes and 4.091 seconds! He stated that the distance traveled by a planet would be equal to the circumference of a great circle illumined by the sun in a yuga, which he defined as equivalent of 43,20,000 years.
He had a unique system of representing large numbers by alphabets in poetry, with amazing brevity of expression, a factor which might have proved too difficult for his followers. Nevertheless, he had generations of admirers and followers. The most noted among them is bhaskara, who composed his commentary of Aryabhatium in 629 AD. Bhaskara on his own made significant contributions to mathematics and astronomy. He obtained the mean celestial longitudes of planets by a simple and short-cut technique. His works have been well known astronomers for centuries, especially in kerala.
Aryabhatta’s rational ideas did not find favor with the conservatives of his time. The conservative current was so strong that even in later years, Aryabhatta had his share of opponents. Varahamihira (born in AD 587) and Brahmagupta (AD 628) criticized him. But eventually truth prevailed and Aryabhatta shines as a diamond forever!
About 12 centuries before Isaac Newton (1642-1727), Bhaskaracharya, an astronomer of the 6th century AD calculated the time taken by the Earth to orbit the sun to nine decimal places, almost accurate by modern measurement.
Ideas on the Earth’s orbit changed slowly in Europe. The Polish astronomer, Nicolaus Copernicus (1473-1543) observed that the earth rotated on its axis and went in an annual orbit around the planets as circular. A Danish astronomer, Tycho Brahe (1546-1601) pointed out that the planets did not move in perfect circles. His assistant, Johannes Kepler (1571-1630) studied the ideas and stated that the orbits of planets were elliptical. He also held that as the altitude increases, a planets takes longer to complete an orbit and that the planetary speed also varies with the distance from the sun.
Newton was the first scholar to suggest an artificial Earth satellite. In his Principia (1687), he explained how an object could go into orbit around the Earth, if it maintained adequate speed to counter the Earth’s gravity. He imagined a very tall mountain above the atmosphere where the friction of the air would not offer resistance. He then imagined cannon at the top of the mountain, firing cannon balls parallel to the ground. After each shot with increasing black powder, the path of the ball would lengthen and eventually the ball would come back to its starting point and begin an orbit of the Earth, if no other force acted on it(See Image below).
Newton pointed out that his law of universal gravitation pervades Nature and governs the motions of all objects. In 1870 Edward Hale (1822-1909), a Clergyman in Boston, USA, suggested use of an artificial satellite to assist in navigation, it was then considered science fiction.