Chapter 12: Earth, Moon, and the Sun

The Earth spins about an imaginary line through the North and South Poles, called its axis. This spinning is called rotation. The Earth rotates from West to East, and one full turn takes about 24 hours. Because we ride on the turning Earth, the Sun appears to rise in the East, climb across the sky and set in the West, just as the trees seemed to slide past you on the merry-go-round. This is an apparent motion: it looks like the Sun moves, but it is really our own turning. A pole's shadow is long early in the morning, short at noon and long again in the evening, because the Sun's apparent position keeps changing. In the middle of the nineteenth century Leon Foucault hung a long pendulum with a heavy bob to give the first simple demonstration of the Earth's rotation; a 22 m Foucault pendulum hangs in the new Parliament building in New Delhi.

At any moment the Sun lights up only the half of the Earth that faces it. That half has day; the other half has night. Since the Earth rotates West to East, a place on it moves into the lit half (sunrise) and later out of it into darkness (sunset). You can see this with a globe and a torch in a dark room: the torch is the Sun and the globe is the Earth; turn the globe slowly West to East. Places in the east turn into the light before places in the west, so sunrise comes earlier in the east: the Sun rises in Arunachal Pradesh earlier than in Gujarat. Countries on different sides of the Earth also differ: when it is afternoon in India it can be night in the USA. One full day-night cycle takes about 24 hours.

Not only the Sun but also the Moon and the stars appear to rise in the East and set in the West. The reason is the same: the Earth rotates from West to East. Try the Big Dipper (Saptarishi) activity: on one night, draw where the Big Dipper is compared with the Pole Star (or a fixed tree or building) and repeat after every two hours. It seems to circle around the Pole Star. In the Northern Hemisphere the Earth's axis points very close to the Pole Star, so the Pole Star seems almost fixed while all the other stars appear to go around it. A camera with its shutter open for a long time records the stars as arcs of circles called star trails. Ancient Indian astronomers such as Aryabhata noticed this too: in his book Aryabhatiya, around the fifth century CE, he compared it to a man in a moving boat who sees fixed objects moving backward, and his value for one rotation, about 23 hours 56 minutes, is close to today's.

Revolution is the motion of an object around another object, and the path it follows is its orbit. While spinning on its axis, the Earth also revolves around the Sun. Seen from the top, its orbit is nearly circular (the long, thin picture in some books is a side view). One revolution takes about 365 days and 6 hours, roughly one year. Rotation and revolution are different: rotation is spinning about one's own axis, revolution is going around another object. Revolution changes the night sky. As the Earth moves around the Sun, after sunset we face different directions of space, so the stars seen in the evening change slowly through the year. A particular star group rises a few minutes earlier each night, about two hours earlier after a month, and after one year the same pattern returns to the same place at the same hour. The Bhil and Pawara communities of the Tapi Valley used the appearance of certain star patterns as markers for the arrival of monsoon rain.

The Earth's axis is not upright with respect to its orbit; it is tilted, and it keeps the same tilt as the Earth goes around the Sun. This tilt and the Earth's spherical shape give rise to seasons. In June the Northern Hemisphere is tilted towards the Sun. Sunrays fall more directly on it, so the same amount of sunlight is spread over a smaller area and heats it more; also the day there is longer than 12 hours. Intense sunlight for a longer time brings summer. In December the situation reverses and the Northern Hemisphere has winter: weaker sunlight for a shorter time. The Southern Hemisphere has the opposite season at the same time: winter in June and summer in December. Two commonly given reasons are wrong: that the tilted hemisphere is nearer the Sun, and that the Earth's oval orbit changes its distance a lot. The distance changes only a little; in fact the Earth is closest to the Sun in January.

In the Northern Hemisphere the longest day occurs around 21 June, the summer solstice. After that the days get shorter and the nights longer. The shortest day and longest night occur around 22 December, the winter solstice. Around 21 March and 23 September daytime lasts 12 hours; in the Northern Hemisphere these are the spring equinox and the autumn equinox. On the equator there are always 12 hours of sunlight and 12 hours of darkness, and the sunrays hit with nearly the same intensity all year, so for southern states of India close to the equator the effect of seasons is not very prominent (local geography and nearness to the sea also matter). At the North Pole the Sun rises on 21 March and stays in the sky for six months, setting on about 22 September; there is then six months of darkness. The North Pole has sunlight for all 24 hours in June and none in December. The South Pole behaves in the opposite way.

A solar eclipse happens when the Moon comes between the Sun and the Earth and blocks the sunlight. The apparent size of an object depends on its actual size and its distance. The Moon is much smaller than the Sun, but it is also much closer, so the two look about the same size in the sky and the Moon can cover the whole Sun. Mercury and Venus are bigger than the Moon but much farther away, so they look far too small to block the Sun; when Venus crosses the Sun it appears only as a tiny black dot, a rare event called a transit of Venus. The Moon's shadow falls on a small area of the Earth. In that area, there is a total solar eclipse: no part of the Sun is seen and it turns dark for a few minutes. Where only part of the Sun is blocked, observers see a partial solar eclipse. Because the Earth and the Moon keep moving, the shadow sweeps across the Earth, so totality lasts only a few minutes. Never look at the Sun directly, not even through sunglasses, binoculars or telescopes; join viewing events run by planetaria and astronomy clubs.

A lunar eclipse happens when the Earth comes between the Sun and the Moon, so sunlight is blocked from reaching the Moon. The Earth's shadow falls on the full disc of the Moon. When the Moon is completely inside the Earth's shadow it is a total lunar eclipse; the bright Moon looks dark red until it leaves the shadow. When only part of the Moon is in the shadow and the rest is visible, it is a partial lunar eclipse. Unlike the Sun, an eclipsed full Moon can be watched safely with the naked eye. People watched eclipses and kept records from ancient times; because the reason was unknown, many feared them, and many superstitions grew about eating, cooking or going out. Now that we know the cause, we need not fear them, as long as we never look at the Sun directly. Ancient Indian texts like the Surya Siddhanta, written in rhythmic Sanskrit shlokas, give calculations for predicting eclipses. The free Stellarium app can show upcoming eclipses visible from your place. The Kodaikanal Solar Observatory, set up in 1899 and run by the Indian Institute of Astrophysics, has given data about the Sun for over a hundred years. M. K. Vainu Bappu, the father of modern Indian astronomy, built telescopes in India, discovered a comet and travelled to study solar eclipses.