Earth, Moon and the Sun: questions with answers

A star records as an arc because it was in one place on the film at the start and another at the end. It moved there because the camera stands on the Earth and was carried round during thirty minutes. The arcs are drawn about the direction the axis of the Earth points at. The Pole Star sits almost on that point, so it draws barely any arc of its own.

Two rules settle every statement of this kind. The Earth turns from West to East. So the Sun and the Moon are both seen to come up in the eastward direction. A place further east is carried round to meet the light earlier. That is so whatever a clock in either place reads.

The stars are very far away and have gone nowhere in this time. What has moved is Meera, carried round by the turning Earth. The Earth turns about its axis, and that axis points very close to the Pole Star. So the Pole Star stays where it is. The rest of the sky, including the Big Dipper, appears to turn around it.

Two of the three places sit on the day-night line, one on each side of it. The arrow is the only thing that separates them. The Earth turns from West to East. So the one in the shaded half is being carried into the light, and the one in the lit half is being carried out of it. The third place is deep in the shaded half and is a long way from either event.

Earth rotates from West to East, bringing the eastern place into sunlight first. On the globe, Jharkhand is east of Rajasthan, so a West-to-East turn matches the observed order. Moving the torch around a stationary globe would model the Sun travelling around Earth and does not represent Earth’s rotation.

Half the Earth faces the Sun at every instant and half faces away, so the lit half and the dark half exist at the same time, always. India and the USA are on opposite sides of that divide, which is why one is in daylight while the other is not. Nothing about the orbit comes into it: the Earth's position round the Sun changes over months, and this difference holds every single day.

The two periods are the surest way of telling the two motions apart. One rotation takes about 24 hours and gives us day and night; one revolution takes about 365 days and 6 hours and brings the year round. Neither period depends on where the Earth is in its orbit, because the orbit is nearly circular and there is no long and short side to it.

Two motions, two definitions, two periods, and they do not overlap. Rotation is motion about an imaginary line passing THROUGH the Earth, and one of them takes about 24 hours. Revolution is motion AROUND another body, and one of them takes about 365 days and 6 hours. Either the line it turns about or the time it takes is enough to name the motion on its own.

The Earth maintains the same tilt at every place in its orbit. So all four axes in the figure are drawn tilted by the same amount and in the same direction. What changes is where the Earth is. At one side of the orbit this fixed tilt points the N end of the axis towards the Sun. At the opposite side the same tilt points the N end away from the Sun. The place where the N end points towards the Sun is the answer.

Two things happen together, and neither of them is about distance. The Earth is a sphere, so sunlight arriving more directly on the tilted hemisphere is spread over a smaller area. The same sunlight on a smaller area heats it more. The tilt also puts more of that hemisphere in the lit half as the Earth turns, so daytime there runs longer than 12 hours. More intense sunlight, lasting longer, is summer.

The chapter names the distance explanations and rejects them. The orbit of the Earth is nearly circular. The lean of the axis brings a hemisphere only very slightly nearer. So the differences in distance are very small either way. What the tilt really does is change the angle at which sunlight meets a round Earth. Arriving more directly, the same sunlight covers a smaller area and heats it more. The fact that settles it is that the Earth is closest to the Sun in January, the middle of the northern winter.

Light travels in straight lines, so the Sun is completely hidden only from places that lie on the straight line from the Sun through the Moon. That is a small patch of the Earth, and the observer at Q is on it. The other two are off the line. They see the Moon covering only part of the Sun, which is a partial eclipse.

Being larger than the Moon buys Venus nothing here, because it is also very much further away. Apparent size falls off with distance, so from the Earth Venus looks far smaller than the Sun does. It is small enough to cross the face of the Sun as a tiny dark dot. That crossing is called a transit, and it is not an eclipse: almost none of the sunlight is cut off.

The Moon is small and casts a small shadow on the Earth. So the Sun is completely hidden over only a small area. Around that area the eclipse is partial, and beyond it there is no eclipse at all. The shadow does not stay in one place. The Earth turns and the Moon moves along its orbit. So the shadow moves off that place, and the total darkness there lasts only a few minutes. Covered or not, the Sun is never safe to look at.

The two eclipses both need the Sun, Earth and Moon nearly in a line. What differs is which body is in the middle. The shadow of the Earth is large, so it covers the whole Moon. A lunar eclipse is therefore seen from the whole night side of the Earth. The shadow of the Moon is small, so a solar eclipse reaches only a small area for a few minutes. Only the solar eclipse is dangerous to watch; the eclipsed Moon is safe to see with the naked eye.