The Moon Never Turns Its Back on Us

The Moon is non-luminous — it produces no light of its own. We see it only because sunlight falls on it and is reflected towards the Earth. A bright full Moon is simply a well-lit rock.

This page covers everything in the CBSE Class 7 Science chapter's second half: why the Moon shines, what causes its phases, why the same side always faces us, and how the two kinds of eclipse work.

Half of the Moon is always lit by the Sun. The phases are simply how much of that lit half we can see from the Earth, and that changes as the Moon travels around us over about a month.

At new moon the Moon lies between the Earth and the Sun, so its lit side faces away from us and we see nothing. Over the following days a thin crescent appears and grows — the Moon is waxing — through first quarter (half lit) to full moon, when the Earth is between the Sun and the Moon and we see the entire lit half.

Then it shrinks back — waning — through last quarter to the next new moon, completing the cycle in roughly 29 to 30 days.

For example, the crescent on the night of Eid and the full Moon on Sharad Purnima are the same rock at different points in that cycle.

The misconception to clear: phases are not the Earth's shadow falling on the Moon. That is an eclipse, which is rare — phases happen every month and are just a matter of viewing angle.

Because the Moon takes the same time to rotate once on its axis as it takes to revolve once around the Earth — about a month for each.

The two motions stay in step, so as the Moon travels around us it turns by exactly the amount needed to keep one face pointed our way. The far side is never visible from Earth.

You can model it by walking in a circle around a friend while always facing them: to do that you must turn right round once per lap. Your friend never sees your back — but everyone else in the room sees every side of you.

So the far side is not permanently dark. It receives just as much sunlight as the near side over a month; it is simply never turned towards us.

That distinction — hidden from us, not unlit — is exactly what questions on this objective test.

A solar eclipse happens when the Moon comes between the Sun and the Earth, and the Moon's shadow falls on the Earth, blocking our view of the Sun. It can only occur at new moon.

In a total solar eclipse the Sun is completely hidden for a few minutes, and the sky darkens as though at dusk. In a partial eclipse only a portion of the Sun is covered.

Precautions matter here more than anywhere else in the chapter. Never look at the Sun directly during an eclipse, not even briefly, and not through sunglasses, exposed film, smoked glass, coloured water or a CD. These do not block the harmful radiation, and looking can permanently damage the eye — painlessly, so you would not notice until later. Use only proper certified solar filters, or watch a pinhole projection of the Sun on a screen, which is completely safe.

The pinhole method links directly back to the previous chapter: it is a pinhole camera pointed at the Sun.

A lunar eclipse happens when the Earth comes between the Sun and the Moon, so the Earth's shadow falls on the Moon. It can only occur at full moon.

The most useful contrast is how many people can see each one.

A solar eclipse is visible only from the narrow strip of the Earth where the Moon's small shadow lands. Move a few hundred kilometres away and you see only a partial eclipse, or none at all.

A lunar eclipse is visible from the entire night side of the Earth at once — anywhere the Moon is above the horizon. The Earth is much larger than the Moon, so it casts a far bigger shadow, and it is the Moon being darkened rather than a small patch of Earth being shaded.

A lunar eclipse is also completely safe to watch with the naked eye, because you are looking at the dimmed Moon, not at the Sun.

So: solar eclipses are rarer for any given place and dangerous to view; lunar eclipses are widely visible and safe.