Every calendar humans have made comes from watching the sky. A day is one spin of the earth, a month roughly one cycle of the moon, a year one journey round the sun.
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Two motions, two results. Confusing them is the commonest error in the chapter.
🔄 Rotation and revolution
| Motion | Meaning | Time | Gives us |
|---|---|---|---|
| Rotation | spinning on its axis | about 24 hours | day and night |
| Revolution | going round the sun | about 365¼ days | the year and the seasons |
Seasons are not caused by distance from the sun. They are caused by the earth’s tilt of about 23.5°. The hemisphere tilted towards the sun gets more direct rays and has summer — which is why India has summer while Australia has winter.
The leap year: the extra quarter-day each year adds up, so every fourth year gains 29 February and has 366 days.
The moon makes no light of its own. We see only the part lit by the sun, and how much of it we see changes through the month.
🌗 The phases
| Phase | What you see |
|---|---|
| New moon (amavasya) | no lit part visible |
| Crescent | a thin curved sliver |
| Half moon | half the disc lit |
| Full moon (purnima) | the whole lit face |
Why we always see the same face of the moon. It takes the same time to spin once as to go once round the earth — about 27 days — so the same side always faces us. The far side is not "dark"; it simply never faces the earth.
The moon has no light of its own. It reflects sunlight. Saying it "glows" is wrong, and this is asked directly.
An eclipse happens when three bodies line up and one casts its shadow on another.
🌑 Which is which
| Eclipse | Order | Happens on |
|---|---|---|
| Solar (sun hidden) | sun — moon — earth | a new moon day |
| Lunar (moon hidden) | sun — earth — moon | a full moon day |
Never look at a solar eclipse directly, not even through sunglasses or smoked glass. It can permanently damage the eye. Use a proper filter or watch a projected image.
Remember by what is in the middle. In a solar eclipse the moon is in the middle and blocks the sun. In a lunar eclipse the earth is in the middle and its shadow falls on the moon.
12 questions on the earth, moon and seasons. 🍀
