Why the Moon has phases

The Moon is a ball. The Sun lights one half of it, just as it lights one half of the Earth. From here we see different portions of that lit half as the Moon travels around us.

At new moon, the Moon is roughly between the Earth and the Sun. The lit half faces away from us, so the Moon is a dark disc in the daytime sky and easy to miss. A slim crescent appears as it moves on, with the lit sliver on one edge.

Half is always lit

At first quarter, half of the disc we see is lit. The name quarter refers to the quarter of the orbit, not to a quarter of the disc. Then comes a gibbous moon, more than half lit, and then the full moon, when the Earth is roughly between the Moon and the Sun and we face the whole lit half.

The sequence runs back through gibbous and last quarter to a crescent and another new moon. The trip takes about 29.5 days, a synodic month, a little longer than the Moon’s orbital period because the Earth has moved on around the Sun.

The month-long walk

A lunar eclipse happens only at full moon, when the Moon passes through the Earth’s shadow. A solar eclipse happens only at new moon, when the Moon’s shadow falls on the Earth. They do not happen every month because the Moon’s orbit is tilted.

The dark part of a crescent moon is sometimes faintly visible. That is earthshine: sunlight reflected from the Earth’s clouds and oceans onto the Moon, then back to us.

Half is always lit

The Moon does not make its own light. We see the sunlit half, and the angle changes through the month. The Moon is a ball. The Sun lights one half of it, just as it lights one half of the Earth.

Put those two statements side by side. The first sets the condition. The second says what happens once that condition is met. Why the Moon has phases needs both. Drop the first and the second has nothing to explain. Drop the second and the first is only the scene.

The month-long walk

The Sun lights one half of it, just as it lights one half of the Earth. From here we see different portions of that lit half as the Moon travels around us.

The order is the explanation. What the first sentence names has to be there before the result in the second sentence can show up. The piece does not ask you to accept the result first and hunt for a cause afterwards.

A check you can make

From here we see different portions of that lit half as the Moon travels around us. At new moon, the Moon is roughly between the Earth and the Sun.

A fair check is sequential. Confirm the first statement. Then see whether the second is the result of it, rather than a separate fact parked in the same piece. If the result shows up without the condition, this account does not cover what you are looking at.

The limit of the claim

At new moon, the Moon is roughly between the Earth and the Sun. The lit half faces away from us, so the Moon is a dark disc in the daytime sky and easy to miss.

The two statements do different jobs. One says what is there, what it is made of, or what acts on it. The other says the change that follows. The title names the change. It is not an inventory of parts.

The same steps, in order

The lit half faces away from us, so the Moon is a dark disc in the daytime sky and easy to miss. A slim crescent appears as it moves on, with the lit sliver on one edge.

Nothing in the second statement cancels the first. It carries the first forward or marks the limit of it. Remember only the result, and the cause goes missing. Remember only the cause, and you will not know what the piece says happens next.

Why the order matters

A slim crescent appears as it moves on, with the lit sliver on one edge. At first quarter, half of the disc we see is lit.

Read them as one path. Start at the first full stop and do not skip to the outcome. The words between the two statements are the mechanism. The title is the name of that path, not a second claim.

Condition, then outcome

At first quarter, half of the disc we see is lit. The name quarter refers to the quarter of the orbit, not to a quarter of the disc.

The first statement can be true and still not finish the job. The second statement is the job finishing. Hold both, and the paragraph above stops being a list. It becomes the reason the title is worded as it is.

What the title is naming

The name quarter refers to the quarter of the orbit, not to a quarter of the disc. Then comes a gibbous moon, more than half lit, and then the full moon, when the Earth is roughly between the Moon and the Sun and we face the whole lit half.

If you wanted a shorter version, it would be those two sentences and nothing else. Everything added around them is there so the dependence is obvious: the second does not happen unless the first has happened.

What has to come first

Then comes a gibbous moon, more than half lit, and then the full moon, when the Earth is roughly between the Moon and the Sun and we face the whole lit half. The sequence runs back through gibbous and last quarter to a crescent and another new moon.

Put those two statements side by side. The first sets the condition. The second says what happens once that condition is met. Why the Moon has phases needs both. Drop the first and the second has nothing to explain. Drop the second and the first is only the scene.

The result that follows

The sequence runs back through gibbous and last quarter to a crescent and another new moon. The trip takes about 29.5 days, a synodic month, a little longer than the Moon’s orbital period because the Earth has moved on around the Sun.

The order is the explanation. What the first sentence names has to be there before the result in the second sentence can show up. The piece does not ask you to accept the result first and hunt for a cause afterwards.