# ELI5. I’ve always been told moonrise for a new moon is around dawn and for a full moon around sunset, but also that it takes about 29.5 days for the lunar cycle. Are both these true? Shouldn’t the new moon rise around dawn one month and then around sunset the next month?

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ELI5. I’ve always been told moonrise for a new moon is around dawn and for a full moon around sunset, but also that it takes about 29.5 days for the lunar cycle. Are both these true? Shouldn’t the new moon rise around dawn one month and then around sunset the next month?

In: 6

A month is the time from new moon to new moon; full moon is halfway.

A new moon is, by definition, the moment at which the sunlit side of the moon is entirely hidden from us. This can only be the case when the sun is directly behind the moon, and therefore the moon is closest to the sun from our perspective. The moon would be rising with the sun. When the moon rises at sunset, it is by definition on the opposite side of the earth as the sun, and the entire sunlit side is visible; a full moon.

The moon takes 29.5 days to orbit the earth, so every 29.5 days there’s a new moon, and every 29.5 days there’s a full moon, and new moon and full moon occur about 14 or 15 days apart.

After thinking about this it occurred to me you might be hung up on the “.5” part.

The new moon and full moon are *moments* in the moon’s orbit. There is a precise time at which the sun and moon reach a certain position that it’s *exactly* new moon or *exactly* full moon. These moments will *not* coincide with your *local* horizon. They will coincide with a local horizon *somewhere* on Earth, but likely not where you are, and it changes every month.

When we say “tonight is a full moon” it’s because the moon stays “full-ish” for a day or so, where you can’t tell with the naked eye if part of the near side is in shadow. The exact moment of full moon might occur when it’s over your head, or an hour before it rises or an hour after it sets, but it’s still “full.”

A new moon is technically just a moment in time and is invisible from earth because it is between earth and the sun. It is only illuminated by the sun reflected from the earth. Unless the earth sun and moon line up perfectly in 3D and we get a solar eclipse it is not visible.

Most new moon are not solar eclipses because the moon is slightly above or below the sun. So they line up in 2D but not in 3D

When we start to see the moon it is a waxing crescent. When it begins to be visible it is still very close to the sun in the sky and the result is that moonrise and sunrise are at the same time.

In the same way is a full moon only a moment in time when all of the moons we see is illuminated by the sun. Earth is in between the moon and the sun. If they line up in 3D we get a lunar eclipse.

Because it it the opposite side of the earth the moonrise is at sunset. You can technically see both because the atmosphere bends light.

The lunar cycle of around 29.5 days is the time it take to move once around earth relative to the sun. If you just look at the period it take to move around earth relative to stars far away it is 27.3 days

In 29.5 days earth moves 29.5/365.25 = 8 % of its orbit around the sun. So the moon needs to move 8% more than a single lap to line up the same with the sun. 29.5/27.3 = 1.08

Because a new moon and full moon depend on the position of the moon relative to the sun the time it rises will always be the same relative to the sun

The moon’s position relative to the sun determines when you can see it and what it looks like.

A new moon occurs when the moon is between Earth and the Sun. A day or two later the moon has advanced far enough so that at sunrise the moon has not quite risen. Daylight blanks out the moon sliver. Only after the sun sets in the evening while the moon sliver is still above the horizon, and does the new moon become prominent in the dimming sky.

Before the next new moon, the moon has traversed the whole sky and arrives at the sun again from the east. So in the morning just before a new moon, the moon rises just before the sun while the sky is still dim. The next day, the moon is pretty much lined up with the sun: another new moon.