The Moon you see tonight is a single frame of a loop that never stops turning. Over roughly twenty nine and a half days it goes from invisible to a thin blade of light, swells to a full disc, then thins back down and vanishes. The phase is simply how much of the lit half is turned toward you right now. Nothing about the Moon changes. The Sun lights one side of it the whole time. What changes is the angle between you, the Moon and the Sun, and that angle is what you are reading when you look up.
The eight phases, in order
Astronomers split the loop into eight named steps. New moon is the dark one, the Moon riding close to the Sun with its lit face turned away. Then comes the waxing crescent, a sliver growing on the right. First quarter is a clean half disc, lit on the right. The waxing gibbous is swollen and almost full. Full moon is the whole face lit, the Moon opposite the Sun and up all night. Then it reverses: waning gibbous, last quarter as a half disc lit on the left, and the waning crescent, a thinning sliver before it goes dark again.
Waxing means growing and waning means shrinking. In the northern hemisphere the light fills in from the right and empties from the right, so a crescent lit on its right edge is growing toward full and a crescent lit on its left is fading toward new.
Why the cycle is 29.5 days, not 27
The Moon circles the Earth in about twenty seven days. But while it does, the Earth is moving around the Sun, so the Moon has to travel a little farther each lap to catch the same angle to the Sun again. That extra stretch is why the phase cycle, called the synodic month, runs to about twenty nine and a half days. It is the reason a calendar month and a moon cannot quite be made to agree.
Full, new, super and blood
A full moon is the Moon directly opposite the Sun, lit edge to edge, rising as the Sun sets. A new moon is the opposite, dark and lost in the Sun's glare. A supermoon is a full moon that lands near perigee, the point where the Moon's slightly oval orbit brings it closest to Earth, so it looks a touch larger and brighter. A blood moon is a total lunar eclipse, the one night the Earth's shadow does fall across the full Moon and bends red sunlight onto it. Each of those is just a full or new moon caught at a particular moment of the geometry.
How to read tonight
Look at which side the light is on and how much of the disc is lit. Light on the right and growing means you are between new and full. A half disc is a quarter moon. A fat lopsided disc is a gibbous. If the Moon is up in the middle of the night and fully round, you are at or near full. If the sky has no Moon at all after dark, you are near new. The live page tracks the exact phase and the dates of the next full and new moon for you.
Know the sky before you walk outside.
The phase tells you what the Moon is doing. Magna Earth tells you what the whole sky is doing over your exact coordinates, the Moon, the planets near it, the aurora when a storm reaches your latitude, so you are outside at the right hour instead of hearing about it the next day.
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