Plate 12 · Space

Why the Moon Changes Shape

The Moon never changes shape, and its phases are not the Earth’s shadow nibbling at it. What changes is how much of its sunlit half we can see from here.

Try it

Chapters · 7

01Always Half Lit
02Once Around
03Right Side First, Then Left
04Not the Earth’s Shadow
05Always the Same Face
06Tonight’s Moon
07Your Turn

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01

Always Half Lit

The Moon makes no light of its own. Like the Earth, it always has one half facing the Sun, in daylight, and one half facing away, in night.

At every moment, exactly half the Moon is lit. Its phase is simply how much of that lit half happens to face us.

Sun, Earth and Moon, schematic · sizes and distances not to scale

02

Once Around

The Moon goes around the Earth about once a month. When it passes between the Sun and the Earth, its lit half faces away and we barely see it: a new moon. On the far side of the Earth, the whole lit half faces us: a full moon.

From one new moon to the next takes 29.5 days on average. The small window at bottom right shows the view from the Earth.

03

Right Side First, Then Left

After new moon, the Moon fills out day by day — waxing. First comes a thin crescent; about a week later we see half its face lit — the first quarter; a week after that, it is full. Then it wanes, shrinking through the last quarter back to new.

In the Northern Hemisphere, a waxing Moon is lit on the right and a waning Moon on the left. In the Southern Hemisphere, people see the Moon upside down, so left and right swap.

04

Not the Earth’s Shadow

Many people think the missing part of the Moon is the Earth’s shadow. But the Earth’s shadow always points straight away from the Sun, and at first and last quarter the Moon is nowhere near it.

Only at full moon can the Moon pass into the Earth’s shadow — a lunar eclipse. The Moon’s orbit is tilted about 5 degrees to the Earth’s path around the Sun, so most full moons slip just above or below the shadow. In a total eclipse the whole Moon turns a dusky red, and the whole thing is over in a few hours — a phase takes weeks.

Earth’s shadow and the Moon’s orbit · tilt drawn three times steeper than the real 5°; not to scale

05

Always the Same Face

As the Moon goes once around the Earth, it also turns once on its axis, so it keeps almost exactly the same face toward us. From the Earth, we never see its far side.

But the far side is not a “dark side.” At new moon, the face toward us is in night while the far side is in full sunlight.

06

Tonight’s Moon

This Moon is set for today’s date, from the phase times published by the US Naval Observatory. Below, the Moon for each day of the coming month, as seen from the Northern Hemisphere.

On average, the Moon rises about 50 minutes later each day. A full moon rises around sunset; a first-quarter Moon rises around midday, which is why you often see it in the afternoon sky.

Phase times: US Naval Observatory · UTC

07

Your Turn

Drag the days and watch the Moon go around the Earth while the window shows its phase. Switch to the Southern Hemisphere to see the Moon people see there.

Try thisSwitch to “View from the south”: five days after new moon, which side is lit?

What the model leaves out

The sizes of the Earth and Moon and the distance between them are not to scale: the real Moon is about 30 Earth-widths away. The Moon’s orbit is really an ellipse, so actual phases come as much as half a day or more earlier or later than the average; the dates on this page use the US Naval Observatory’s times. “Lit on the right in the north” holds at middle latitudes; near the equator a crescent often lies on its back like a boat. The tilt of the Moon’s orbit is drawn at 15°, three times the real 5°, so you can see it. Because the Moon wobbles slightly, over time we glimpse a little past its edges — about 59% of its surface.

A common misreading

The missing part of the Moon is not the Earth’s shadow; it is the Moon’s own night. The Earth’s shadow falls on the Moon only in a lunar eclipse, which can happen only at full moon. And the far side of the Moon is not always dark: at new moon, it is in full daylight.

Sources

Data

Moon phase times: US Naval Observatory, Astronomical Applications API. Moon: NASA Scientific Visualization Studio, CGI Moon Kit (LRO LROC WAC colour mosaic; LOLA elevation). Earth: NASA Earth Observatory, Blue Marble Next Generation, October 2004 (Reto Stöckli); city lights, NASA Earth Observatory Black Marble 2016 (Joshua Stevens, Miguel Román, Suomi NPP VIIRS); clouds, NASA Blue Marble 2002 (Reto Stöckli, NASA GSFC). Elevation (globe relief): Terrain Tiles on AWS Open Data; ETOPO1, NOAA; GMTED2010 and SRTM, USGS. Stars: Yale Bright Star Catalogue, 5th revised edition (Hoffleit & Warren 1991), via CDS VizieR. Milky Way: NASA/Goddard Space Flight Center Scientific Visualization Studio, Deep Star Maps 2020; Gaia DR2: ESA/Gaia/DPAC.