Why the Moon changes shape.

The Sun always lights half the Moon. What changes is how much of that lit half is turned toward us. Drag the Moon around its orbit and watch both figures update.

Day of the lunar cycle

New1st qtrFull3rd qtrNew

Tilt vs. the Sun

TotalPartialPenumbralMisses

Earth's shadow Southern Hemisphere

Fig A · From above the North PoleDrag the Moon!

Lit halfHalf facing Earth// Shadow squeezed past this mark

UmbraEarth hides the whole Sun.

PenumbraEarth hides part of the Sun.

AntumbraEarth looks smaller than the Sun, leaving a ring of light. The Moon never gets this far.

Fig B · From EarthNorthern Hemisphere

What you'd see looking upMoon photo: NASA

Fig C · Side viewOrbit tilted 5°

Heights are to scale. Distances are squeezed sideways.

Current phase

Waxing crescent

Cycle day: 4.5 of 29.5
Disc lit: 20%
Rises around: 9 am
Next full moon: No eclipse

Why it looks like this

Note 1

There isn't a dark side of the moon.

Like Earth, the Moon always has a day side and a night side, but it changes. The Sun lights the half facing it wherever the Moon is in its orbit. Because the moon is tidally locked with the Earth, we only ever see the half facing us. That's why it feels like the moon has a dark side that never sees daylight.

Note 3

A Lunar Eclipse is the Earths shadow passing over the Moon.

During total and partial Lunar eclipses, the Moon passes through at least part of the Earth's shadow. When this happens, the Moon takes on a rusty, reddish color. That's light from the sun being filtered through Earth's atmosphere which affects the color of the light.