Keyboard: Left and Right arrow keys move the Moon along its orbit by three degrees. Hold Shift for fifteen degrees. Press Space to let the Moon orbit by itself or to hold it still. Press L to toggle the light rays.

The Observatory · Moon Lantern

The Moon Lantern

Crescent

growing

Field notes

The Moon makes no light of its own — it reflects the Sun’s light, like a mirror made of grey rock.

The Sun always lights up exactly half of the Moon, no matter where the Moon is.

The shapes we call phases are just how much of that lit half faces Earth as the Moon travels around us — nothing on the Moon actually changes.

In here the lamp never moves, so one lap around Earth is one whole run of phases: about 29.5 days. Out in the real sky Earth is travelling around the Sun as well, so the Moon’s lap measured against the stars is a bit quicker — about 27 days — and it needs the extra couple of days to catch the sunlight up.

Follow the middle ray: it crosses the sunlit space in front of Earth, then stops — Earth is solid. Behind Earth, away from the lamp, it leaves a long, narrow shadow. So why isn’t the Moon in that shadow every Full Moon? Its path is tilted a little, so it almost always passes just above or below. On the rare month it slips right through, we get a lunar eclipse.

Lit part0.30

Day0.0d

Angle55.0°

PhaseCrescent

Speed 1.00d/s

The Moon Lantern

This lab hit a snag