The phases of the Moon

The Moon makes no light of its own — it reflects sunlight. Exactly half of it is always lit; the phase you see just depends on how much of the lit half is turned towards you.

• New Moon — the Moon is between the Earth and the Sun; the lit face points away from us, so we see nothing.
• Waxing crescent → first quarter → waxing gibbous — the lit fraction grows.
• Full Moon — the Moon is opposite the Sun, so the whole lit face is turned towards us. It rises at sunset and sets at sunrise.
• Waning gibbous → last quarter → waning crescent — the lit fraction shrinks back to new.

Misconception alert: phases are not the Earth's shadow falling on the Moon. That happens only in a lunar eclipse — a rare event at full Moon. Phases are simply a viewing-angle effect.

Where is the Moon?

Sidereal month vs synodic month

There are two ways to time one "orbit" of the Moon, and they give different answers:

• Sidereal month = 27.3 days — one full 360° orbit measured against the background stars.
• Synodic month = 29.5 days — one full cycle of phases, e.g. new Moon to new Moon.

The synodic month is longer because while the Moon goes round, the Earth has moved about 27° along its own orbit. The Moon must swing a bit further to line up with the Sun again — which takes about another 2.2 days.

Everyday consequence: the Moon rises roughly 50 minutes later each day (1440 min ÷ 29.5 ≈ 49 min), so it slides steadily eastwards against the stars night after night.

Your turn — the Moon's daily motion

Your turn — moonrise

Solar and lunar eclipses

An eclipse needs the three bodies in an almost perfect straight line.

• Solar eclipse — at new Moon. The Moon passes in front of the Sun and its shadow lands on the Earth. Inside the small dark umbra you see a total eclipse; in the wider penumbra you see a partial one. If the Moon is near apogee its disc looks slightly too small to cover the Sun, and you get an annular eclipse (a ring of Sun).
• Lunar eclipse — at full Moon. The Moon moves into the Earth's shadow. It is visible from the whole night side of the Earth at once and lasts hours, so lunar eclipses are seen far more often than total solar ones. The Moon usually turns coppery red, because sunlight refracted through the Earth's atmosphere still reaches it.

Contacts: Edexcel names the stages of a total eclipse by umbral contacts. For a total solar eclipse: first contact — the Moon first touches the Sun's disc (partial phase begins); second contact — the Sun is fully covered (totality begins); third contact — the Sun starts to reappear (totality ends); fourth contact — the Moon leaves the disc. For a total lunar eclipse the same four contacts mark the Moon entering and leaving the Earth's umbra.

Why not every month? The Moon's orbit is tilted about 5° to the Earth's orbital plane (the ecliptic). Most months the shadow passes above or below its target. Eclipses only happen when a new or full Moon happens at a node — where the two orbital planes cross.

Which eclipse, which phase?

Why so rare?

The tides

The Moon's gravity pulls hardest on the water nearest to it and least on the far side of the Earth. The result is two tidal bulges — one facing the Moon, one on the opposite side. As the Earth rotates beneath them, most coasts get two high tides and two low tides in roughly 24 hours 50 minutes.

• Spring tides — the biggest range (highest highs, lowest lows). They happen at new and full Moon, when the Sun and Moon pull in line and their effects add.
• Neap tides — the smallest range. They happen at the quarter Moons, when the Sun and Moon pull at 90° and partly cancel.

The Sun also raises tides, but although it is far more massive it is also far further away, so its tide-raising effect is less than half the Moon's. That is why the Moon sets the timing of the tides and the Sun only makes them bigger or smaller.

Nothing to do with spring the season: spring tides happen twice every synodic month, roughly every 14.8 days — about every fortnight, all year round.

Your turn — how often are spring tides?

Which eclipse is it?

Tap a statement, then tap the eclipse it describes.

Match the description to the term

Tap a description on the left, then its matching term on the right.

The dark heart of the shadow

Annular eclipses and the "supermoon"

Total eclipses are possible only because of a coincidence of relative sizes and distances: the Sun's diameter (1.4 × 10⁶ km) is about 400 times the Moon's (3500 km), and the Sun (1 AU = 1.5 × 10⁸ km) is also about 400 times further away than the Moon (380 000 km), so both discs appear about ½° across. For comparison the Earth is about 13 000 km across — roughly 4 Moons wide and 1/109 of the Sun.

The Moon's orbit is an ellipse, so its distance varies from about 363 000 km at perigee (closest) to about 405 000 km at apogee (furthest). Its angular size therefore changes by roughly 10% through the month.

• A full Moon at perigee looks slightly bigger and brighter — the "supermoon".
• If a solar eclipse happens when the Moon is near apogee, its disc is too small to cover the Sun completely. A brilliant ring of photosphere remains visible all round it: an annular eclipse. (You must still never look at it directly.)

Why a total eclipse is so short: the Moon's umbra reaches the Earth as a spot only about 100-270 km wide, sweeping across the surface at over 1500 km/h. Totality at any one place lasts a maximum of about 7½ minutes — usually far less.

A ring of fire

The big ideas to know

That is The Earth-Moon-Sun System covered for Edexcel GCSE Astronomy. Press Finish to see your score.