Moon Phases and Tides

Why does the Moon change shape, and why are some tides much bigger than others? Look down on the Sun, Earth and Moon from space and move the Moon through its 29.5-day cycle.

Day
4.0 of 29.5
Phase
Waxing crescent
Lit part
17%
Tides
In between

The maths, live

Moon angle θ = 360° × day ÷ 29.5θ = 360° × 4.0 ÷ 29.5 = 49°
Lit part = (1 − cos θ) ÷ 2= (1 − 0.66) ÷ 2 = 17%
Tide = Moon's pull + Sun's pull (46%)together: 104% of the Moon's alone

Not to scale: the real Moon is about 30 Earth-widths away and the Sun 400 times further still, and the ocean bulges are hugely stretched. The angles and shapes are right. Drag the Moon round its orbit!

How it works

The Sun always lights up half of the Moon. As the Moon travels round the Earth, we see different amounts of that sunny half: none at new moon, half at the quarters and all of it at full moon. One cycle takes 29.5 days.

The Moon's gravity pulls the oceans into two bulges, one facing the Moon and one on the far side, and the Earth spins through them to give two high tides a day. The Sun makes smaller bulges too. At new and full moon they line up and add together into big spring tides. At the quarters they pull at right angles and we get small neap tides.

Try this

  1. Play a full cycle. How many days from new moon to full moon?
  2. Find the two phases with the biggest tides. Where is the Moon then?
  3. At first quarter, which side of the Moon is lit as seen from Earth?