Shape, size and movements
- Sun: a star, about 109 times Earth's width, about 150 million km away (1 astronomical unit). Its light takes about 8 minutes 20 seconds to reach us.
- Earth: almost a sphere, slightly flattened at the poles (polar radius about 6357 km, equatorial about 6378 km). It rotates once in about 24 h (day and night) and revolves round the Sun in about 365¼ days, with its axis tilted 23.5° (seasons).
- Moon: Earth's only natural satellite, about ¼ of Earth's width, about 384 000 km away. It goes round Earth in 27.3 days (relative to the stars).
The 3D model is not to scale: at true scale the Moon would be 30 Earth-widths away and the Sun a tiny dot far off screen.
Why the Moon has phases
The Moon makes no light; it reflects sunlight. The half facing the Sun is always lit. What changes is how much of that lit half faces Earth.
- New Moon: Moon between Earth and Sun; the lit side faces away. We see nothing.
- Waxing crescent → first quarter (half disc, about day 7) → waxing gibbous.
- Full Moon: Earth between Sun and Moon; the whole lit side faces us (about day 15).
- Waning gibbous → last quarter (about day 22) → waning crescent → New Moon again.
One cycle of phases (a synodic month) is 29.5 days, two days longer than one orbit (27.3 days), because Earth has moved along its own orbit and the Moon must go a bit further to line up with the Sun again.
Phases are not caused by Earth's shadow – that only happens in a lunar eclipse.
Why we see only one face of the Moon
The Moon spins on its axis in exactly the same time it takes to go round Earth (27.3 days). This is synchronous rotation, caused long ago by Earth's gravity slowing the Moon's spin. So the same side always faces Earth. The "far side" is not always dark; it gets sunlight at New Moon.
Tides
The Moon pulls the side of Earth nearest to it a little more than Earth's centre, and the centre more than the far side. The oceans stretch into two bulges: one facing the Moon and one on the opposite side. As Earth spins through both bulges, most coasts get two high tides and two low tides about every 24 h 50 min.
- Spring tides (largest range): at New and Full Moon, when Sun and Moon pull in a line.
- Neap tides (smallest range): at first and last quarter, when they pull at right angles.
The Sun's tidal effect is a little under half the Moon's, because tides depend strongly on distance.
Solar and lunar eclipses
| Solar eclipse | Lunar eclipse | |
|---|---|---|
| Order | Sun – Moon – Earth | Sun – Earth – Moon |
| Phase | New Moon | Full Moon |
| What happens | Moon's shadow falls on a narrow strip of Earth | Moon passes into Earth's shadow and often turns copper-red |
| Who sees it | Only people in the shadow path | Everyone on the night side |
| Safe to watch? | Only with eclipse glasses / projection | Yes, with bare eyes |
The Moon's orbit is tilted about 5° to Earth's orbit, so usually the Moon passes a little above or below the Sun–Earth line. Eclipses occur only when a New or Full Moon happens near the crossing points – a few times a year. A total solar eclipse is possible because the Sun is about 400 times wider than the Moon but also about 400 times farther away, so both look almost the same size.
Earth: a planet fit for life, and how we explore space
Earth has the right conditions for life:
- A distance from the Sun where water stays liquid (the habitable zone).
- An atmosphere with oxygen, that keeps heat in and burns up small meteors; an ozone layer that blocks harmful UV light.
- A magnetic field that turns away charged particles from the Sun.
- A large Moon that keeps Earth's tilt steady, giving stable seasons.
We explore with telescopes on the ground and in space (for example Hubble and James Webb), with probes and rovers, and with crewed missions. Examples: Apollo 11 put people on the Moon in 1969; India's Chandrayaan-3 landed near the Moon's south pole in 2023; Aditya-L1 studies the Sun from a point 1.5 million km away; China's Chang'e missions brought back samples from the Moon's far side.
Try it: model the phases
In a dark room put a lamp on one side. Hold a ball at arm's length – that is the Moon, your head is Earth. Turn slowly on the spot, keeping the ball in front of your face. Watch the lit part grow from crescent to half to full and shrink again. When is the ball's lit side fully facing you? Where must the ball be for a "solar eclipse" of the lamp?
Key formulas and definitions
- Moon's orbit (sidereal month) ≈ 27.3 days; phase cycle (synodic month) ≈ 29.5 days
- Earth–Moon distance ≈ 384 000 km; Earth–Sun distance ≈ 150 million km (1 AU)
- Light time = distance ÷ speed of light (3 × 10⁵ km/s)
- Fraction lit ≈ (1 − cos θ) ÷ 2, θ = angle travelled since New Moon
- Solar eclipse: New Moon, S–M–E · Lunar eclipse: Full Moon, S–E–M
Worked examples
1. Today is New Moon. Roughly when are the next first quarter and Full Moon?
A cycle is 29.5 days. First quarter ≈ 29.5 ÷ 4 ≈ 7.4 days later; Full Moon ≈ 29.5 ÷ 2 ≈ 14.8 days later.
2. How long does moonlight take to reach Earth?
t = 384 000 km ÷ 300 000 km/s = 1.28 s.
3. Why can a lunar eclipse never happen at first quarter?
At first quarter the Moon is at right angles to the Sun–Earth line, nowhere near Earth's shadow. Only at Full Moon is the Moon behind Earth.
4. The Sun is 1.39 million km wide and 150 million km away; the Moon is 3474 km wide and 384 000 km away. Compare their apparent sizes.
Sun: 1 390 000 ÷ 150 000 000 ≈ 0.0093. Moon: 3474 ÷ 384 000 ≈ 0.0090. Almost equal, so the Moon can just cover the Sun.
5. A high tide is at 06:00. Estimate the next two high tides.
High tides come about every 12 h 25 min, so about 18:25 and then about 06:50 the next day.
6. What phase do you see when the Moon is 135° past New Moon, and roughly how much is lit?
135° is between first quarter (90°) and full (180°): waxing gibbous. Lit ≈ (1 − cos 135°) ÷ 2 = (1 + 0.71) ÷ 2 ≈ 85%.
Common mistakes
- Saying Earth's shadow causes the phases. The phases come from our viewing angle; Earth's shadow only causes lunar eclipses.
- Thinking there is an eclipse every New and Full Moon. The 5° tilt of the Moon's orbit prevents most of them.
- Calling the far side of the Moon the "dark side". It gets as much sunlight as the near side.
- Thinking only the Moon causes tides. The Sun matters too; together they give spring and neap tides.