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IGCSE Physics · Lesson · Unit 6: Space Physics

What causes day and night, the seasons and the Moon’s phases?

Day and night come from the Earth turning on its axis about once every 24 hours. The seasons come from its axis being tilted at about 23.5°, not from its distance to the Sun. The Moon’s phases come from how much of its sunlit half faces the Earth as it orbits.

From the lesson The Earth and Moon, in The Earth and the Solar System. By the ConceptOrbit team · Updated

The lesson: The Earth and Moon

Earth

In this section

  • Earth as a planet.
  • Rotation and revolution.
  • Basic gravitational environment.

The Earth rotates once on its axis in approximately 24 hours, and that rotation is what produces day and night: the half facing the Sun is in daylight, the half facing away is in darkness. The Sun appears to move across the sky from east to west because the Earth turns from west to east.

It also orbits the Sun once in approximately 365¼ days. The quarter is why a leap day is added every four years, to stop the calendar drifting against the seasons.

The seasons are not caused by the Earth being closer to the Sun in summer — the orbit is very nearly circular, and the Earth is in fact slightly closer during the northern winter. They are caused by the axis being tilted at about 23.5°. When a hemisphere is tilted towards the Sun, sunlight arrives more nearly overhead, so a given beam is concentrated on a smaller area, and the days are longer. Both effects deliver more energy per square metre, and that is summer. Six months later the same hemisphere is tilted away, and it is winter.

Key points

  • One rotation ≈ 24 hours gives day and night; one orbit ≈ 365¼ days gives the year.
  • Seasons come from the 23.5° axial tilt, not from changing distance to the Sun.

Moon

In this section

  • Natural satellite of Earth.
  • Orbital motion.
  • Phases and basic observational relationships.

The Moon is a natural satellite of the Earth, held in orbit by gravity, and it takes approximately 27 days to complete one orbit.

It produces no light of its own. We see it because it reflects sunlight, and the phases are simply how much of the lit half is turned towards us as it goes round. A full moon is the lit face seen head on; a new moon is the lit face turned away.

The Moon also keeps the same face towards the Earth at all times, because its rotation period and its orbital period are equal.

Its gravitational pull on the Earth’s oceans is the main cause of tides, raising a bulge of water on the near side and, through the same pull acting less strongly on the far side, another on the opposite side.

Key points

  • The Moon is a natural satellite, orbiting in about 27 days.
  • It shines by reflected sunlight; phases show how much of the lit half faces us.

Now try it

Phases of the Moon

Phases of the MoonPhysics · Space physics · drag the Moon
SunlightNew moonFull moonFirst quarterLast quarterEarth
Seen from EarthWaxing crescent
Lit part facing Earth12%
Task · find both half-lit Moons0 of 2 found

Half the Moon is always lit by the Sun. The phase is how much of that lit half is turned towards us. (Seen from above the North Pole, not to scale.)

Open Phases of the Moon on its own page

Common mistakes

  • Thinking the phases are the Earth’s shadow falling on the Moon: half the Moon is always lit, and we see different amounts of that half.
  • Placing the full Moon between the Earth and the Sun: there it is a new moon, with its lit half facing away from us.

More lessons, each with its simulation