The Earth, the Sun and the Moon
Days, months and years
- A day, a month and a year all come from motion in space.
- The Earth spins, the Moon circles us, and we circle the Sun.
- Let's see what each motion gives us — and how to find an orbital speed.
The spinning, orbiting Earth
- The Earth rotates on its axis once in about $24$ hours — giving day and night.
- It orbits the Sun once in about $365$ days — one year.
- Its axis is tilted, so each half of the Earth leans toward the Sun for part of the year — giving the seasons.
What causes day and night?
The Earth rotates once in about 24 hours, so each place faces the Sun (day) then faces away (night).
What causes the seasons?
The tilted axis means each hemisphere leans toward the Sun for half the year (summer) and away for the other half (winter).
The Moon
- The Moon orbits the Earth once in about one month.
- As it goes round, we see different amounts of its lit side — the phases (new, half, full).
- From shortest to longest: day (Earth's spin) < month (Moon's orbit) < year (Earth's orbit).
Put these from the shortest time to the longest.
Day (~24 hours) < month (~1 Moon orbit) < year (~365 days).
The phases of the Moon happen because we see different amounts of its lit side as it orbits.
As the Moon orbits Earth, the fraction of its sunlit side that we can see changes, giving new, half and full moon.
Orbital speed
- One full orbit covers the circumference $2\pi r$, where $r$ is the orbit radius.
- The orbital speed is that distance divided by the period $T$ (the time for one orbit):
- The same equation works for planets, moons and satellites.
A satellite orbits at radius $r = 7.0 \times 10^6\ \text{m}$ with a period $T = 5800\ \text{s}$. What is its orbital speed, in m/s?
$v = \dfrac{2\pi r}{T} = \dfrac{2\pi \times 7.0 \times 10^6}{5800} \approx 7600\ \text{m/s}$.
Compared with planets close to the Sun, the outer planets orbit:
The Sun's gravity is weaker further out, so the outer planets move more slowly in their orbits.
You've got it
- Earth spins → day/night; orbits the Sun → year; tilt → seasons
- the Moon orbits Earth in ~1 month, giving its phases
- day < month < year
- orbital speed $v = \dfrac{2\pi r}{T}$