The Earth and the Moon in Motion

≈ 20 min

Three bodies, three motions, and almost everything we call "time" comes out of them.

Motion	How long it takes	What we get from it
The Earth rotates on its axis	about 24 hours	Day and night
The Earth revolves around the Sun	about 365¼ days	The year (and the extra quarter-days give us a leap year every four years)
The Moon orbits the Earth	about 27.3 days against the stars, and 29.5 days from one new Moon to the next	The month

The Moon is Earth's only natural satellite — an object held in orbit by the pull of a larger body's gravity. It is about 384 400 km away, and its diameter (about 3 475 km) is roughly a quarter of Earth's (about 12 742 km).

The Moon has almost no atmosphere. That single fact explains most of what it is like there: no wind, no weather, no sound, no blue sky, craters that survive for billions of years because nothing erodes them, and a surface that swings from roughly $+120\ ^\circ$ C in sunlight to about $-130\ ^\circ$ C in darkness with nothing to hold the heat in.

Worked example — how far is the Moon, really? Imagine you could drive to the Moon in a minibus at a steady 100 km/h without ever stopping.

\text{time} = \frac{\text{distance}}{\text{speed}} = \frac{384\,400\ \text{km}}{100\ \text{km/h}} = 3\,844\ \text{hours}

That is $3\,844 \div 24 \approx 160$ days — more than five months of non-stop driving, roughly Johannesburg to Cape Town about 270 times. And the Moon is our nearest neighbour in space.

Why can we see the Moon shining at night?

It is a small star and gives off its own light

It reflects light from the Sun

It glows because it is very hot

Streetlights on Earth light it up

Name the movement of the Earth that causes day and night.

Craters made billions of years ago are still sharp and clear on the Moon, while craters on Earth wear away. What is the main reason?

The Moon has almost no atmosphere, so there is no wind or rain to erode them

The Moon is made of much harder rock than the Earth

The Moon is hit by far fewer objects than the Earth

The craters on the Moon are repaired by moonlight

The Moon is about 384 400 km from Earth. If a minibus could drive there in a straight line at a steady 100 km/h without stopping, how many hours would the trip take?