01. Science & Space
Today at a glance
Three new videos on the Moon tell one story: the tides between it and Earth keep one face turned towards us and are slowly pushing it away.
- Tidal Notebook explains why we only see one face: the Moon spins once for every trip around Earth, a state called tidal locking.
- Lamplight Physics shows how lasers aimed at mirrors left by Apollo crews reveal the Moon drifting about 3.8 centimetres further from Earth each year.
- Backyard Apogee draws the link: total solar eclipses need the Moon to look at least as big as the Sun, so its drift will one day end them.
TTidal Notebook · 14 min
Why we only ever see one side of the Moon
How Earth’s pull slowed the Moon’s spin until one face was left looking our way.
- The Moon turns once on its axis in the time it takes to orbit Earth, so the same side always faces us.
- Earth’s gravity raised a slight bulge on the young Moon, and its pull on that bulge slowed the Moon’s spin over time.
- An apparent wobble, called libration, lets us see about 59 per cent of the Moon’s surface from Earth over time.
- The far side is not dark. It gets as much sunlight as the side we see; it simply never faces us.
LLamplight Physics · 22 min
The mirrors we left on the Moon
Reflectors left by Apollo crews still let us measure the Moon’s distance, and it is growing.
- Apollo 11, 14 and 15 crews left panels of reflectors on the Moon. Observatories fire lasers at them and time the light’s return.
- The round trip takes about two and a half seconds, and timing it gives the Moon’s distance with great precision.
- The measurements show the Moon moving away from Earth by about 3.8 centimetres a year.
- Tides on Earth tug the Moon slightly forward in its orbit, lifting it higher, while Earth’s spin slows and days grow very slightly longer.
1 more video in this section