The moon moves away from Earth about 3-4 cm/year. Why?
The differential force of the moon on the Earth raises tides in Earth’s ocean. The tides also have a significant effect on the moon – the mass of the water exerts a force on the moon. The bulges will move ahead of the moon since the Earth rotates once a day and the moon orbits once a month – this causes the bulges to get ahead of the moon and to pull it forward in its orbit.
Angular momentum is conserved. The force of the bulges on the moon actually adds angular momentum to the moon’s orbit.
This next idea may seem counter-intuitive, so you’ll have to take my word for it.
When an orbiting object gains angular momentum, it begins to move faster in its orbit, so it moves farther away from the object it is orbiting. The same is true for satellites and space shuttles orbitting Earth. If they want to increase their distance from Earth, they just fire their thrusters to jump into a higher orbit. Cool, right?
You may argue that angular momentum has to be conserved. Don’t worry – it is. As the moon gains orbital angular momentum, the Earth loses its rotational angular momentum. This means the Earth is rotating slower now than it did billions of years ago.
A dinosaur day was only 23.5 hours long!
So the moon is moving away from Earth right now. What did it look like when dinosaurs roamed?
Looks pretty similar! That’s because although the moon is moving away every year, in 85 million years it will have moved 2000 miles away. But this is only 0.8% of the total distance of the moon. So it won’t look different.
Do not believe those sci-fi movies that show the moon to be super huge during the time of the dinosaurs. It would look the same.
Pretty cool.
unravelingthesky 