The Moon is slowly siphoning off the Earth’s rotational energy, in a phenomenon called tidal acceleration. This moves the Moon’s orbit farther from the Earth. According to Kepler’s laws of planetary motion, the Moon must orbit more slowly as it gets farther away. Well, which is it? Is the Moon speeding up, or slowing down?…… Continue reading Simple orbital mechanics
The dwarf planet Eris is sometimes closer to Earth than Pluto is. That’s no secret. Wikipedia’s article on Eris even has a chart similar to the one below. But I feel like this fact has been strangely ignored in the whole is-Pluto-a-planet debate. Eris is a tiny bit smaller in diameter than Pluto, though it’s…… Continue reading When will Eris be closer than Pluto?
Maybe you’ve heard of the Milankovitch cycles, one of which involves changes to the eccentricity of the Earth’s orbit, as it is perturbed by other objects in the Solar System. Suppose you want to depict this with a diagram, using a circle, and an ellipse of exaggerated eccentricity. You could just draw any old random…… Continue reading How to draw an elliptical orbit
[I suggest reading my earlier post, The Big Bang, before this one.] The observable universe (OU) is the part of the universe that we can, at least in principle, observe, from a designated vantage point at a given place and time. This post is my attempt to figure out in more detail exactly what the OU…… Continue reading The observable universe
In cosmology, the term the Big Bang has several subtly different meanings. It means the moment in time, it means the “explosion” event, and it means the all the events and states of the early universe. How early is early? Could be lots of things. Maybe just the first fraction of a second, maybe the…… Continue reading The Big Bang
Suppose you are a passenger in a spaceship, you look outside, and you notice that, from your vantage point, the Sun and the Earth appear to be almost exactly the same size. Question: Is it possible that you are currently in orbit around the Earth? Suppose instead that it’s the Earth and Moon that appear the…… Continue reading Hill Spheres and apparent radii
As seen in the Earth’s sky, the Moon and Sun are very close to the same size. I’ve seen it asserted a handful of times that this situation is unique in the Solar System (example). But is that actually true? It always sounded a little suspicious to me, so I decided to take a closer look.…… Continue reading Solar eclipses on other planets