Ways things can rotate

This is an overview of some of the kinds of rotation an object can undergo, with a focus on astronomical objects. Standard warning: I'm starting to realize that I don't really know enough about physics to write about this. But I try not to let things like that stop me. Things to note: I'm discussing …

Earth portrait photos

This is a pretty good photograph: … Though it's so ridiculously overused and cliché that it surprises me that graphic designers, etc., still seem to like using it. Maybe we've been so thoroughly conditioned by it that other photographs of the Earth just don't look like we think the Earth is supposed to look. If …

When was the Earth formed?

When was the Earth formed? Modern estimates are that it formed by accretion, 4.54ish billion years ago. But I'm not going to worry about its absolute age; what I'm wondering is where we choose to draw the line between Earth-not-quite-formed and Earth-now-formed. We don't really know how long it took for the Earth to form. …

How fast does the Sun move?

How fast does the Sun move? I don't mean though the galaxy, or the universe, but simply how fast does it move across the sky? I've seen it stated categorically that the Sun moves at 15 degrees per hour. But, for one thing, 15.000000° can't be perfectly correct at all times, because the Earth's orbit …

Contaminating space

In April, 2019, Israel's Beresheet spacecraft landed on crashed into the Moon. It later emerged that it was secretly carrying a supply of live tardigrades -- microscopic creatures known for their hardiness. Naturally, many observers, joking or not, brought up the idea of accidentally contaminating the Moon with destructive Earth life. I worry about a …

Black holes – supplement

Here are a few notes about black holes that I think are interesting, but didn't fit in my previous post on the topic. What would happen if… I asked as an example question "What would happen if you tried to escort a small black hole into a large black hole?", but I didn't answer it. …

Black holes

Black holes are well understood by physicists, but I've read too many oversimplified, confusing, and seemingly-contradictory descriptions of them. Over time, I've settled on my preferred mental model of a black hole, one that usually helps me to work out the right answers to questions that non-physicists have about black holes. (An example of such …

Simple orbital mechanics

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? …

When will Eris be closer than Pluto?

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 …

How to draw an elliptical orbit

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 …