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. … Continue reading When was the Earth formed?
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 … Continue reading How fast does the Sun move?
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 … Continue reading Contaminating space
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. … Continue reading Black holes – supplement
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 … Continue reading Black holes
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