[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
Maybe you remember that viral internet graphic from a few years ago showing how the other 7 major planets in the Solar System could fit neatly between the Earth and the Moon. I’m not sure if I’m supposed to be surprised by that, but it’s a mildly interesting coincidence that the distances are so close.…… Continue reading Could all the planets fit between the Earth and Moon?
I didn’t want my first real post here to be about file formats, so I’ll go with the next best thing: space aliens. I don’t know whether extraterrestrial intelligent life (ETI) exists in the observable universe. A lot of very smart people think it does. But based on humanity’s current observational and theoretical knowledge, it…… Continue reading We’re probably alone