Hello, SFIA audio listeners, in this month's Nebula exclusive, Big Alien Theory,

we're asking the reason alien civilizations might be rare is because most aliens are huge.

To hear it and every episode early and add free, plus hours of bonus content,

check out go.nebola.t.v.tv slash Isaac Arthur, and use my code, Isaac Arthur.

For centuries, humanity has used the stars to navigate our own world. slash Isaac Arthur, and use my code, Isaac Arthur.

For centuries, humanity has used the stars to navigate our own world, but our future is to navigate to those stars and the worlds around them.

We were deep into the holiday season this week, and I remember when I was a kid there was

a lot of media attention given to trying to find

or backtrack the location of the Star of Bethlehem or Christmas Star mentioned as

guiding the Magi in the Gospel of Mark. There have been a lot of theories advanced as to what

that might have been, star, comet, Nova, and so on, but it stuck in my head as a kid, having

taken up a deep interest in astronomy that the stars move relative to each other and us. Our sky changes slowly, but it does change,

some of it sickly, some of it not. It's something folks often don't appreciate about trying

to send a ship, or even a message, to another star. If you aim for it, it won't be there when

you or your message arrive.

Today we'll be looking at interstellar navigation, but for completeness, we won't just look at

slower than light travel today, but also faster and light navigation issues and intergalactic

navigation issues, and even problems with popping forward or backward in time through a warmhole.

But even without time travel, the galaxy shifts a lot over time, and not in the regular and repeating way we see in solar systems.

While objects do orbit the galaxy, much as moons orbit planets and planets orbit stars,

there's very little recurring pattern to this.

The Earth has orbited

...