air inside the train car moves with the train car, air outside the train car does not.

if you're standing on the roof, you've got some friction to hold you in place against the air rushing past. jumping while on top of the roof causes all that rushing air to smash into you totally unimpeded, pushing you back

It's not really like that, since tau and pi only differ by a factor of 2.

A useful equation is, for example d = 1/p, where distance is measured in parsecs and p is measured in arcseconds. If you used meters, it's not just a factor of 2, it's something like 3.086×10^(16) d = 1/p. With distances as big as parsecs, there's no intuition you can use to understand the size, so it really doesn't matter what unit you use. It just makes the most sense to use the natural unit instead of the arbitrary one

it's defined as the average distance

metric is actually fairly arbitrary. AU and pc are so much more useful in astronomy because they're derived from actual physical quantities that affect the measurements we make. it's so so easy to write down fundamental astronomical equations in terms of parsecs, whereas if you used metric you would need to include some sort of conversion factor

> Light minutes, on the other hand, are a unit of time, not distance. They are used to measure the time it takes for light to travel a certain distance. For example, it takes about 8 minutes for light to travel from the Sun to the Earth, so the distance from the Sun to the Earth is about 8 light minutes

You've contradicted yourself in this paragraph. Light minutes are not a measurement of time, they're a measurement of distance. You know this intuitively because you explained it correctly the final sentence here

Astronomers don't use light minutes within the solar system because they don't really care about light within the solar system, AU is just a more convenient unit.

This isn't an explanation, so I can't post it as a top-level comment, so I'm going to borrow yours

> It still calculates them

This is true, and there actually exist extensions for both Firefox and Chrome (and other chromium-based browsers) that return the dislike counters. If anyone reads this and happens to want the dislikes back, go looking for an extension called "return youtube dislikes"

If your driverless train fails, it slowly comes to a stop on the tracks and everyone gets out. If your pilotless plane fails, you've got 200+ people dead.

Music used to be so much better!

I don't think that's quite right. The Earth is still tilted, just sideways with respect to the sun. So, for example, at the vernal equinox, the sun would just slightly to the north and set just slightly to the south

> But does the sun take the same path through the sky if the observer is on the North Pole vs the Equator?

No: if you're on the equator the sun will be directly overhead, if you're at the north pole the sun will always be in the southern part of the sky.

> Also, is there any day / location where the sun rises directly in the east, goes directly overhead, and then sets directly in the west?

The closest you could get to this is being exactly on the equator during an equinox. It's still not perfect, the tilt of the earth makes what you're describing impossible, but it's pretty close.

Key point: this is referring to the observable universe

Light travels at a speed and the universe has an age. How far away we're able to see is determined by how long the light has had to reach us. Our observable universe is a sphere with a radius of 93 billion light years. if you travelled a billion light years away from earth, you would still only be able to see a sphere around you with a radius of 93 billion light years. Your observable universe would look different but it would be the same size and it would be centered on you no matter where you went.