stosb

wearer of programming socks

  • she/her

mid 20s | bisexual | programmer | european


profile pic: a picrew by Shirazu Yomi
picrew.me/en/image_maker/207297
i use arch btw
xenia the linux fox -> πŸ¦ŠπŸ³οΈβ€βš§οΈ
the moon
πŸŒ™

8akesale
@8akesale

and by the end of today's space fact y'all are gonna know exactly why it's called that.

Turns out, there's a lot of sky! Exactly 4*pi steradians, to be precise. (We'll get to what those are in a bit i promise) There are a bunch of different ways of telling someone where something is up there that are way better than "that bright dot by the moon", and we're gonna talk about a few of them today.

Starting at the beginning :3


8akesale
@8akesale

The original method used to point out areas on the sky is something everyone is familiar with - constellations!

Constellations aren't just the stars that make up a shape in the sky - a constellation is an entire region of the sky, containing the recognizable stars as well as all the minor stars and anything else that happens to be in that direction. Together, all 88 constellations cover the entire sky. My favorite is Draco :3

Some things are named based on the constellation that they happen to be located in! For example, Ξ² Virginis and the Virgo Cluster are both in the direction of the Virgo constellation.


8akesale
@8akesale

Image: Cmgee, CC BY-SA 3.0

This is the complete constellation map (under an equirectangular projection if you're into maps) - notice how every part of the sky is covered by exactly one constellation!

However, sometimes that isn't good enough. You can't just say "alright, go to the middle of Draco and then a bit left" to a telescope operator and expect to get an observation of the right objec. Even something like Neptune is usually far too dark to spot by eye, so if you wanted to point at Neptune, you'd need something a lot more precise.


8akesale
@8akesale

Here is where we'll sidebar into steradians - they're not super necessary to know what's coming up but I think they're really neat! We're gonna be talking some math here so feel free to skip this post if you're not into that

So you know how you can measure distance with meters, and then area with square meters?

Well, we can measure angles with radians, right? Is there an equivalent "area" for angles?

Yup! It's called solid angle, and it's measured in steradians. A regular radian is defined as the angle that cuts out a length equal to a circle's radius on the circle's circumference, and the steradian cuts out an area equal to the square of the sphere's radius on the surface of the sphere. It's neat! There's 4pi of them and it very useful to measure things like received flux and whatnot.

Ok back to the actual topic - stellar coordinates


8akesale
@8akesale

So, ok. You're trying to look at Neptune with your telescope, but you can't really see Neptune through your rangefinder. You know it's somewhere in Scorpius, but that's not much help - constellations are pretty big, all things considered.

Well, chances are your telescope has two axes to spin on, one that points it up and down, and the other that spins it around in a circle. Turns out that you can use those axes to define a coordinate system!

The two things that you can know for sure will help us find reference points for this. The spot directly above you, let's call that 90 degrees altitude (or elevation. I like altitude :3). Why 90? Well, if you go 90 degrees away from it, you'll be exactly at the horizon, so it makes sense to make that 0. Ok, that tells us how far up or down to point, but we still have no idea how far left or right...

The Earth gives us a really nice way of defining that kind of direction - just take North and call it 0 degrees! This is the azimuth angle, which almost looks like a yinglet said it. Then just look up the alt-azimuth coordinates of Neptune based on where your current location, and bam! There you go, one telescope pointed directly at Neptune even though you couldn't see it.

For about two minutes.


8akesale
@8akesale

Yeahhhh turns out the Earth giveth and the Earth taketh away - that bitch is spinning and you're stuck to it.

It spins, and it spins your telescope, which changes the direction it was pointed, and now you've got to fiddle with both of your angles to keep track of it and it is SUCH A PAIN IN THE ASS ASK ME HOW I KNOW

Also, the alt-az coordinates of any given object are changing constantly, both in time and also based on location. Even if you could teleport, moving any significant distance would change the alt-az you'd need to find it.

Can we fix that? Can we fix any of this?

YES AND I LOVE IT


8akesale
@8akesale

So, let's go back to those reference points we picked. Clearly, we didn't make very good choices, but can we do better?

Absolutely.

Because of the geometry of Earth's rotation, turns out that Polaris - the North Star - doesn't actually "move" at all. So... let's use that?

Alright, Polaris is now 90 degrees... wait, things can be further than 90 degrees away from it. Eh, that's fine, just call it positive 90 degrees and let that number go below 0, no worries. The celestial "equator" is at 0, and the South Star (if we had one) would be at -90. This number, effectively describing the distance from Polaris, is called declination.

We still have the problem of the "left-right" angle though...

This one feels pretty arbitrary but it's standardized so it works well enough. Where's the Sun on the March equinox? That's 0.

0 what? It's not degrees.


8akesale
@8akesale

Turns out it's in hours. Since the Earth turns once every 24 hours, we decided that there should 24 hours in a full circle around a given declination. Yeah, it's weird, but you do get used to it i promise. This is gonna be your right ascension.

This is called an equatorial coordinate system, and i am in love with it.

Get yourself a fancy telescope with an equatorial mount, look up Neptune's RA and Dec, and there you go. As the Earth rotates beneath you, just move your right ascension knob and you'll follow it perfectly. It's so nice.

So, this equatorial system is the most common one you'll see in astronomical names. Back to the title of this thread :3

PSR B1257+12. It's a pulsar, with right ascension 12 hours 57 minutes, and declination of +12 degrees. Nice and easy :3


8akesale
@8akesale

Abyzz

a yinglet wearing a lab coat, talking with ears raised

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in reply to @8akesale's post:

i don't remember enough about stars to get much from this image (also it's going on 11 and I can't get myself to read lots of text) but I do like how the ecliptic line has the two astrological signs that share their names with Earth's tropics both about, uh. ...Fuck, I forget math.

I like that the funny wave has cancer and capricorn both just before maximum amplitudes.

in reply to @8akesale's post: