Distances: Crash Course Astronomy #25

How do astronomers make sense of the vastness of space? How do they study things so far away? Today Phil talks about distances, going back to early astronomy. Ancient Greeks were able to find the size of the Earth and from that the distance to and the sizes of the Moon and Sun. Once the Earth/Sun distance was found, parallax was used to find the distance to nearby stars, and that was bootstrapped using brightness to determine the distances to much farther stars.

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Chapters:
Introduction 00:00
How did we calculate the Earth's Size? 1:07
THE Astronomical Unit (AU) = 149,597,870.7 km 3:12
Depth Perception & Parallax 5:39
Light Years & Parsecs 7:31
Brightness Indicates Distance 9:07
Review 10:30
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PHOTOS/VIDEOS
Venus & Mercury [credit: Phil Plait]

[lhunt]

I remember first watching this video when it came out, at the time I was still in school doing my exams but now I'm in the 2nd year of my PhD in astrophysics:)

[IXPrometheusXI]

My favorite thing about this episode is that it focuses on the how, not the what. I love learning about the methods people actually used to figure these things out. It makes them seem so much... Nearer.

[brfisher]

Putting 4.2 light-years in perspective: seeing that this video was uploaded 4 years ago (July 16, 2015) we're seeing the alpha centauri system as it was around the time this video was first uploaded back when Phil Plait was giving us awesome Crash Course Astronomy videos! That is if we're viewing it on 2019.


I enjoyed seeing Phil Plait on Crash Course Astronomy back in 2015!

[hornchief]

Phil is a great teacher, I actually found him on TV. I really hope he keeps doing more Crash Course stuff!

[LydiaIsSweet]

I hate school books, they're almost always written in such a difficult way as if they assume you've been studying the topic for years - and I'm talking undergrad level.
So, so grateful for these crash courses that are much easier to grasp and actually try relating some of this stuff which may feel abstract to everyday life examples. And it's not even in my first language yet it's much more pedagogic and understandable than the books I have in my home language.
Thanks for being creative and reminding students why they chose their field!

[brentsander]

So figuring out the AU turned out to be pure gold.

[TheFireflyGrave]

This series is so great. I knew a parsec was roughly 3 lights years, but I had no idea that it was devised in a completely unrelated way. The ways scientists over the centuries came up with various measurements is fascinating.

[juliocervantes]

This might be my favorite CrashCourse Astronomy lesson. Love learning how people figure stuff out.

[LPSwimmer]

This series has probably become my favorite CrashCourse series. Thanks Phil!

[mcsmaria]

5:58 - well, I never really understood math until I got older, because ironically I went into a field that required math. Even though I hated it in school. I wish I had as good math teachers as I had in the other subjects. It may not have seemed so scary.

[viasaliha]

i get chills almost every time i watch this series bc DAMN the universe is so big and beautiful and just wow

[scarletstar]

OMG THANK YOU FOR UPLOADING THIS!!!! I was sooo confused in class today!! And this helped me a lot!!!! THANK YOU THANK YOU THANK YOU THANK YOU!!!!

[speedanderson]

I was watching a documentary about The Sun last night, and I saw Phil Plait on there, thinking "Hey! The Crash Course guy!". I really like this guy as a host for this.

[MestR]

I LOVE that last picture of the Andromeda Galaxy you used. I could stare at that, and its own stars, all day.

[ffggddss]

Handy way to remember (how to compute) what a parsec is:
par-sec == parallax-second

1 parsec = 1 AU / 1 arcsecond
where the arcsecond is expressed in radians:
1 arcsecond = 1º/3600 = (π/180)/3600 = π/648, 000

So:
1 parsec = 1 AU · 648, 000/π = 206, 264.8 AU

[toastynotes]

Love the breakdown on how we learned the relative size of our galaxy. From the size of Earth, to how far away the Moon is, to how far away the Sun is. Learning how the puzzle was put together can be just as fun as looking at the finished result.

[Quokkat]

Crash Course Astronomy is my favorite one of the entire crash course series! amazing job!

[HSCrimson]

When I was very young, I actually though the moon was chasing me...

[herpsenderpsen]

this whole video basically summed up my physics class and it was hell!

[TerenceClark]

I've taken college level astronomy coursework and been a space geek my whole life, but I'd never heard how we determined the AU.  That's awesome!  Thanks, Phil!  And thanks crash course!