November 9, 2019 100

Distances: Crash Course Astronomy #25

Distances: Crash Course Astronomy #25

Oh. Hey! Sorry, I don’t mean to be rude. I’m just
trying to figure out how far away my thumb is. How? Parallax. Centuries ago, people thought the stars were
holes in a huge crystal sphere, letting through heavenly light. It wasn’t clear just how big
the sphere was, but it was pretty dang big. I have some sympathy for them. By eye, and
for all intents and purposes, the stars are infinitely far away. If you drive down a road
you’ll see trees nearby flying past you, but distant mountains moving more slowly.
The Moon is so far it doesn’t seem to move at all compared to nearby objects — and
it’s easy for your brain to think it’s much closer, smaller, and actually following
you, which is a bit creepy. Sometimes people even think it’s a UFO tailing them. Finding the distance to something really far
away is tough. It’s not like you can you can just pace off the distance. Or can you? The ancient Greeks knew the Earth was round
and there are lots of ways to figure that out. For example, ships sailing over the horizon
seem to disappear from the bottom up, as you’d expect as they slip around the Earth’s curve. But how big is the Earth? Over 2000 years
ago, the Greek philosopher Eratosthenes figured it out. He knew that at the summer solstice,
the Sun shone directly down a well in the city of Syene at noon. He also knew that at
the same time, it was not shining straight down in Alexandria, and could measure that
angle. There’s a legend that he paid someone to
pace off the distance between the two cities so he could find the distance between them.
But more likely he just used the numbers found by earlier surveying missions. Either way,
knowing the distance and the angle, and applying a little geometry, he calculated the circumference
of the Earth. His result, a little over 40,000 km, is actually amazingly accurate! For the very first time, humans had determined
a scale to the Universe. That first step has since led to a much, much longer journey. Once you know how big the Earth is, other
distances can be found. For example, when there’s a lunar eclipse, the shadow of the
Earth is cast on the Moon. You can see the curve of the Earth’s edge as the shadow
moves across the Moon. Knowing how big the Earth is, and doing a little more geometry,
you can figure out how far away the Moon is! Also, the phases of the Moon depend on the
angles and distances between the Earth, Moon, and Sun. Using the size of the Earth as a
stepping stone, Aristarchus of Samos was able to calculate the distances to the Moon and the
Sun as well as their sizes. That was 2200 years ago! His numbers weren’t terribly accurate, but
that’s not the important part. His methods were sound, and they were used later by great
thinkers like Hipparchus and Ptolemy to get more accurate sizes and distances. They actually
did pretty well, and all over a thousand years before the invention of the telescope! And
I think it also says a lot that these ancient thinkers were willing to accept a solar system
that was at least millions of kilometers in size. But at this point things got sticky. Planets
are pretty far away and look like dots. Our methods for finding distances failed for them. For a while, at least. In the 17th century, Johannes Kepler and Isaac
Newton laid the mathematical groundwork of planetary orbits, and that in turn made it possible,
in theory, to get the distances to the planets. Ah, but there was a catch. When you do the
math, you find that measuring the distances to the other planets means you need to know
the distance from the Earth to the Sun accurately. For example, it was known that Jupiter was
about 5 times farther from the Sun than the Earth was, but that doesn’t tell you what
it is in kilometers. So how far away is the Sun? Well, they had
a rough idea using the number found by the Greeks, but to be able to truly understand the solar system,
they needed a much more accurate value for it. To give you an idea of how important the distance
from the Earth to the Sun is, they gave it a pretty high-falutin’ name: the astronomical
unit, or AU. Mind you, not “an” astronomical unit, “the” one. That’s how fundamental
it is to understanding everything! A lot of methods were attempted. Sometimes
Mercury and Venus transit, or cross the face of the Sun. Timing these events accurately
could then be used to plug numbers into the orbital equations and get the length of an
AU. Grand expeditions were sent across the globe multiple times to measure the transits,
and didn’t do too badly. But our atmosphere blurs the images of the planets, putting pretty
big error bars on the timing measurements. The best they could do was to say the AU was
148,510,000 km — plus or minus 800,000 km. That’s good, but not QUITE good enough to
make astronomers happy. Finally, in the 1960s, astronomers used radio
telescopes to bounce radar pulses off of Venus. Since we know the speed of light extremely
accurately, the amount of time it takes for the light to get to Venus and back could be
measured with amazing precision. Finally, after all these centuries, the astronomical
unit was nailed down. It’s now defined to be 149,597,870.7 kilometers.
So there. The Earth orbits the Sun on an ellipse, so
think of that as the average distance of the Earth from the Sun. Knowing this number unlocked the solar system.
It’s the fundamental meterstick of astronomy, and the scale we use to measure everything.
Having this number meant we could predict the motions of the planets, moons, comets,
and asteroids. Plus, it meant we could launch our probes into space and explore these strange
new worlds for ourselves, see them up close, and truly understand the nature of the solar
system. And it’s even better than that. Knowing the
Astronomical Unit meant unlocking the stars. We have two eyes, and this gives us binocular
vision. When you look at a nearby object, your left eye sees it at a slightly different
angle than your right eye. Your brain puts these two images together, compares them,
does the geometry, and gives you a sense of distance to that object. And you thought your teacher lied when she
said math was useful in everyday life. We call this ability depth perception. You
can see it for yourself by doing the thumb thing: as you blink one eye and then the other,
your thumb appears to shift position relative to more distant objects. That shift is called
parallax. The amount of shift depends on how far apart your eyes are, and how far away
the object is. If you know the distance between your eyes
— we’ll call this the baseline — then you can apply some trigonometry and figure
out how far away the object is. If the object is nearby, it shifts a lot; if it’s farther
away, it shift less. It works pretty well, but it does put a limit on how far away we can
reasonably sense distance with just our eyes. Stars are a bit beyond that limit. If we want
to measure their distance using parallax, we need a lot bigger baseline than the few
centimeters between our eyes. Once astronomers figured out that the Earth
went around the Sun rather than vice-versa, they realized that the Earth’s orbit made
a huge baseline. If we observe a star when the Earth is at one spot, then wait six months
for the Earth to go around the Sun to the opposite side of its orbit and observe the
star again, then in principle we can determine the distance to the star, assuming we know
the size of the Earth’s orbit. That’s why knowing the length of the astronomical
unit is so important! The diameter of Earth’s orbit is about 300 million kilometers, which
makes for a tremendous baseline. Hurray! Except, oops. When stars were observed, no
parallax was seen. Was heliocentrism wrong? Pfft, no. It’s just that stars are really
and truly far away, much farther than even the size of Earth’s orbit. The first star
to have its parallax successfully measured was in 1838. The star was 61 Cygni, a bit
of a dim bulb. But it was bright enough and close enough for astronomers to measure its
shift in apparent position as the Earth orbited the Sun. 61 Cygni is about 720,000 astronomical
units away. That’s a soul-crushing distance; well over 100 trillion kilometers! In fact, that’s so far that even the Earth’s
orbit is too small to be a convenient unit. Astronomers came up with another one: The
light year. That’s the distance light travels in a year. Light’s pretty fast, and covers
about 10 trillion kilometers in a year. It’s a huge distance, but it makes the numbers
easier on our poor ape brains. That makes 61 Cygni a much more palatable 11.4 light
years away. Astronomers also use another unit called a
parsec. It’s based on the angle a star shifts over the course of a year; a star one parsec
away will have a parallax shift of one arcsecond—1/3600th of a degree. That distance turns out to be
about 3.26 light years. As a unit of distance it’s convenient for astronomers, but it’s a terrible
one if you’re doing the Kessel Run. Sorry, Han. The nearest star to the sun we know of, Proxima
Centauri, is about 4.2 light years away. The farthest stars you can see with the naked
eye are over a thousand light years distant, but the vast majority are within 100 light
years. Space-based satellites are used now to accurately
find the distance to hundreds of thousands of stars. Still, this method only works for
relatively nearby stars, ones that are less than about 1000 light years away. But once
we know those distances, we can use that information on more distant stars. How? Well, like gravity, the strength of light
falls off with the square of the distance. If you have two stars that are the same intrinsic
brightness—giving off the same amount of energy—and one is twice as far as the other,
it will be ¼ as bright. Make it ten times farther away, it’ll be 1/100th as bright. So if you know how far away the nearer one
is by measuring its parallax, you just have to compare its brightness to one farther away
to get its distance. You have to make sure they’re the same kind of star; some are more luminous than
others. But thanks to spectroscopy, we can do just that. A star’s distance is the key to nearly everything
about it. Once we know how far it is, and we can measure its apparent brightness, we
can figure out how luminous it is, how much light it’s actually giving off, and its
spectrum tells us its temperature. With those in hand we can determine its mass
and even its diameter. Once we figured out how far away stars are,
we started to grasp their true physical nature. This led to even more methods of finding distances.
The light given off by dying stars, exploding stars, stars that literally pulse, get brighter
and dimmer over time. All of these and more can be used to figure out how many trillions
of kilometers of space lie between us and them. And we see stars in other galaxies, which
means we can use them to determine the actual size and scale of the Universe itself. And all of this started when some ancient
Greeks were curious about how big the Earth was. Curiosity can take us a great, great distance. Today you learned that 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. Crash Course Astronomy is produced in association
with PBS Digital Studios. Head over to their YouTube channel to catch even more awesome
videos. This episode was written by me, Phil Plait. The script was edited by Blake de Pastino,
and our consultant is Dr. Michelle Thaller. It was directed by Nicholas Jenkins, edited
by Nicole Sweeney, the sound designer is Michael Aranda, and the graphics team is Thought Café.

100 Replies to “Distances: Crash Course Astronomy #25”

  • Robert Barnes says:

    I just found your channel. Excellent in every respect. Teaching — conveying information so that it hits the target brain and sticks — is a true art, and you appear to have nailed it. I'd like to make one suggestion, however. I know that the YouTube gurus tell you to aim for a higher degree of animation and expressivness than you would in everyday conversation, to keep people engaged. It's possible to go overboard to the point of making the presentation distracting rather than entertaining or educational. Specifically, scrunching your face into a parody of joy, waggling your fists and proclaiming "YAAYYY!!" rings the bell on the Dork-O-Meter, and the distraction overwhelms the content. I guess a good rule of thumb is "Anything that would get you a wedgie in middle school should probably be edited out." Your content and ability to explain will stand on their own. Otherwise, really excellent. Please keep it up.

  • cvt cvt says:

    What about bends in light and time?

  • Aval Sirithanawat says:

    Does anyone here knows how the angle can be measured practically using the trigonometric parallax method? I know the more the apparent position of the star shifts, the greater the angle but how do I acquire the actually number of arc seconds? I'm doing a project and will have to use a telescope to do this experimentally. Thanks

  • Orion6699 says:

    Pretty pathetic that there are all these flat earth idiots today and 2000 plus years ago the Greeks with their technology could figure it out.

  • Marcus Gaillard says:

    Stellar Parallax assumes that the Earth orbits the Sun in the first place. Your 'science' is a circular reasoning joke. Lucky people are waking up to this pseudoscience and bad astronomy. The earth has actually NEVER been shown by scientific empiricism to be actually moving in 'space'. True.

  • Denis Bourque says:

    Impossible because solar noon can’t be at both places at the same time. But nice try

  • Erick Ricardo Flores says:

    I love 7:23

  • Damien Bull says:

    He wasn't the first to do it. The Ancient Cro Magnon man (Caucasian) was able to do it thousands of years before.! 👍

  • Abril Guevara says:


  • Bianca Mitchell says:

    Okay listen, I've read about parallax so many times and only ever sort of got it, not fully understanding why the math works. I always thought it seemed oddly arbitrary and why use opposite points on the orbit? Phil all you had to say was "two eyes" and "trigonometry" and now I'm sitting here so f*cking annoyed that it's that simple, it's just a goddamn triangle oh my god. I'm so mad at myself.

  • joshua spurgeon says:

    Kindly explain if the base line for the star distance calculation is earth 6months apart.. I suppose stars travels at very high speed so by 6 months the star is not in its original position and the results of parallax could be just the star actually moved from point A to B and we could be completely wrong.
    Kindly throw some light into my doubts.
    Thank you

  • joshua spurgeon says:

    Again I just did a maths by myself…
    If the base line is earth orbit or say 2 au and the nearest star 4.3 light years away. The trigonometric triangle is very very skinny that would be 0.0002 km to 4.3 km approx … That's lot of negligent values

  • Sketchstuffs says:

    You use kilometers? That's cute, I use freedomiles.

  • Harshil Jain says:

    Big fan of your's as I have been knowing about you from the programme -How The Universe Works. I have gained too much knowledge from that programme as I want to become an astronomer like you in future at NASA

  • Kevin Lampen says:

    Full freedom of speech allows for curiosity… as freedom allows difficult questions to be asked …

  • Ivan Ooze says:

    omg i want some kerbals

  • Ivan Ooze says:

    is it keel-oh-me-teer or kill-om-it-tur?

  • vibra64 says:

    Great explanation. Thanks!!

  • Krisha Arya says:

    Wow! Man.. Subscribed

  • Kanzu999 says:

    Did they make a video on red shift?

  • saladdogger says:

    149000000. Is that the distance to the centre of the sun or the surface ?.

  • samzie samzie says:

    That was excellent, thanks

  • قناة آية الثقافية says:

    how to measure parallax distance

  • John Howard says:

    We need that guy with an accent so thick you can't understand him!!!

  • Riyansh Gupta says:

    Aryabhatta told radius of Earth before birth of Christ

  • toastynotes says:

    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.

  • BürgerZorn says:

    funny that they mention hipparchus & ptolemy … the first dude seems to be a jewish student and the 2nd is just a cheap faker (wiki) … i mean Ptolemy would have said anything to get fame. Next Guy Kepler – alot of cringe storys about him – he just steal this telescope from a geocentric scientist, who "accidentally" die : / btw Ptolemy name is rly similiar to Ptolemy I. who "take jerusalem while sabbat" and become a friend of jewish ppl. just sayn it.

  • Indiana Joe says:

    So basically, if you thought you was just a spec, LoL! You wish!

  • oliver batson says:

    everything this man says is a lie, flat earth for life

  • Rob Cosentino says:

    149,597,870.7 kilometers = 92,955,807.273 miles

  • Richard Groves says:

    Man, this guy is an amazing smart arse!

  • Ravi Adluru says:

    the parsec can be used to measure the Kessel run. It isn't a race

  • EdgyVenoM says:

    If we calculated the distance by using speed of light , then how did we calculate the speed of light?

  • rod sturgiss says:

    My brain hurts….

  • Terrence Murrin says:

    Here's another thumb thing you can do. Sit on one thumb while sucking on the other and your mind will open up to a whole new universe.

  • SpottedSharks says:

    This guy was great on Mythbusters.

  • 5950155 says: many mixed reviews..I rather enjoyed it.took me back about 38 yrs to grade 11 physics

  • Conjuring K says:

    Stupid westerners understand only basic dodo knowledge of greeks when the world outside knew million folds more than greeks who were merely knowledge recipents from east… What today is being claimed as greek was already originated somewhere else even before they ever existed.the truth is that greeks only copied from others & passed it down.. Due to rise of western civilization in 19th century after british empire & america after ww2 there is massive upsurge in greco romanian supremacism over science, rationality & thought although much of it is minuscule(compared to knowledge traditions of other civilization back then), acquired from others or blatantly falsely attributed to greeks.

  • Amira Lozse says:

    0:44 next time look at the road whilst driving!!

  • zeroomens says:

    Yeah Mr. White… SCIENCE!

  • JayCub says:

    I wish Phil was there with me in my exam

  • PotatoWalrusFilms says:

    You lost me at “the earth is round” :/

  • Reggie Anderson says:

    Flat earthers while in school must have done horrible in geometry, trigonometry and history.

  • Prem S says:

    I have a genuine doubt, maybe it is silly. Parallax method can be applied when my thumb is stationary and vision through different eyes… How can we be sure that the stellar object does not move in 6 months? I would appreciate if someone clarifies it… thanks

  • Mo Fa says:

    my teacher didn't lie when "she said math was useful in everyday life … " because I didn't have a female math teacher. Female math teachers are pretty unusual, actually … (Why IS that ?)

  • Richard Castillo says:

    why does the guy driving in the car look like Hunter S. Thompson from Fear and Loathing in Las Vegas?

  • vliegendehollander says:

    Parallax's accuracy is still limited to several thousand light years. For measuring the longer distances at millions to billions of light years, look up the Candle Method.


    i learned more watching this video than the last four classes.

    plus, where is john green?

  • Saud ali Khan says:


  • SnoopyDoo says:

    No mention of Cephid variable stars. Downvoted.

  • Pete Wilson says:

    Newton looks like Rodney McKay on Stargate Atlantis

  • Brent Weeks says:

    Host is American. Stop using kilometers cause we use miles!

  • Subscribe For No Reason says:

    Take that Flat Earthers!

  • Tom McMorrow says:

    Raoul Duke: "Is that moon following me? By God I swear it is, man!"

  • Carter Soelberg says:


  • KM says:

    So show some of this work. He keeps saying, "and you can do a little math and figure out X". So show us.

  • Savage Dude says:

    It's so annoying that the audio and visuals aren't perfectly in sync

  • Noe Diaz says:

    Mission accomplished + respect

    Achievement unlocked: unlock solar system

  • EulogizeMe43 says:

    I feel like I'm getting a college level education for free. I love these videos.

  • Rich D says:

    4 light years may sound close but that means you need to travel near the speed of light to reach it in about 4 years…

  • Sterling Crockett says:

    No offense Phil, but I wish Michelle Thaller was the one hosting the videos. She's not just smart, she's pretty too!

  • ثريا says:

    Loving this crash course alotttttt

  • No Fame Here says:

    Astronomers are a gift to the world!

  • Davy Mckeown says:

    This is slightly misleading, the first parallax used the diameter of the earth as its baseline to measure the distance to the moon. Henrietta Swan Leavitt identified the first standard candle, Cepheid variable, a variable star whose period she calculated in 1908.

  • Code Tutorial says:

    in your intro u can see kerbals the star trek enterprise wow
    how much more can you likethis guy

  • Jamell Washington says:

    Phil plait has a unique way of teaching love this

  • mjimih says:

    over 55? then an AU is 93 million miles.

  • Kile B says:

    I think he could be a younger Walter white . Love these vids

  • Bill Boyd says:

    Wow what a huge achievement by men, almost exclusively! I guess if we had left it to women we would still be casting horrorscopes!

  • Naouali Nizar says:

    Egocentrism in action… Interesting!!!

  • dartagnanx1 says:

    The stars are in a sphere. It's just a fifth dimensional sphere. Troglodytes!

  • Brent Sander says:

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

  • Joseph Casey says:

    I enjoy these videos. But I have to say, I just despise the use of metric measurements by American scientist. It just seems so un-American to me. Why don't you use what 99.9% of us grew up using, instead of the dumbed down version of math (ie. 10, 100, 1000, etc…)? If your biggest audience was from Peru, would your videos be in Russian? Of course not. So why do you snobby scientist insist on using metric in videos aimed at the lay person? Do you blame your progressive professor for this flaw?

  • James M says:

    I wanted to know how the Cathlic church fitted into this story as Galileo was torchered for even suggesting the the Earth was not the centre on the solar system. Also, the ptolemeic system puts the Earth at the centre.

  • chanctonbury63 says:

    9.31 Not quite. If the light is interrupted by gas clouds, for instance, the value is worthless.

  • Lizard King says:

    What does the term 'average' mean for an elliptical orbit? Exactly midway between the closest and furthest distance? Or the distance you'd get if you took lots of measurements randomly over time and took the mean?

  • xxYetterxx says:

    so, where did you get those KSP figurines? I want them so bad

  • Daniel Rafferty says:

    Thanks for this video. Dude I know who's a flat earthier tried to ask how come we can't perceptibly see the stars in the night skies parrallax. Having a basic understanding all I could tell him was "because they're really far away" glad I knew that much at least.

  • Donald Martinez says:

    My mind was blown and I'm not even high..

  • LeroY says:

    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!

  • John Freeman says:

    During the times of ancient Greece: yeah, Earth is round.

    Present day 'merica: flat earthing intensifies

  • flat world says:

    The ship stopped disappearing since the p1000 come out the world is flat you understand what i sayyyyyy😂😂

  • Jonathan Ward says:

    If there was no such thing as religion we would be a lot more advanced now because we would have progressed since the ancient Greeks. 😔. Ahh well soon enough as it is now, people can do as they please without worrying about being a heiritic.

  • youtube Rana says:

    Q re hindi me bol…
    Khadi boli me…

  • Kenneth Sanoy says:

    Haha😂 I'm sorry but did i see the Tagline of Selena in MLBB?😂

  • Subramaniam Chandrasekar says:

    When planets are constantly moving in orbits, how is it possible to claim accurate distances than X or Y method etc?

  • TASFIA MIM says:

    0.25 s didn’t pass and I was already pressing the subscribe button.

  • Slow Mo says:

    “We can’t stop here, this is bat country.” Thanks for that!

  • Dr. Laird Whitehill's Fun with Astronomy Channel says:

    Phil is great. I'm a fan. And I teach Astronomy too. Come to my class. No prerequisites.

  • jackson clevenger says:

    It’s kilometers not kilometers

  • loki Alhat says:

    What is original direction? I mean 0 gravity.

  • fair business says:

    Ancient Greeks? Lol. You mean the ancient Africans. The Greeks went to Africa to learn

  • Tim Storey says:

    Is this guy good, or what?

  • Brandon Fisher says:

    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!

  • Lobsta12874 says:

    i still feel uncomfortable when astrophysicists neglect or gloss over the errors in distance measurements based on apparent magnitudes and spectroscopy. Its the reason why you find so many "break throughs" and "paradigm shifts" based on the next space probe or observatory. Bruh tech hasnt gotten that much better than 1980, they just stopped making wild conjectures base on horrible estimates. half a light year error means you run out of fuel well before your destination, or your galaxy model is misleading. What I truly hear is less scientific fact and more opinions, and the fact that Phil starts this off by holding up his thumb is about how accurate most of these astronomical measurements actually are. If you pull this cornerstone of shikatery out all the work on conjectures from universe expansion and mass measurements takes a hit in plausibility.

  • Bob James says:

    Dude, you just brightened up my evening!

  • Tuna Free Dolphin Meat says:

    Glad to know I am not the only one who drives down the road as trees fly past and with the moon following me. From content and host to visuals and sound, an interesting and well produced series.

  • C w Wagstaff says:

    Wow someone I can talk to

  • velazquez armouries says:

    well your thumb is around 2 and a half shaku from your face

  • Grant Johnston says:

    Love the HST tribute in the cartoon Red Shark.

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