How to Measure The Universe One Step At A Time 

IMO, there’s not nearly enough of this sort of “how we know what we know” educational content available to most people. Carl Sagan did a good job of presenting pieces of it, like the story of Eratosthenes and the initial measurement of the Earth’s diameter, but I don’t think I’ve seen anyone build out the whole distance ladder in a single clear explanation like this. Bravo.

There's one more thing to say about the distance ladder: it's why astronomers have a bunch of weird different units (AU, Parsecs, red-shift, etc...). Each one corresponds to a different method (AU to orbital mechanics, parsecs to parallax from the earth's orbit, light shift from the expanding universe). To convert it back into something conventional like metres you have to convert it step by step because that's how the calibration works (eg, to convert parsecs into metres, you first have to know how many metres are in an AU). But the calibration is often inaccurate and is improved in time. So it's best to just leave results in the "natural" unit it was measured in, and if someone wants to translate it into metres at some later point, they can use the most accurate calibration values available then. This is much easier than republishing every table of astronomical distances every time there's a tiny improvement in any of the data at lower rungs of the distance ladder. Giving published work a longer shelf life is just common sense, even if it means using particularly weird and unconventional units. For example, we can measure red-shift of distant galaxies pretty accurately now, a couple of significant figures or so. However, converting red-shift to metres is HUGELY difficult. First you need a good value for the Hubble Constant; we don't have that, there are even two different values computed different ways that are incompatible with each other. Secondly, you have to either assume the Hubble Constant constant (which it isn't, because the universe is accelerating), or you have to estimate what it was at every instant in time between when the light was emitted from the observed galaxy and when we observed it. But that is very hard and estimates vary considerably when we get to very high red-shifts. So we know distances to far away things in terms of red-shift pretty accurately, but have only a very loose idea for what that distance is in metres or equivalent units.

The distance ladder has always fascinated me. Especially the standard candle of type 1a supernovae. Such a specific set of circumstances yet it is common enough to be used as a standard. Mind blowing how big our universe is.

Me: "This galaxy is a few million light years away" Other dude: "How do you know that?" Me: *inhales deeply*

It's mind boggling to realize that there are people still alive today who were born when the vast majority of astronomers thought the Universe was little bigger that our own galaxy, the Milky Way.

The accelerating expansion of the universe was being discovered just as I started school, so to me it's what I've always read about. I was so surprised to hear your experience of it, since I usually forget how recent it is.

6:27 "the calculations for these would take a few years" and here I was pouting and racking my brain studying engineering with a scientific calculator on hand. These ancient astronomers are really, really amazing!

Interesting. I always thought that it was a little strange to choose the earth sun distance as a unit, but it makes more sense now that I know that there was a time when it was useful unit but it was not known exactly how big it was.

7:52 Gaia is about to have its 3. Data release next month. It isn't as glamorous as something like Hubble is with its beautiful pictures, but the huge amounts of basic measurements is incredibly important as a foundation for a lot of astronomy.

Fun fact, us astronomers still don't actually know exactly what causes type Ia supernova! As mentioned in the video, they could originate from accreting white dwarfs exceeding the Chandrasekhar limit. Alternatively they might be caused by the mergers of double white dwarfs that slowly spiraled into one another due to gravitational radiation. Either way, they can still be used as standard candles as the underlying mechanism doesn't really matter when it comes to the luminosity of a type Ia supernova.

"Space is big. You just won't believe how vastly, hugely, mind-bogglingly big it is. I mean, you may think it's a long way down the road to the chemist's, but that's just peanuts to space." (Douglas Adams) Happy Towel Day!

Scott, given that the far galaxies are red-shifted - does that mean that we also observe them in slow-motion? I guess - yes, but I was never able to find a number of how much slower time runs in these galaxies compared to ours. And even more mind-boggling thing is that they also observe us red-shifted and therefore for them the time in our galaxy is slower...

Thank you for the detailed description. I've always been curious of how we could possibly measure such great distances

Brilliantly done. Fantastic summary of the history of our understanding of the scale of the Universe! Thank you.

Great summary of a lot of important concepts in astrophysics. No wonder your channel is where I prefer to go than television!

I was waiting for this video for years. Scott, you do not disappoint

Really great video Scott - this is like a synopsis of all the astronomy books I've read over the years.

Fantastic video!! Most of these things I had no idea about. Science is amazing and thank you for bringing it to us!

As always, very informative and very interesting. Thank you.

Many thanks for this amazing video. This brings back a lot of memories from my first semester as an astronomy student in the introduction to astronomy lectures.

Thank you for putting this thought-provoking video together. Concise, but so many dropping-off points for so many rabbit-holes! Cheers!

David Butler has a FANTASTIC video series on youtube called "How far away is it", covering these topics. Highly recommended for anyone who enjoys the science of astronomical measurement and likes to look at gorgeous photos from Hubble while listening to classical music and unintended asmr narration.

David Butler's "How far away is it" series (here on youtube) is a really well-put-together collection of videos that goes into this subject in much more detail. It's well worth a watch if this stuff interests you.

Excellent summary thanks!

Thank you, Scott Manley!!!

I love it when a teaching resource just plops into my lap. Cheers Scott!

Another brilliant video... Thank you sir!

The ladder is such an interesting concept, it seems really unusual until you remember that it's exactly the same thing that we do on the somewhat smaller, nm - m range. The recently released book, "The End of Everything" by Katie Mack has a nice section on it too! (And is also a wonderful read)

Yes! Love the astronomy focus in addition to the rocket science !

Wow that’s super fascinating thanks for the breakdown. Love it

Elite Dangerous is such a great visual demo of just how immense the distances are.

I literally was researching this exact exact topic yesterday, this is so freaky. Wow. Of course watched the video through as always. Wish it came out yesterday, would've saved me a lot of time!

Jason Kendall also explains all this in a very easy and detailed way. Thank you Mr. Manley, is always great to learn from you, scientists.

Men! This is soo good. So packed and dense. I've had to go back over and over trying to understand.

Thanks for making this video! I have been seeking to learn about exactly this topic!

Thanks for the great video, Scott! This would be a good jump-off point for a future video on Hubble Tension problem ;)

Great idea for a video, I enjoyed it!

Loved it! Thanks! Helped me remember a lot of this...

Fantastic episode!

I learnt a lot. Thank you Scott.

great history lesson...thanks!

One of those videos RU-vid didnt show me when it was new, but recommends it 3 weeks later. 🤷 Awesome video Scott!

This was excellent!

Love this exposé!

Awesome video, thank you

Those images of Venus 's transit .... I'm sad I missed it :) Excellent video as usual Mr Manley, TY.

I feel young again listening to and watching your videos. It takes me back to wonderful days of arguing in science class over the potential truth of whatever newest idea our teacher would unpack for us. I gave up arguing physical science for human years ago, but you bring it back to life in an aging brain so well. Seems like sometimes I WAS on the winning side of those old debates. Thank you.

That is a very interesting breakdown of the discoveries of very bright people on our planet over time!

Wow. Thanks Scott!

Very good. Thank you.

Brilliant outline of my A-Level physics astronomy module from '97 - although the accelerating expansion was too new to be on the syllabus, as you say 25 years ago!

Thanks for the astronomy lesson Scott.

I learned in the undergraduate years that these concepts are fundamental to astronomy: excellent summary, Scott!

The cosmological distance latter might be my favorite part of astronomy. It’s like the universe is trying to straight up tell us how big it haha

I kinda know this stuff but the Manley treatment makes it so much more juicy.

Love that "FLY SAFE " shirt!!

Fantastic job of taking an incredibly complex series of observations and calculations and making them reasonably understandable to the slightly above average Joe.

Great video!

Good episode sir. I'll watch it again. Some when over my head. Any way, I want to talk about that Sectional Chart on the desk next to you.

4:08 One key to understanding why heliocentrism took so long to be accepted is that naked-eye observations did not provide accurate enough observations. The figures obtained could be used in calculations supporting geocentrism, heliocentrism, and the even odder Tychonic system that had the Sun orbiting the Earth but the rest of the solar system orbiting the Sun. Scott touches on this at 6:26. Of courser the religious objections were a big impediment, but that issue is too well known to get into fully. The aspect of the planets being held in perfect crystalline spheres was certainly appealing philosophically and was supported by naked-eye observations that placed the planets in perfect circles. This was taken to support the idea of the perfection of God's creation. Then along came telescopes and the unpalatable observations that the planets moved in eccentric orbits.

Scott, what a terrific explanation of how we measure space distances. I loved it, even though I knew most of it in broad strokes. Your focus on details and concrete examples was wonderful. Also, highlighting those efforts by women in the sciences from the earliest days. The Manhattan project used "computers" people that did calculations and many of them were female.

My head exploded. It felt good! Thank you Scott! 🤣

And now, after 5 years I finally noticed that video is inverted horizontally "FLY SAFE"...

that was a nice journey. thanks.

Excellent! A free, astronomy tutorial. And I even understood most of it! (My spell checker thingy changed 'astronomy' to 'astrology'!!)

I really love your videos, your knowledge and all the information you give us. But one proposal: Could you insert overlays when you mention numbers? I'm not a native English speaker and listening to you and then figuring out the numbers gets sometimes very hard. All the best!

This is one of the greatest story of discovery!

A good supplemental series to this video is "How far away is it" by David Butler. It's here on RU-vid and is fantastic.

First I wanna say, this is an absolutely fascinating trip through time, following the evolution of astronomy!!! Second, I want that " "Fly Safe" shirt to ad to my collection!!!🤟

Most excellent.

Great video

This, in my opinion, is the best kind of science film - about the history and development of ideas. My childhood hero was Jacob Bronowski in the BBC series The Ascent of Man.

Keep up the good work! Can’t wait to see someone on the mün!! Don’t loose that accent ;-)

bangin' great video

My whole family says "Hello! It's Scott Manley here!" except my cat, but we're working on teaching her.

Scott giving us Astronomy 101: How we know anything. (P.S. That cheeky 'Fly Safe" sign on Eve Citadels....)

Follow up video on how the CMB allows a "top down" measurement (two actually, because you can do baryon acoustic measures from it iirc). And how the two measures don't quite agree. However they're still amazingly close, which is still mind-blowing.

Jolly good!

Yet another in the long line of education videos on a topic I ma interested in that leaves me behind about half way through and I have to just sit back and enjoy the ride. We go from easy to hard to "...okay" to '...and I'm out.'

Always fun when I notice you slip in something that came up in the forums.

Nice to see the sectional on the desk. How's the flying going?

Alan Hirshfeld's book Parallax is a great historical examination of measuring the cosmos. Recommended for space nerds.

Great summary of something that normally takes hours to present.

That animation of the stars spiraling around was super cool

This should be a Ted Talk. Well done Scott, excellent use of both litterature to prove your points, yet simple enough to understand. Brilliant! Been a fan for years. This was really top-notch!

Is that unique identifier like a MAC address in that moving the transceiver to a different aircraft moves the ID to that aircraft? Or is the ID more like a VIN in that the plane with that ID will always have that ID?

I knew most of this already, but somehow never realized that Type 1a supernovas weren't used until the 90s, I would have guessed much earlier.

Love dark energy sounding spooky at the end. I think its all tied into the true nature of gravity and spacetime itself. We still don’t know *why* it works at all. It’s funny studying civil engineering and you immediately look past the most basic assumption of 9.8 m/s^2 pointing straight down, but it’s amazing the complex structures we have designed with that simple assumption at play. But why exactly that steel beam wants to hug the core of the earth so bad is still completely unknown. We are at a weird but awesome time with science.

Just before I watch the video, that's a great hitch-hikers reference there at the start 🙂

me from canada,brilliant episode, more funding all around is key.

I wonder how far we could get parallax to be good for if we sent space telescopes out to far orbits? The balance would be between how great a separation you could get versus compromises in resolution and data rate for telescopes that you could actually send that far away with a practical launch vehicle (even taking gravity assists into account).

Cracking good video

I've heard the term Dark Gravity used too.

Yes please, a more descriptive name for "dark energy" for "it" 😄 Douglas Adams mentioned how mind-mindbogglingly big the universe is.

Beautifully explained for this slower boy from Alabama. Oh to have had your relative as my physics professor.

Next time my relatives doubt my ramblings about astronomy, I'm going to send them a link to this video. Thanks Scott ! 😀

David Butler RU-vid channel has a playlist called How Far Away Is It in which this measurements are explained in detail. You start using trigonometry in your backyard, and end up measuring the known universe. :-)

It’s always good to be reminded of how smart people were thousands of years ago. They weren’t stupid, they just lacked information.

very thorough explanation! now we just need a farther standard for the next level

Afaik Tycho Brahe did not propose a "classic" geocentric model but some kind of hybrid.