How do they know Earth’s circumference?

[@bhs128]

Love your new channel! Keep it up! One question though- you mentioned that gravity is weaker farther from the center of the earth when talking about the pendulum, but then in your gravity map, you mention that gravity is weaker when you are closer to the center.

[@Howtown]

Good question - the geoid is a bit tricky to talk about. @Antzen10 and @chrisray1567 had similar questions. Let’s imagine that the Earth was entirely made of water and not spinning at all. It WOULD be a perfect sphere because the water on the surface will “fall” until every place on the surface is equidistant from the center of mass and the surface itself is orthogonal to gravity at every point. If there was a bulge anywhere, that water would be “uphill” and spread out until there was no bulge. Ok, now we add spinning. This causes the equator to displace outward - now it’s further from the center of mass - gravity is weaker. But what if the water somehow had different densities - mass was unequally distributed throughout. (That’s what’s happening with the stuff Earth is made of!) Water that was above a patch of higher mass would experience greater gravity than expected for that point on the ellipsoid - that area of the surface would now effectively be “downhill.” Water would flow downhill - and the surface above that high density patch would rise until the gravity was the same at the surface of this bulge as all other points around it. You can see this happening with actual water in the ocean that sits over a giant mountain on the ocean floor - due to the increased gravity (the rock of submarine peak is denser than the water around it) it pulls more water to it and the surface of the ocean bulges upward. So the ellipsoid is caused by spinning, which changes gravity - but the geoid is CAUSED by differing gravity. When I say where gravity is stronger, the surface bulges out - I mean where higher densities are making gravity stronger than expected at a given point in the ellipsoid - the earth sorta bunches up. The geoid is actually the earth kind of “trying” to create a level surface relative to gravity it experiences! In every episode we debate how complex to get - and in this case, this was one rabbit hole that we decided not to go down :)

[@bhs128]

@@Howtown that helps a lot! One of the things about this channel I most appreciate is that you dig deeper into the ‘how do we know’ part of the answers that so often get glossed over on other media. Of course there is a balance to be struck, I get it, (but my two cents would be to err on slightly too much detail. 😊 ). Keep doing your thing & thanks again!

[@veronicamcghie5238]

Clicking this video and absolutely hyped to hear about those Greek dudes and their trigonometry

[@joshsatterwhite1571]

Wait, you've already had Cleo, Hank, and now Simone on your channel in the first month? Impressive connections you must have. Much more impressive than the algorithm taking this long to let me know you existed. Awesome video! I like the pacing, the info, and the presentation style. It's not everyday my ADHD-addled brain will sit through 14 minutes of things I (mostly) already know just because I enjoy the way it's shown. I look forward to whatever - and whoever - you bring to the table next!

[@VishnuPadmanabhan]

The research, the visuals and the explanation, absolutely wonderful! Brilliant!

[@sebastiantovarmarquez1902]

Imagine my surprise when I found out that you're living in my home city Bogotá, awesome experiment, awesome channel, subscribed from day 1

[@cmasjoan7333]

How are the bike lanes there? I know Bogotá had a big shift towards bike infrastructure under Peñalosa and his predecessors, and I'm curious if it's as good as the internet says.

[@sebastiantovarmarquez1902]

@@cmasjoan7333 Some areas of the city are made really well to allow bikers to move around. U can find bike lanes from the 170 to the 100 (roughly 70 blocks) both in the Séptima and Novena avenues in the north, but this is also truth in the 26 avenue that has a lane that goes almost from the airport to Bogotá's centre and many other examples. I would say is as good as the internet make it seem but you have to beware of the insecurity and weather

[@germanlondono8700]

@@cmasjoan7333 Also the last administration made an effort to have more bike lanes. But, yeah, compared to other cities and taking into account the sheer size of this city, they are pretty good. Always could be better, but they are good, I think.

[@nebulan]

Friend-shaped

[@JansthcirlU]

drawfee moment

[@tomato1656]

You explained this without making it complex ,and you were able to keep me curious throughout the video. You guys are fantastic storytellers. Thankyou. It is crazy that you two are perfectly situated to perform that experiment.

[@leahvashevko]

Love the visuals! One thing: maybe hold for one more beat after an explainer animation finishes playing before cutting away. Like at 7:38, give a second to look at the different lengths before cutting to Adam.

[@geeksdo1tbetter]

Agree. Ty!

[@oblivious_waffle]

10:29 cat cat cat cat cat cat cat cat cat

[@geekgee]

Noticed a thumbnail with Simone in my recommendations, watched, liked and subscribed. 👍👍

[@abh1kr]

You mentioned that gravity is weaker in some places and stronger in others. Why and how is this possible?

[@alexanderstubbs1127]

It varies because the planet is not perfectly spherical (some places are closer to the centre than others) or uniformly dense (lots of different types of rock but also water). In addition, gravity is weaker at the equator due to centrifugal forces produced by the planet’s rotation. It’s also weaker at higher altitudes, further from Earth’s centre, such as at the summit of Mount Everest. Mount Nevado Huascarán in Peru has the lowest gravitational acceleration, at 9.7639 m/s2, while the highest is at the surface of the Arctic Ocean, at 9.8337 m/s2.

[@Howtown]

What Alexander said!

[@chrisray1567]

It’s a bit paradoxical to me because it seems like being further from the center of the earth would means there’s more mass under you, and more mass would mean a stronger gravitational force, not less. I think the resolution to this paradox is that the strength of gravity is only linearly proportional to mass (doubling the mass doubles the gravitational force) while it is inversely proportional by the square of the distance (the gravitational force is four times weaker if you double the distance). In other words, while there is more mass under you, the increased distance has a greater effect (weakening it) on the total gravitational force you experience.

[@WaitWhat99]

Stumbled across this channel from shorts. Immediately subscribed 😁 Loving it, cant wait for more!

[@needamuffin]

I clicked because of Simone, disappointed that she's in the video for all of 9 seconds.

[@Lyndiloo]

9 seconds of Simone garners trust more than a verified checkmark. 😂

[@geeksdo1tbetter]

Nods. I think adding a follow-up where the guest gets to remark on the explanation is better, like with Hank.

[@revealist8477]

These animations, sound, writing...dang. Great explanations created with words and images

[@powerverwirrt]

I loved this video but I noticed that the official captions are off a lot of the time, so the automatically transcribed ones ended up being much more accurate. Maybe something to look into?

[@kendyxx]

This channel is mindblowing!

[@vinny142]

"Why is it so close to exactly 40k" The answer in this case is "because that's what we based the meter on", but otherwise it does raise the very interesting question of "why _do_ people care about these coincidences?" Why is it that we think we rolling two sixes with dice is much harder than to roll a 3 and a 4?

[@ChristopherYeeMon]

You and the other Vox alumni are keeping the Vox video heritage alive

[@kw43562]

The visual effects in your videos are incredible!!

[@Howtown]

Trivia Q: what place on Earth’s surface is furthest from the center of the planet?

[@That_AuCh_Geek]

The Himalayas?

[@jeremiahrosen1242]

Must be a mountain along the equator due to the equatorial bulge?

[@briantorres6614]

Chimborazo! (A volcano in Ecuador.) Sí o no Adam?

[@Howtown]

@@briantorres6614🗻

[@sebas11tian]

How close would t 14:35 he sun have to be, for the calculations o of the experiment to match?

[@amirclasic1582]

I noticed that you didn't mention Al-Biruni, who was a pioneer in this field. Around the 11th century, he developed a method that was incredibly accurate for its time. By measuring the height of a mountain and using trigonometry to observe the horizon, he calculated the Earth's circumference to within a small margin of error compared to today's measurements. His work is a great example of the advanced scientific thinking that was happening in the Islamic world long before many European scholars tackled similar problems."

[@michaelmonsa9702]

Boy, I hope this channel booms cause your content is fire 🔥

[@nebulan]

My brain hearing hbomberguy say "oblate"

[@pretendtobenormal8064]

I learned about Eratosthenes' work on measuring the Earth's circumference from Carl Sagan in the original Cosmos.

[@h3ct0r13]

The analogies you guys make are just awesome!!! So gald I found this channel from the beginning. Sub since 1st video!!❤️

[@katjarozantseva8069]

Quick note before watching, new thumbnail “how do they know the Earth circumference” is way better than old one. I do know that Earth is not a sphere, that doesn’t make to have a thought about clicking

[@mgevirtz]

Placing Eratosthenes in time was a good touch.

[@jeouxgentry7733]

Super-impressed with the artful effects. The overlapping text to illustrate a face particularly caught my eye.

[@tpain525]

I have notifications one for this channel, and with this graphic 1:00 did not disappoint

[@TS-cs2pe]

0:45 flat earthers got excited for a while

[@Rulerofwax24]

The interesting thing about the estimated geoid equation is that it's specifically very accurate around the United States because they were the ones to come up with the formula. There are other equations that are slightly more accurate in other parts of the world, but because of the US's dominance with GPS mapping, the US-centric equation is what stuck.

[@isacami25]

the first homework we gave to my students back in 2020, was to measure the circumference of the earth. we were able to ask that of them because Chile is pretty much a straight line and because of the pandemic we had students from all over the country.

[@matthewb3113]

The title is going to excite flat-earthers. “If only they had a brain”

[@jakeplease31]

This channel is so, so fantastic. Keep up the great work!

[@timothypatrick9476]

Thanks! Loved this episode!

[@PeggyKTC]

This was more interesting than I expected! I had no idea that the Earth was so lumpy.

[@HowToNguyen]

New Howtown video?? It’s for sure gonna be a good day now

[@claudia-ayuso]

This was so much fun to watch and so interesting and my brain is kinda melted

[@lucianolizana446]

Love this!

[@MattWinchell]

I have absolutely been loving the content you all have been releasing. Keep it up!

[@Howtown]

Thanks Matt!

[@WaiManSun]

Here's something to fact check. According to the British TV show "QI" and a few other sources, there's no evidence that humans in history thought the earth was flat. And that the flat earth theory is a recent thing.

[@Howtown]

Hmmm ... I know that Homer (from Greece, not Springfield) describes a shield that shows the whole world - and it definitely sounds like he's talking about a flat disc ... in other places he describes the sky as a dome. It does make sense to me that the default position -- before you start to really pay attention -- would be that the earth was flat (as one of the commenters on this video said it's "obviously flat"!). But I do know that the modern Flat Earth movement really got going in the 19th century.

[@sudosu078]

Can you please do a video on how stampedes work? And how to save yourself if you are in one? Stampedes are scary and unpredictable and I believe its good to understand the well enough!!

[@nikiwalker25]

BRB training to become a bematist - Y'ALL this is SO good!

[@Zavorio]

Hi, I just wanted to say I truly LOVE your first three videos and I just singed up to contribute on your Patreon. I think the incentives for creators have made it so that a lot explainer videos lately are extremely shallow, prioritize cool editing and storytelling over substance and a significant chunk of their run time are sponsors. I know it is hard to make it out there independentely so I can't blame people for relying on sponsors. But the substance is just not there either, there are no insights or interesting connections being made. These first three videos had excellent pacing, they were packed with insights and they are really making me think about stuff. What you present is broken down into manageable bits but you are not treating the audience like dummies. It is just great work really and we need more of it.

[@Howtown]

Thanks so much - I'm glad that's coming through because we're trying our best!

[@Xlash95]

Great one! Keep the videos coming.

[@KellenProctor]

How do they know which plants are native and which ones are invasive?

[@palladiumbc7576]

Great Video!

[@geeksdo1tbetter]

Hooray! Simone is the best!!

[@jd-zr3vk]

They used a really long measuring tape.

[@muza-productions]

I knew parts of the story, but I was not expecting to learn so many fascinating new details in this video

[@aaronnoble6812]

Does the argument of earth’s shape includes the water of our oceans?

[@zumabbar]

i really like how to scientist's punk rock band background relates to the invention of the meter. I thought you were just being generous in introducing him, and after that I kept wonderinf when is he going to show up again giving his opinion or lecture on the topic, why did you show him so early lol.

[@Rkcuddles]

More howtown!

[@DigitalicaEG]

JOSS IS LIFEEEEE

[@MaxLenormand]

I see a video about maps & satellite images, I click

[@mjwu786]

I want a video like this to explain every question I have lol

[@ToastyOs]

Hi. Love the channel have been subbed since the first video. I want to see the channel succeed, and it is with that mindset I tell you I think it was a mistake making the original title/thumbnail about the earth not being a sphere. I understand perhaps you thought people would be intrigued by the idea, but you lost the pull of the original question the video seeks to answer: how do we know the circumference of the earth? It is questions like these that inspire the very existence of this channel - things we take for granted but don't actually know. The pull is not some other related question that we never asked ourselves before. Trust in the appeal of the simple, universal questions that everyone can understand. Don't lose sight of what makes this channel what it is. But hey, that's just my opinion 🤷🏻‍♂️❤

[@ihssene_]

Amazing

[@abhishekbalakrishnan8755]

Really well made! Love it

[@ScannerOfficial]

This channel is impressing me

[@tmuller9905]

Hey, I think there's a typo in the subtitles at 2:42 it says "inside" and I'm guessing that it's supposed to be "in Syene"? Sorry if I'm mistaken but it seems like a typo. Love the video though! Edit: oh and at 4:17 it says "our city's" instead of "Eratosthenes"

[@trollimusprime8521]

Fantastic content!

[@robrook8617]

Best channel on this site.

[@Antzen10]

At 10:30, I think you guys got the spectrum flipped; gravity is weaker at places where the surface bulges out and stronger at places where the surface sags. Awesome video and visualizations though, and I really enjoyed going through the different historical techniques!

[@AnyNotEverything]

I don't remember where I heard this but I thought the extra few kilometers was caused by a miscalculation

[@Howtown]

Yep - it was a combination of some small errors (that forever tortured some of the meter’s creators - read “Measure of All Things” by Alder) and differences between the reference ellipse we now use and the ideal ellipse assumed in the 18th century

[@gimmerain4days]

I wish you would have mentioned how Erastothanes synchronized the time of his observations, nontrivial for him. I assume he chose summer solstice solar max because solar max is easy to measure and is at the same time along longitude lines. Also would love to hear about error analysis, sounds boring but so crucial! How much variation can be detected in each measurement? How many decimal places are you accurate to?

[@LoganYouTube]

Did most of your videos released thus far get recorded on the same day or did you guys settle on the same outfits for each episode as a consistency sort of thing?

[@Howtown]

I just don’t have that many different clothes -Adam

[@ellisemoon9019]

Is the 40,007.863 number an average based off that standardized ellipsoid? Or is it a measurement at a particular path, such as the prime meridian? Wouldn't you get slightly different numbers for polar circumference if you measure at different meridians?

[@Howtown]

40,007.863 km is the polar circumference of the reference ellipsoid, which was chosen to minimize the distance to the geoid around the entire globe (in 1984 - the geoid does change). That ellipsoid has the same polar circumference at every meridian. The geoid would have a slightly different circumference depending on which meridian you chose.

[@elizabethfreed472]

So cool! I love that the math makes sense the way you explain it, thank you!

[@jeevesh1294]

curious how the experiment would work, if the earth was flat. nice cliffhanger 🤓

[@Howtown]

The upshot is you could have shadows of the lengths we observed if the Earth was flat and the Sun was ~4,000 km (~2,500 mi) above the surface (closer to Adam than to Joss). On patreon.com/howtown we chat about a few ways you could modify the experiment to determine if this is what’s actually going on

[@rickkwitkoski1976]

But... if the sun were close and small, those same angles could also work on a FLAT earth! A third point would clinch the earth as a globe because all three measurements would not simultaneously conform to a flat earth model. On a globe, they do.

[@AgroRanco]

Brilliant 🎉

[@TheBoringAddress]

What I've always wanted to know: How did the ancient Greeks managed to figure out all of that trigonometry and geometry? What sort of instruments and thinking processes did they use?

[@hapis9870]

actually simplified of ny 3 yrs bachelor degrees of geodesy 😁 am happy to recall

[@bradleyisgladley]

I’m assuming they knew they had a north south relationship because of the stars. Is that correct?

[@c97f]

The whole "earth isn't a sphere" thing is a bit misleading. Nothing is a perfect sphere. The earth is pretty darn close to a sphere. Stating otherwise is a bit pedantic.

[@tallpauldoll]

No credits for the Jean Riché voice actor?!

[@gibberishname]

wait, Simone Giertz and Cadamole? what channel IS THIS?

[@AhammedShaheedhudheen]

New Favorite channel

[@gimmerain4days]

Love what you guys are doing. Maybe a dumb question, but does it matter that the angle of the shadows don't intersect at the center of the earth? The geometric construction srrmed rushed to me, or maybe im just slow.

[@Howtown]

Not dumb - we flew through that pretty fast. On a perfect sphere, if you extended a line straight down from the obelisks (or Pringles cans), those lines would meet at the center of the Earth. That’s the assumption we made, and Eratosthenes made. But since the Earth isn’t a perfect sphere, those lines wouldn’t quite meet at the center - introducing error into the calculation. Does that answer your Q?

[@gimmerain4days]

@@Howtown Thank so much! That clarifies it for me. It could be an optical illusion from the crop/framing, but the intersection of the lines don't appear to me to be the center of the arc shown (at approx. 2:13 and 2:25). Maybe a dot at the center or a wider crop would help?

[@neoroxx]

Eratosthenes is smarter than all the flat earthers out there 😂

[@Jdbfyskxuek]

At 7:28 you said since the equator is further from the center of the mass than the pole, it has weaker gravity. However, at 10:20 you also said when the earth bulges out, the gravity is stronger. Isn't this contradictory? @Howtown

[@marnoster]

I hope you talked to Johnny H before you did a map related video

[@Joe-nb3fs]

If you want to learn way too much about the measurement of the kilometer, I suggest reading "The Measure of All Things: The Seven-Year Odyssey and Hidden Error That Transformed the World". If you want to keep your sanity, I suggest you don't 🤣It's ... lengthy.

[@thurian7144]

At 10:24, i dont seem to understand this map they show. To my limited logic it doesn't make sense how a region that is lower down (and therefore closer to the center of the earth) experiences less gravity than a place on the earth that "bulges out" and that is therefore farther away from the center.

[@qr6QRbMBG6hjGpZhnWqG]

I was also confused because the opposite would be true if you were above a black hole (gravity is inversely proportional to the square of distance). Farther away means less gravity. But in this case, the amount of mass is also changing. If you are lower down, there is less "stuff" beneath you (less mass), so you experience less gravity. If you are at a bulge, there is more earth and more gravity.

[@thurian7144]

It seems to me that the distance would still have a larger effect overall seeing as it is squared. But is it that, because the volume (meters cubed) of earth changes beneath your feet whether you're lower down or higher up, that in fact the amount of earth pulling in meters cubed on you has a larger effect than the distance in meters squared between you and the center?

[@qr6QRbMBG6hjGpZhnWqG]

@@thurian7144 That's right, the volume (r^3) has a greater effect than distance (r^2).

[@thurian7144]

@@qr6QRbMBG6hjGpZhnWqG Noted, Thank you very much for your help

[@TMWriting]

the ADR is strong in this one

[@cyborgzulu2011]

What about water? The geoid doesnt include water right?

[@Skunktruck]

for the algo

[@naninano8813]

why is Simon here? answer! answer!

[@rishi.filmss]

Woaah Woah❤‍🔥🔥

[@bbmcrae]

*Whoa Whoa

[@jonahplayscello]

what

[@xaviersgiantego1453]

Algorithm

[@makhalid1999]

9:37 PAKISTAN MENTIONED 🇵🇰🇵🇰🇵🇰🇵🇰🇵🇰🇵🇰🔥🔥🔥🔥

[@Ishanjaber]

I'll take as many breaths as many likes I get in this comment

[@user-lb4gd7vh4s]

1st

[@cloidless]

Nice try. The Earth is obviously flat. Good video and cool theory though

[@cyborgzulu2011]

Our universe seems to be flat though. Nice try. The earth itself is a sphere.

[@cyborgzulu2011]

Your a lizard. Humans aren't real. You also plug into a lizard matrix. Good try though

[@robin_holden]

Care to provide us with an accurate map of your flat earth?

[@Timothy_VV]

The only thing that's flat is your brain. So much evidence and you have so little (none). Stop sticking your head into the sand. 😂 And show some respect for those who prove you very wrong. Be a man, admit you're wrong.

[@TeagueChrystie]

This is fine except it feels phony.

[@foulwin9719]

Reality doesn't care about your feelings

[@pleggli]

nice try globe heads

[@leanderthal2689]

Damn, I hate the music in the background of these videos 😤

[@cangen]

First