Neil deGrasse Tyson Explains the REAL Length of Day

[Music] i'm back in your face chuck well virtually yes [Laughter] just because you move closer to the camera [Laughter] it's not as intimidating as it is in real life and you're you're a pretty big guy so in real life that'd be intimidating you know but virtually it's just like you're you're like making a funny face to me just that's a you know when i grew up in the street if people want to get in a fight but if we're going to get into a fight but you didn't want to fight the person you pretend you were angry and then you were badass but you'd say hold me back right exactly and you run behind people to have them hold your back nobody's touching you dude what are you talking about i said hold me back man that's my i'm telling you yeah yeah i need you to hold me back right so he don't hit me right so what am i talking about i'm gonna talk about earth okay okay i like that place okay so good good so how long does earth take to rotate on its axis from what i've been told 24 hours 24 hours okay in fact we kind of defined 24 hours to mean that right so that's how you we divide up time okay so as you might have guessed it doesn't take 24 hours um it actually takes 23 hours 56 minutes and four seconds okay really yes let me just be clear about what i mean 23 hours get rid of the entire just get rid of the entire solar system okay and watch earth rotate okay and stand there and time it it will that spot that's in front of you will come back around in 23 hours 56 minutes and four seconds one full revolution okay okay that's called a sidereal day a second i mean stars starting however we don't base our lives on when stars return to the same spot in the sky in the night sky we'd base our days on when the sun which also happens to be a star returns to its spot on the sky okay it turns out that takes longer than 23 hours and 50. why i tell you why because in the time it took earth to rotate 23 hours and 56 minutes it actually moved almost one degree in its orbit around the sun so it rotates back to where its previous line but it has to turn a little bit extra a little bit definitely to get back to the same thing a little bit extra to put the sun back in the same spot on the sky right and that's a little bit yeah one minutes extra that's fantastic and why didn't i say god that's so so that little extra four minutes so i said we move a degree in our orbit each day you know i didn't just pull that out how many how many days are there in a year uh 365 okay and how many degrees in a circle 360. yeah so it's about a degree it's about a day a day yeah we to 10 is one degree and five 360 sixty-fifths oh wow have a degree but it's basically a degree a day and in that degree you have to turn that little extra so just imagine you keep having to turn your head just a little bit extra amount okay because you're gonna move just a little bit so now you gotta look back just a little bit correct cool well let's keep going okay all right earth's orbit around the sun is not a perfect circle oh boy by the way so the first day is a sidereal day the second day is a solar day that's okay so for obvious reasons and they don't they're not the same all right now earth's orbit around the sun is not a perfect circle right some which means sometimes we're farther away sometimes we're closer when we are closer we are moving faster in our orbit than when we are farther away okay okay that's how gravity works so it turns out that extra four minutes is not the same if we're farther from the sun than if we're closer to the sun because we moved more than our allocated fraction of a circle when we're close to the sun and less when we're farther away from the sun that's it okay so the length of the solar day is changing continually throughout the year wow and sometimes it's less than 24 hours sometimes it's more than 24 hours so sometimes the sun gets to its highest point in the sky before clock noon right and sometimes it gets so what we do is we just average that over the whole year and say son you are average 24 hours and there you go wow by the way if you ever look at sundials there's a map on this sundial that corrects for the sun being early and the sun being late you either add or subtract up to 14 minutes of the day from what the sun time reads in order to get your clock time wow all right every sundial has it so it's a figure eight it's called an analemma it's got a name and a lemon but i don't want to talk about analytics right now so what you have on your clock is the average length of a day over the 365 days of the year all right so now we define that as 24 hours each hour 60 minutes each minute 60 seconds got it okay well what is defining this well it's the rotation of the earth right okay makes sense well well all right well how stable is that even in the perfect average that we're taking uh it's house well you'll never know will you because you're using earth to define the the time frame right exactly if earth is your measure and you are slowing down you will never know this just there's no way to know it right okay now back when i was growing up watches they would boast this is how old i am they would say get this watch it's so accurate it's accurate to two minutes a month okay what that that was like accurate that is not a good advertising okay but then it was because you know cheap watches were 10 minutes a month you'd have to re you'd set your watch every day why do you think heist movies would always say let's synchronize our clocks right before they played on the heist that's cool they knew that the clocks they were not keeping good time earth was keeping the best time of them all okay so now you pose the question maybe earth is not the best time heating device let's offload the responsibility of keeping time to something else like a vibrating atom so we did that so the ccm 137 there's a electron transition between two energy levels that has a very precise frequency okay very precise you can measure what's good about that is any lab can get some ccm and measure this and then define the length of a second in their lab okay okay when you do this you multiply it by 60 you multiply that by 60 you get the hour you multiply that by 24 you do you do this and you find out that earth is slowing down [Music] well you know it's kind of old i'm just insane it's been added it's tired that's a lot of tired really if you're four point six billion years old you might slow down a little bit you might be tired i'm just saying you might lose a step on your own that's very sweet of you that's very it's very geriatrically sensitive of you so you only would know that earth was slowing down once you offload it to something else that tracks time right and what we found out is that the sloshing of tides on the ocean floor on the beach fronts actually works to slow down the rotation of the earth oh wow okay because the moon is causing these tides right okay and we're rotating faster than the moon is orbiting us so the moon is tugging on tides backwards on our attempt to rotate right okay so it's almost like there's a a counter weight the counterbalance exactly that's pulling against what against us so tides are slowing down the rotation of the earth and in response the moon is spiraling away from us by a couple inches a year okay that's in response to earth slowing down it all relates to what's called the conservation of angular momentum but it's a big ballet okay and so we've been slowing down and chuck that's so we what we could do is say let's redefine the second the length of the second so that we always have 24 hours and 60 minutes and 60 seconds but that's messy every year here's a new definition of the second pope folks go to your lab redefine the cesium atom it's much easier to accept the fact that you know we're breaking up with the moon but you know it's just taking some time breaking up is hard to do everybody everybody has to be on the same page so the breakup can be as amicable uh so that's uh that that's that's a nice way to think about it so so this so we the way we compensate for this is how we add a leap second when when do we oh well it has to be every four years then we had a no no no no no no no did i mention years at all dude days every four days no no no no what i'm saying is we monitor when we have fell behind a second okay so when the second when we know that we've lost a second based on our atomic measurement correct then internationally we say time to throw in a leap second now by convention we throw it in on june 30th or on december 31st and we can throw in one at each time if we needed it okay since 1973 or four or something like early 70s there's been 23 24 leap seconds added to the calendar sweet so when they're gonna add a leap second they choose which of those days it will be when they do so that final minute has 61 seconds in it nice yes it's very cool that's very cool and so so this is what we do that's the largest minute of yes it's the longest minute because it's actually 61 seconds okay so newspapers like having fun with they say this year will be slightly longer than other years right because you have to throw in the leap second that leap second happens on greenwich time so for us it would take place at like uh i guess 7 p.m right because we're five hours uh behind them when the christmas mean so i was at a dinner party once in the june 30th leap second at 7 pm that's when you're having dinner and i said pause i got out the atomic clock and we just watched that sucker tick 61 seconds went by we toasted it with a sip of champagne and then went on our way okay why are you walking around with an atomic clock no no i have wait just don't have access don't just let's not just i got people okay right on it all right so a couple more things uh other things that can change the rotation of the earth not just the title swatching right okay but here's one for you you ready good right you've seen skaters who uh want to speed up their speed so what do they do their hands start out extended and they start with a slight rotation they bring in their hands and they spin faster right right what they've done is they changed where the mass is relative to their rotation axis right so the farther away the mass is the slower they're going to rotate the closer it is to the rotation the faster they'll turn right okay if earth has an earthquake and one of the continental slabs shifts north okay then there's mass on earth's surface that used to be closer to the equator the day before and has now moved closer to the pole that will have the effect of speeding up the rotation of the earth right large animals that migrate okay will change where the mass of the earth is from the northern climes down to the south and they're farther away from the rotation of the axis so not only does do tides affect the rotation of the earth my seasonal migration of animals affects the rotation of the earth and so does earthquakes so do volcanoes and anything else that's remapping the surface of the earth you know what else to change it the melting of land glaciers uh-oh we're in trouble okay that you got uh antarctica has has land glaciers as it melts what used to be right there near the pole now melts and goes closer to the equator that will slow down the rotation of the earth because that's like the skater bringing their hands away right so all of these factors combined basically the earth's day has been increasing rather than d and principally could decrease depending on what's going on right did i i then i had this diabolical idea we'd go set up jet engines um anchor them to the earth face them either due east or due west and ignite them and then the exhaust would help uh speed up or slow down the earth i thought we might do that then i did the math on that and no it's not going to work it's not enough you would have made a good bond villain though yeah it's just you know it's like lex luthor saying let me um create an earthquake to um create trees from property in california so yes geo engineering on on a huge scale um but in principle you could do that but if you take the greatest engines and fire them the mass of the earth is so huge you just haven't essentially no effect no effect at all yeah so i'm just saying the rotation rate of the earth is a is is susceptible to all these small effects that can add up and we deal with it we you know deal with it we throw in leap seconds 23 hours 56 minutes and four seconds that's our rotation rate to the start they start it's exactly 24 hours on average and that average is changing right oh man that is that's so cool man yeah just to show you that that smart people think about this stuff yeah and solve it so you don't have to because everyone you just think everything oh just it works oh the computer tells me what the right time is and everything and the bank knows what time it is and my gps we've got people thinking about this stuff so before the atomic clock nobody knew what time it was that's why they synchronized clocks at the beginning of heist movies everyone had different time dude oh my god that's crazy oh man i wish i lived back there and i'd never be late for anything it'd be like chuck you're 10 minutes late not according to my watch and as far as i can tell don't nobody know what time it is i'm not 10 minutes late you 10 minutes early that's what the problem is so that's all i got to say and it's a fascinating thing that is fascinating and i love thinking about it just because of how many different branches of of the science and technological world had to come together to figure this out sweet well i gotta go i'm i'm i'm preparing for the next leap second so all right dude by the way if we have a leap second announced we'll do a special show and maybe celebrate that and i'll bring out my atomic clock and we can watch it that'd be so cool i'm all about it okay yes and make sure don't forget the champagne because you said you had champagne the last time so don't even try to skip out on the stream forget that that's right okay i'm just saying all right this is star talk the explainer video check good day oh it's a pleasure neil degrasse tyson here you

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Keywords: startalk, star talk, startalk radio, neil degrasse tyson, neil tyson, science, space, astrophysics, astronomy, podcast, space podcast, science podcast, astronomy podcast, niel degrasse tyson, physics, Chuck Nice, length of day, sidereal day, solar day, 24 hours, length of day explained, Neil deGrasse Tyson explains, atomic clocks, sun dials, Earth, Moon, tides, Earth’s rotation, Earth’s rotation explained

Id: w7lad5a4XhI

Length: 16min 56sec (1016 seconds)

Published: Tue Jan 12 2021