Why Some Days Aren’t 24 Hours

Why Some Days Aren’t 24 Hours


In the distant future, aliens who live on
asteroids near the center of the galaxy get in touch and want to come visit you. And so you tell them, “Of course! I’m free any day this week.” But they don’t know what that means – they
live in an asteroid belt and have a totally different kind of calendar and, to them, the
concept of a “day” is very… alien. So you tell them that a day is how long it
takes for the Earth to complete a full rotation about its axis. And as they input that into their computer
simulation, you notice a fatal flaw in your explanation: as the earth rotates relative
to the distant aliens, it moves a little bit around the sun, and by the time it makes its
way to the other side of the sun, our “daytime” and “nighttime” have somehow switched,
with the sun directly overhead when one day changes to the next, rather than in the middle
of the day! This is not what we mean when we talk about
calendar days. What you’ve actually described to the aliens
is called a Stellar day and it’s measured with respect to a distant, more or less stationary
reference point far off in space – but our concept of a day has more to do with the sun,
not the galactic center. So you try again. This time you tell them that when Earthlings
look up at the sky, for each turn of the earth there’s a time when the sun is highest. And you say that a day is the time it takes
for the Sun to get back to the highest point. And so the Asteroid-ians tap away on their
instruments, calibrating them to your insightful specifications until you notice that their
day counter isn’t staying in sync with your clock! It’s starting the new day earlier, and earlier
and earlier each… day. And then later, and later, and later. This isn’t a bug in their programming – it’s
a feature… of the Earth’s orbit. What you actually described to them is called
a Solar day, and it’s not the same thing as a day kept by a clock. Solar days use the sun as a reference point
for when “noon” is, but the length of time between when the sun is highest isn’t
constant – it changes up or down by a minute over the course of the year. This discrepancy is due to the complications
of the earth’s orbit being elliptical and the earth’s spin axis being tilted. If we used solar days in everyday life, we’d
either need to have calendars and clocks that changed the number of minutes and seconds
in a day depending on the time of year, or we’d need to have clocks that changed the
length of a second (or changed the number of seconds in an hour) depending on the time
of year. And sundials kind of automatically do this! But they have other… drawbacks. Anyway, changing the length of a second or
the number of seconds in an hour isn’t particularly appealing for regular – or interplanetary
– use. And so you tell the asteroidaliens that a
day is – more or less – an invented time period that is 24 hours long, where each hour is
33 trillion oscillations of a special kind photon emitted by a cesium atom. And if they want to know why a day is just
defined to be a fixed time period and how that time period actually relates to the rotation
of the earth, you can send them over to our interactive video over on MinuteLabs. It will guide anyone and everyone through
the details of solar, stellar, and standard 24 hour days; how they’re related; and how
the orbit of the earth affects them. Not only that, but it also lets you play around
with different orbits to see how it changes the length of those days! The link is in the video description, or you
can just go to minutelabs.io and look for the “What is a Day?” lab, and you’ll be fully prepared to coordinate
a visit with aliens… no matter what day that may be.

100 thoughts on “Why Some Days Aren’t 24 Hours

  1. I'll click on your link tomorrow at this time… though I don't know when that exact time is?!

  2. DISCLAIMER: Product not intended for use indoors or near tall buildings or trees. Does not work at night. Do not attempt to move after initial installation – relocation will void warranty. Not guaranteed to work on cloudy days, or in dense fog, or wildfire smoke. Requires vertical mounting if installed in the wrong hemishpere.

  3. TBH if aliens wanted to visit and not destroy us I would tell them to come when they can lol I think I could get up early morning to welcome them

  4. Oh man, if only brief flashes of text funnies were not a thing. A swear to god. Haha we got it. Now let us read it if you want us to see how funny you are

  5. pssst if you're going to include a fast-text, pause-so-you-can-read-it gag in another video, it'd be fantastic if it wasn't in exactly the part of the screen that gets cut off by the toolbar-that-never-leaves-when-you-pause-the-video

  6. The false statement is that the planet actually goes around the sun. It would be accurate to use perihelion and aphelion. When I first heard that the high scFOOL tale that the earth revolves around the sun set off major warning bells I was being dumbed down by the public education system to be draft animal in it.

  7. Wow! Such a great explanation of what a day is! I have pondered this many times and never bothered to go looking for the answer, so thanks for putting this together. Your interactive lab really helped me to understand things clearly.

  8. Thank you for being one of the few channels than don’t upload 10min videos for ad revenue and straight to the point

  9. Aliens: are you trolling us?
    Hoomen: no, but you have to understand…

    Universe: let there be an intergalactic war started over an over complicated concept of “day”.

  10. huh….When I saw the title, I thought you were going to talk about how some days are 86401 seconds long, and then the thought of 23 hour days and 25 hour days.

  11. Who else didn't realize the software he was using was an actual thing until he said it at the dead ass end of the video because you were just grooving along to a minutephysics video?

  12. It'll be so much fun when we colonize space and other planets and the entire concept of a 24 hour day will seem nonsensical!

  13. It's the good ol equation of time. The Earth's elliptical orbit and its tilt cause this. You should go over analemmas too.

  14. Get a retired person to say it doesn't matter, what do aliens need to know the our time for anyway.
    No extraterrestrial of going to ring up and make an appointment.

  15. What about "The time it takes the Earth to make a full revolution around the sun, divided by 365"? +some additional maths for the gap years.

  16. Not even gonna watch this video cause time keeps going 😂 and there’s 24 hours in each day Ad about to end✌️

  17. My name doesn't show up very often in ads but I just got one omg

    And it's an off-handed remark about how it's unbelievable that I've "taken up parkour"

    I don't even do parkour and I feel attacked

  18. To be honest, I would like a longer video where you explain a more in-depth. It is kind of hard to wrap my head around so many new concepts in a minute.

  19. why would you put those (*) comments for a split second in the bottom part of the video where it gets covered by Youtube video controls when the video is paused?

  20. Changing the length of a second is total feasible now given computing power… not that I'm advocating for that, 'cause it would suck….

  21. for people who didn't pause at the sundial

    DISCLAIMER: Product not intended for use indoors or near tall buildings or trees. Does not work at night. Do not attempt to move after initial installation – relocation will void warranty. Not guranteed to work on cloudy days or in dense fog or wildfire smoke. Requires vertical mounting of installed in the wrong hemisphere.

  22. You gave us a website that gives us the power to change the earth’s orbit, and you think we’ll use it to not destroy the Earth?

  23. Hey minute earth. On earth the sun's Analemma is a figure 8 oscillating mostly up and down. On venus the Analemma is wildly horizontal. What would this do to the climate and to the days or seasons? My guess is that it would make days in some of the year very long and in other times of the year very short.

  24. What I have trouble understanding is how there is 24 hours of sunlight in some areas of the earth at certain times of the year. How does that make sense?

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