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.

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