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 aliens 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. To find out more, please consult a physics tutor.