How Long Is A
Day and Stuff? How long is a day? That
depends. What's a day? Most people think that a day is the
length of time it takes for the Earth to make exactly one rotation (360
degrees) about its own axis. That's as good a definition as any,
really. In fact, that definition is called a sidereal day,
with sidereal
meaning relative to the stars of the
cosmos. But if you use that definition, then a day is only about
23 hours and 56 minutes long.
(Plus almost 4 seconds, if you want to get really picky.) So why
isn't it 24 hours, like you'd expect?
Well, the reason is that the Earth is also revolving around the Sun. Yeah, I know you knew that, but it isn't commonly known that in a year, the Earth makes, not 365 full rotations about its own axis (forgetting fractions for the moment), but 366 rotations. You see, with each day, the sunlit side of the earth moves around about a degree (actually 1/365th of a circle, but a degree is close enough for this discussion) every day, and the Earth must spin on its own axis that extra degree each day in order to catch up with the creeping advancement of the sunlit side. So in a year, the Earth has added a full rotation relative to the number of actual "days" it has experienced.
So a day is not 360 degrees, but more like 361 degrees of rotation. In other words, a day is generally regarded as the time between, say, two sunsets, regardless of Earth's interaction with the rest of the cosmos. (This is called a solar day.) And it's a good thing, too. Otherwise, our clocks wouldn't keep up with the advancement of the sunlit portion of the Earth, and in six months, noon would become midnight.
But even then, you can't simply say that a day is the length of time it takes for the sun to traverse a relative (meaning, from our perspective) 360 degree arc across the sky, because this time interval changes throughout the year. In the summer (for the Northern hemisphere), the Earth is farther away from the Sun, so it orbits more slowly, thus making the sunlit side of the Earth advance more slowly from day to day. In the winter (again, for the Northern hemisphere), the Earth is closer to the Sun, thus making the sunlit side of the Earth advance more quickly. (Please note that this is a phenomenon resulting from Earth's slightly elliptical orbit, not from the seasons or tilt of Earth's axis.) So the solar day is actually longer in winter than it is in summer since the Earth's rotation requires more time to catch up with the more rapidly-advancing sunlit side during winter. So what do you do? Well, you generally just pick something in the middle that you know won't throw off your clocks after a long time frame.
So 24 hours it is! Except that even that isn't sufficient. Even as we speak, the Moon is robbing angular momentum from the Earth, slowing it down, making each day ever so slightly longer than the one that preceded it. Of course you know about leap years, wherein a day is occasionally added to a year to account for the fact that a year doesn't contain an integral number of days. But did you know that we also have leap seconds? It's true. Occasionally, at the end of the year, we have to add an extra second to the year to account for the ever-lengthening day. We don't have to do it often, but eventually, we'll have to add several seconds each year, though that requirement is still a long, long way off.
Ah, but defining a year is much simpler, because a year is simply the time it takes for the Earth to make a full circle around the sun, right? Well, actually, no. That (the sidereal year) is not how we define a year in our current calendar. The reason is because the Earth wobbles as it spins, and the seasons, which are a consequence of the Earth's tilt, follow the wobbling of the tilted axis. In other words, if we base our calendar on a sidereal year and ignore the wobble, then in about 12,500 years (or so), we'd have summer in December and snow in July (again, Northern hemisphere). So to avoid the utter madness that would ensue as a result of seasonal creep in our calendar (and the inevitable downfall of civilization), we have constructed our calendar based on the tropical year definition, which maintains the solstices and equinoxes at reasonably fixed points from one year to the next. However, although adopting the tropical year instead of the sidereal year avoids seasonal creep in the calendar, it does lead to constellation creep instead. In other words, instead of the summer and winter eventually switching places in our calendar, the December constellations will eventually become our June constellations, and vice versa. How constellation creep is deemed to be preferable to seasonal creep, I'm not sure. But I hope it doesn't lead to utter madness and the downfall of civilization.
And if that weren't complicated enough, we're also moving around the Milky Way's center, bobbing up and down through the galaxy's plane like a horse on a merry-go-round! If our life spans were on the order of millions of years, we could well have calendars that take even our complex motion through the galaxy into consideration.
So the next time you look up at the sky, watch for a moment as the sky slips by the Earth, and ponder the many directions it is heading at once. If you feel a tingling sensation somewhere along your spine, don't worry. That's just a normal response to the realization that you are, quite literally, on the ride of your life.
Well, the reason is that the Earth is also revolving around the Sun. Yeah, I know you knew that, but it isn't commonly known that in a year, the Earth makes, not 365 full rotations about its own axis (forgetting fractions for the moment), but 366 rotations. You see, with each day, the sunlit side of the earth moves around about a degree (actually 1/365th of a circle, but a degree is close enough for this discussion) every day, and the Earth must spin on its own axis that extra degree each day in order to catch up with the creeping advancement of the sunlit side. So in a year, the Earth has added a full rotation relative to the number of actual "days" it has experienced.
So a day is not 360 degrees, but more like 361 degrees of rotation. In other words, a day is generally regarded as the time between, say, two sunsets, regardless of Earth's interaction with the rest of the cosmos. (This is called a solar day.) And it's a good thing, too. Otherwise, our clocks wouldn't keep up with the advancement of the sunlit portion of the Earth, and in six months, noon would become midnight.
But even then, you can't simply say that a day is the length of time it takes for the sun to traverse a relative (meaning, from our perspective) 360 degree arc across the sky, because this time interval changes throughout the year. In the summer (for the Northern hemisphere), the Earth is farther away from the Sun, so it orbits more slowly, thus making the sunlit side of the Earth advance more slowly from day to day. In the winter (again, for the Northern hemisphere), the Earth is closer to the Sun, thus making the sunlit side of the Earth advance more quickly. (Please note that this is a phenomenon resulting from Earth's slightly elliptical orbit, not from the seasons or tilt of Earth's axis.) So the solar day is actually longer in winter than it is in summer since the Earth's rotation requires more time to catch up with the more rapidly-advancing sunlit side during winter. So what do you do? Well, you generally just pick something in the middle that you know won't throw off your clocks after a long time frame.
So 24 hours it is! Except that even that isn't sufficient. Even as we speak, the Moon is robbing angular momentum from the Earth, slowing it down, making each day ever so slightly longer than the one that preceded it. Of course you know about leap years, wherein a day is occasionally added to a year to account for the fact that a year doesn't contain an integral number of days. But did you know that we also have leap seconds? It's true. Occasionally, at the end of the year, we have to add an extra second to the year to account for the ever-lengthening day. We don't have to do it often, but eventually, we'll have to add several seconds each year, though that requirement is still a long, long way off.
Ah, but defining a year is much simpler, because a year is simply the time it takes for the Earth to make a full circle around the sun, right? Well, actually, no. That (the sidereal year) is not how we define a year in our current calendar. The reason is because the Earth wobbles as it spins, and the seasons, which are a consequence of the Earth's tilt, follow the wobbling of the tilted axis. In other words, if we base our calendar on a sidereal year and ignore the wobble, then in about 12,500 years (or so), we'd have summer in December and snow in July (again, Northern hemisphere). So to avoid the utter madness that would ensue as a result of seasonal creep in our calendar (and the inevitable downfall of civilization), we have constructed our calendar based on the tropical year definition, which maintains the solstices and equinoxes at reasonably fixed points from one year to the next. However, although adopting the tropical year instead of the sidereal year avoids seasonal creep in the calendar, it does lead to constellation creep instead. In other words, instead of the summer and winter eventually switching places in our calendar, the December constellations will eventually become our June constellations, and vice versa. How constellation creep is deemed to be preferable to seasonal creep, I'm not sure. But I hope it doesn't lead to utter madness and the downfall of civilization.
And if that weren't complicated enough, we're also moving around the Milky Way's center, bobbing up and down through the galaxy's plane like a horse on a merry-go-round! If our life spans were on the order of millions of years, we could well have calendars that take even our complex motion through the galaxy into consideration.
So the next time you look up at the sky, watch for a moment as the sky slips by the Earth, and ponder the many directions it is heading at once. If you feel a tingling sensation somewhere along your spine, don't worry. That's just a normal response to the realization that you are, quite literally, on the ride of your life.
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2008-01-01: Happy New Year! If you want to read more about this topic (from a REAL astronomer, and everything!), then go to the Bad Astronomer's Super Deluxe Web Site! Man, that guy should be a writer, or something!
Copyright (c) by BSW, 2007. All Rights Reserved.
