And why leap year happens once per 4 years only to address this?
In: 1081
>If Earth makes one complete rotation on its axis every 23 hours and 56 minutes,
It doesn’t really, that’s just a kind of colloquial thing we say because the exact precision isn’t required most of the time. What a day (in common usage) *actually* represents is how long it takes for a given spot on the surface of the Earth that is facing the Sun to be facing the Sun again. That’s closer to 24 hours. This isn’t simply 360 degrees of rotation later, because the Earth will have moved along its orbit around the Sun during that time. Measuring a day based on the time between a spot on the Earth facing the Sun takes into account both the rotation of the Earth, and its movement along its orbit. If we measured a day based on exactly 360 degrees of rotation, then you’re correct, exactly what you said would happen would happen.
Edit to add: [The Wikipedia page](https://en.m.wikipedia.org/wiki/Earth%27s_rotation) does a surprisingly good job at explaining all the ways you can calculate a day and how they’re all different, pretty good read if you’ve got the time!
Remember that 1 year is about 364 days.
That means that each time a day of rotation happens, the earth *also* moves about 1/364th of the way around the sun.
So “noon” today isn’t exactly where “noon” was yesterday. It’s now about another 0.98 degrees further “east” than it was before.
So the Earth has to rotate a little bit further for the same spot to reach the “noon” position again. That “little bit further” to move about another 1 degree or so takes about 4 more minutes.
Thus the “length of a day”, measured as the time it takes to go from “noon” today to “noon” tomorrow, is 4 minutes longer than the time it takes to actually rotate the Earth back to the same exact spot.
That’s the reason the rotation is short of 1 day by about 4 minutes.
You want the hours align with the sun, not with the stars.
Since earth is rotating around the sun as well, after it aligned back to the stars after a rotation, it needs to rotate a bit further to align to the sun again.
Aside all the explanations, there are multiple ways to correct this, not just “the classic 4 leap year”.
The leap year is every 4th year, where year is divisible by 4, so after 3 years of not counting the remaining fragment of the incomplete year, they are added in a full day, 29th, in February.
However, that day fragments are not a perfect 1/4 day, they are a little more that that so with every leap year we are overcorrecting this system, therefore, every 100 years, instead of adding a leap day, we subtract a leap day.
So, if we add a leap day in every year that is divisible by 4, we subtract a leap day in every year that is divisible simultaneously by 4 and by 100 and that means that in 1700, 1800, 1900 we didn’t added a leap day.
BUT .. but taking out a day, we are overcorrecting in the opposite direction so every 400 years, we ARE adding a leap day, even if the year is divisible simultaneously by 4 and 100 and that’s why 2000 had a leap day and the rule is: we add a leap day if the year is simultaneously divisible by 4, 100 and 400.
And ofc, if you think that this is complicated, google for the leap second 🙂
Because Earth is moving around the sun, so the sun “moves” in the sky 4 minutes per day, taking about 365 days to make a complete “rotation” around the Earth (with minor corrections.)
The earth makes a full 360 rotation every 1436 minutes. However, it takes the sun 1440 minutes to return to the same location in the sky as the previous day.
This happens because the earth rotates about its axis but also orbits the sun: every day, it moves about 2.6 million km. For this reason, to us, relative to the stars, the sun appears to move in the sky, slightly eastward.
So a day isn’t measure why a full rotation of the planet, but by how long it takes for the sun to return to the same longitude.
If the Earth stayed fixed in space on one side of the sun, yes, noon at one point on the surface would be midnight six months later.
But the Earth also revolves around the sun, which exactly compensates for the Earth being on the opposite side of the sun once per year. So the six months net change in direction from a four minute shorter revolution (than the length of the “day”) means the same side of the Earth points towards the sun at noon, six months later.
The earth rotates like a top, and it also orbits the sun. If we went “up” from the North pole and looked down on our solar system, it would be orbiting the sun counter-clockwise and also spinning counter-clockwise.
If the Earth didn’t rotate at all, then the sun would still rise and set, but it’d go backwards, rising in the West and setting in the East, and a day would be a year long — six months of light, six months of darkness. So it’s like there’s one backwards-day built in, just because the Earth goes around the sun.
It’s easiest to visualize by literally just making a fist with one hand and calling it the sun, and using the finger on your other hand and making it go around the sun. If you lived on the fingernail side, it’d spend six months in darkness, but as it got around to the other side of the sun (your fist), then it’d be in daylight for six months.
So our days are (on average) 24 hours even though it only takes 23 hours and 56 minutes to rotate. That 4 minutes of extra time to complete a day is to counteract that built-in backwards day caused by Earth going around the sun. And that’s why extra 4 minutes a day every day for a whole year roughly equals that one backwards-day’s worth of time.
Leap years aren’t really related… They exist because the time it takes Earth to make one orbit isn’t an integer-number of days. It’s not 365 days — it’s actually about 365.2422 days. That’s very close to 364.25 days, so adding an extra day to the calendar every 4 years keeps us roughly in line with the actual length of a year. But it’s still not quite right, so we skip a leap-year every 100 years, which brings it down to 365.24 days per year on average, which is closer. But that’s not quite right, so we add a leap year every 400 years, which brings us up to 365.2425 days, which is *very* close to 364.2422 days. Theoretically if we keep the calendar for long enough, it’ll still drift, and maybe we have to remove a leap year once every three thousand years to keep it even closer.
—
EDIT:
Incidentally, Venus is kind of in that “not rotating at all, just orbiting” situation. Not perfectly — it does rotate, albeit the wrong direction. So a day on Venus would be about six (Venusian) months long.
Mercury is somewhat similar, completing 3 rotations for every 2 orbits of the sun. That means a day on Mercury is longer than a year on Mercury… every 2 mercury-years, it makes 3 rotations, but has two backwards-days to cancel out from orbiting the sun, so a mercury-day ends up about 2 mercury-years long.
Checking Wikipedia, a mercury-year is 88 earth-days long, mercury-days are 176 earth-days long. So you could literally outrun a sunrise (ignoring the part where there’s nothing to breathe)
Is it possible for earth to revolve on its axis or rotate around the sun at a slower or faster speed ever? What kind of event or force of nature can make this happen if it’s possible?
>If Earth makes one complete rotation on its axis every 23 hours and 56 minutes, how does day and night not being flipped on our clocks after six months?
Because that’s a *sidereal* day, rather than a solar day.
Remember that the Earth also orbits around the Sun. Thus, if you define a day as the time it takes for the Sun to start at noon, and then get *back* to noon, this time will be a little longer than the sidereal day as the Earth has to spin a little bit further than 360 degrees for the Sun to be back in that same location relative to the sky.
The length of the solar day varies by as much as 51 seconds over the course of a year, but averages out to pretty damn close to 24 hours.
Latest Answers