40
$\begingroup$

The Earth takes 24 hours to spin around its own axis and 365 days to spin around the Sun. So in approximately half a year the Earth will have spun around its axis 182.5 times.

Now take a look at the following picture:

enter image description here

Assuming that the Earth is in the position on the left is, say, on 1st of Jan. 2017 and in the position on the right, half year after. The Earth will be roughly on the opposite side of the Sun given that half a year passed, is that correct? If at noon, half a year earlier, that part of the Earth was facing the Sun, then why wouldn't the opposite part of the Earth be facing the Sun now, after 182 complete rotations and the Earth being on the opposite side of the Sun? We expect the noon-time to occur on the dark side instead of the lighted side.

Shouldn't this cause the AM/PM to switch, the rotations made are consistent with 182 passing days. Assuming it's noon at both dates, why does the Earth face the Sun at the same time on both sides of the Sun?

$\endgroup$
  • 51
    $\begingroup$ What takes 24 hours is not the earth's rotation, it is the time from solar noon to solar noon at a fixed point on earth. It that time it rotates more than 360 degrees. It rotates about 361 degrees. $\endgroup$ – Mike Dunlavey Dec 22 '16 at 1:06
  • 12
    $\begingroup$ simple.wikipedia.org/wiki/Sidereal_day $\endgroup$ – Pieter Dec 22 '16 at 1:11
  • 20
    $\begingroup$ The concept of "noon" was invented by people who knew nothing about celestial mechanics (any theories they had were wildly incorrect) and had no accurate clocks (and in some cultures, no clocks at all). In that situation, "the time in the day when the sun is highest in the sky" seemed important enough to need a name, so they gave it a name. Your argument about "how the concept should be defined" in terms of modern knowledge is therefore back-to-front. $\endgroup$ – alephzero Dec 22 '16 at 1:59
  • 8
    $\begingroup$ A 'half year' is closer to 182.625 days since a year is closer to 365.25 days. (That's where leap-days come from every four years.) $\endgroup$ – user2338816 Dec 22 '16 at 12:54
  • 3
    $\begingroup$ The Earth spins around its axis ~366 times in a year, not ~365 times. $\endgroup$ – Samuel Dec 22 '16 at 19:00
86
$\begingroup$

The earth takes 24 hours to spin around it's own axis.

Depending on the specifics (such as what it means to "spin around"), this is incorrect. To spin around exactly once with respect to distant stars (aka Sidereal day) requires 236 seconds less than 24 hours. Over half a year, this nearly 4 minute difference every day adds up to about 12 hours, the time it takes to rotate half way around and face the sun again.

24 hours is the length of the average solar day (Synodic Day), the time it takes the earth to rotate so that (on average) it is facing the sun at the same angle. Because the time period derives from a sun-referenced rotation, not a star-referenced rotation, the same spot on the earth faces the sun at approximately the same time every solar day. (Ignoring additional changes from axial tilt and orbital eccentricity)

$\endgroup$
  • 4
    $\begingroup$ Those "additional changes" go under the name of the "equation of time". $\endgroup$ – David Hammen Dec 22 '16 at 2:27
  • 1
    $\begingroup$ If you ignore tilt and eccentricity, why still say "approximately"? Isn't "same spot always faces sun at the same time of day" the very definition of "solar day"? $\endgroup$ – Bergi Dec 22 '16 at 19:16
  • 3
    $\begingroup$ @Bergi, in addition to eccentricity and tilt, there's annual variations in the Earth's rotation speed (most significantly, seawater on the equator during northern summer becomes snow around the pole during the winter), monthly variations due to the rotation of the Earth-Moon system, irregular variations in the Solar System's center of gravity (the Sun is not the center of the Solar System, just close to it), and probably others I've forgotten. $\endgroup$ – Mark Dec 22 '16 at 20:01
  • 3
    $\begingroup$ I intended the parenthetical to explain part of the reason that it's an approximation, not exclude it. But since we keep time by the mean solar day and not the actual one, there are additional deviations not covered by the equation of time. $\endgroup$ – BowlOfRed Dec 22 '16 at 20:01
16
$\begingroup$

Our clocks are set so that 24 hours is the time for the Sun to appear in the same part of the sky. What this means in terms of the Earth's orbit and rotation is that the Earth does slightly more than a complete rotation in 24 hours.

Let's say that your picture is drawn from the perspective above Earth's north pole. Earth rotates and orbits counterclockwise. Draw a line on the right-hand side Earth from the point closest to the sun (where it is noon) towards the sun. After 24 hours, the Earth will have moved about 1/365 of the way around it's orbit, and the line will have rotated just a bit more than 360 degrees so that it is pointing at the sun again.

The time where the line from Earth is parallel to the original line before rotation is called a sidereal day, and is 23 hours and 56 minutes long.

$\endgroup$
13
$\begingroup$

The earth takes 24 hours to spin around it's own axis.

It almost does.

How would you pin point an exact full rotation? It's much easier to say "when the Sun is at the same spot in the sky" than to say "after one full rotation."

By referring to the Sun, a "full rotation" would be exactly 360 degrees only if Earth wasn't moving around it.

But it does. After Earth has rotated once, it has also moved a bit. Thus it has to rotate a tiny bit extra to have the Sun in the same spot in the sky again. A "full rotation" is then a tiny bit more than 360 degrees.

You can never reach a situation, where you would say "now it's noon" without having the Sun at the same spot in the sky - not after half a year either.

$\endgroup$

Not the answer you're looking for? Browse other questions tagged or ask your own question.