Why is it never dark at 12:00 noon?

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jimg44

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<p><font size="2">There must be a simple answer to this, but I can't figure it out.</font></p><p><font size="2">My question is this: Why is it never dark at 12:00 noon?</font></p><p><font size="2">Imagine looking down on the solar system from a perspective high above it.</font></p><p><font size="2">The Sun is in the middle and the Earth orbits around it in a roughly circular orbit.</font></p><p><font size="2">Imagine as you are looking down that the Earth is at the leftmost point in its orbit.</font></p><p><font size="2">Someone standing on the right side of the Earth (from your perspective) will see the sun directly overhead.</font></p><p><font size="2">Let's say it is 12:00 noon for that person.</font> </p><p><font size="2">The Earth rotates once in 24 hours, and at noon the next day that person is again on the right side of the Earth from your perspective, and that person sees the sun directly overhead.</font></p><p><font size="2">Every 24 hours the Earth rotates once and at 12:00 noon that person is on the right side of the Earth from your perspective.</font></p><p><font size="2">But as the Earth orbits around the Sun, the right side of the Earth does not always face the sun.</font></p><p><font size="2">Six months from now, the right side will face away from the Sun.</font></p><p><font size="2">Therefore, it should be completely dark for that person at 12:00 noon. Why isn't it?&nbsp; </font></p><p>&nbsp;</p>
 
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MeteorWayne

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<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>There must be a simple answer for this, but I can't figure it out.My question is this: Why is it never dark at 12:00 noon? Imagine looking down on the solar system from a perspective high above it. The Sun is in the middle and the Earth orbits around it in a roughly circular orbit. Imagine as you are looking down that the Earth is at the leftmost point in its orbit. Someone standing on the right side of the Earth (from your perspective), will see the sun directly overhead.Let&rsquo;s say that it is 12:00 noon for that person.The Earth rotates once in 24 hours, and at noon the next day the person is again on the right side of the Earth and the Sun is directly overhead. Every 24 hours Earth rotates once and that person is on the right side of the Earth from your perspective.But as the Earth orbits around the Sun, the right side of the Earth does not always face the Sun. Six months from now, the right side will face away from the Sun. Therefore, it should be completely dark at 12:00 noon. Why isn&rsquo;t it? <br />Posted by jimg44</DIV><br /><br />Welcome to Space.com</p><p>The simple answer is that noon is defined at each place on earth by when the sun is at it's highest point in the sky (+/- a few minutes). When it is noon at one point on the earth, it's midnight on the opposite side of the earth.</p> <div class="Discussion_UserSignature"> <p><font color="#000080"><em><font color="#000000">But the Krell forgot one thing John. Monsters. Monsters from the Id.</font></em> </font></p><p><font color="#000080">I really, really, really, really miss the "first unread post" function</font><font color="#000080"> </font></p> </div>
 
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BrianSlee

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Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>There must be a simple answer to this, but I can't figure it out.My question is this: Why is it never dark at 12:00 noon?Imagine looking down on the solar system from a perspective high above it.The Sun is in the middle and the Earth orbits around it in a roughly circular orbit.Imagine as you are looking down that the Earth is at the leftmost point in its orbit.Someone standing on the right side of the Earth (from your perspective) will see the sun directly overhead.Let's say it is 12:00 noon for that person. The Earth rotates once in 24 hours, and at noon the next day that person is again on the right side of the Earth from your perspective, and that person sees the sun directly overhead.Every 24 hours the Earth rotates once and at 12:00 noon that person is on the right side of the Earth from your perspective.But as the Earth orbits around the Sun, the right side of the Earth does not always face the sun.Six months from now, the right side will face away from the Sun.Therefore, it should be completely dark for that person at 12:00 noon. Why isn't it?&nbsp; &nbsp; <br />Posted by jimg44</DIV><br /><br />I think what might be hanging you up is that you are trying to associate the rotational period of the planet with&nbsp;its orbital period around the sun.&nbsp; <div class="Discussion_UserSignature"> <p> </p><p>"I am therefore I think" </p><p>"The only thing "I HAVE TO DO!!" is die, in everything else I have freewill" Brian P. Slee</p> </div>
 
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DrRocket

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<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>I think what might be hanging you up is that you are trying to associate the rotational period of the planet with the orbital period around the sun.&nbsp; <br />Posted by BrianSlee</DIV></p><p>What Wayne said is absolutely correct.&nbsp; In addition it is dark at 12:00 noon at the&nbsp;right time of year if you are north of the arctic&nbsp; circle or south of the antarctic circle.&nbsp; It is also light at midnight at times.</p><p>http://geography.about.com/od/physicalgeography/a/longestday.htm</p> <div class="Discussion_UserSignature"> </div>
 
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aphh

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<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>I think what might be hanging you up is that you are trying to associate the rotational period of the planet with&nbsp;its orbital period around the sun.&nbsp; <br /> Posted by BrianSlee</DIV></p><p>That's exactly what&nbsp; he means, but he still didn't get the definitive answer.</p><p>Because Earth follows a curved line, not a straight line, each complete revolution that the Erath completes on a curved path makes the vantage point in direct line of sight on the equator move slightly constantly.</p><p>I'm not an expert and didn't do any research for this, but I think the compensation is made so that we "pretend" that one compelete revolution lasts 24 hrs, but in reality the time it takes to revolve around Earth's axis is slightly different. Hence the vantage point in direct line of sight in relation to sun and the clock is a match or near match each day. </p><p>I'm sure somebody or the wikipedia has better analysis.&nbsp;</p>
 
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MeteorWayne

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Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>That's exactly what&nbsp; he means, but he still didn't get the definitive answer.Because Earth follows a curved line, not a straight line, each complete revolution that the Erath completes on a curved path makes the vantage point in direct line of sight on the equator move slightly constantly.I'm not an expert and didn't do any research for this, but I think the compensation is made so that we "pretend" that one compelete revolution lasts 24 hrs, but in reality the time it takes to revolve around Earth's axis is slightly different. Hence the vantage point in direct line of sight in relation to sun and the clock is a match or near match each day. I'm sure somebody or the wikipedia has better analysis.&nbsp; <br />Posted by aphh</DIV><br /><br />That's also correct. It takes 24 hours for the a spot on the earth to point back at the sun, it takes the earth 23 hours and 56 minutes for the spot to point back at the same spot in the sky. In other words, the sky rises 4 minutes earlier each day. The clock time doesn't, because it is referenced to the sun. <div class="Discussion_UserSignature"> <p><font color="#000080"><em><font color="#000000">But the Krell forgot one thing John. Monsters. Monsters from the Id.</font></em> </font></p><p><font color="#000080">I really, really, really, really miss the "first unread post" function</font><font color="#000080"> </font></p> </div>
 
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aphh

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<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>That's also correct. It takes 24 hours for the a spot on the earth to point back at the sun, it takes the earth 23 hours and 56 minutes for the spot to point back at the same spot in the sky. In other words, the sky rises 4 minutes earlier each day. The clock time doesn't, because it is referenced to the sun. <br /> Posted by MeteorWayne</DIV></p><p>That's it. But also position of other stars varies slightly throughout the year even if your vantage point is on the equator, because we follow a circle who's diameter is 2 Astronomical Units.</p><p>Because the stars are very far away, the effect on the equator is miniscule, but noticeable and just enough to make a trigonometric caluclation to determine the distance to a star.&nbsp; </p>
 
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derekmcd

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<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>There must be a simple answer to this, but I can't figure it out.My question is this: Why is it never dark at 12:00 noon?Imagine looking down on the solar system from a perspective high above it.The Sun is in the middle and the Earth orbits around it in a roughly circular orbit.Imagine as you are looking down that the Earth is at the leftmost point in its orbit.Someone standing on the right side of the Earth (from your perspective) will see the sun directly overhead.Let's say it is 12:00 noon for that person. The Earth rotates once in 24 hours, and at noon the next day that person is again on the right side of the Earth from your perspective, and that person sees the sun directly overhead.Every 24 hours the Earth rotates once and at 12:00 noon that person is on the right side of the Earth from your perspective.But as the Earth orbits around the Sun, the right side of the Earth does not always face the sun.Six months from now, the right side will face away from the Sun.Therefore, it should be completely dark for that person at 12:00 noon. Why isn't it?&nbsp; &nbsp; <br /> Posted by jimg44</DIV></p><p>I understand the dilemma you're having, but you are overthinking it.&nbsp; We measure the solar day as 24 hours.&nbsp; In other words, it takes 24 hours for the sun to reach it's apex in the sky.&nbsp; </p><p>24 hours isn't really how long it takes for the Earth to make a complete rotation about it's axis.&nbsp; It's rotation is slightly less than 24 hours (23 hours, 56 minutes).&nbsp; If you used that time to define our clocks, then it would eventually become dark at 12 noon. </p> <div class="Discussion_UserSignature"> <div> </div><br /><div><span style="color:#0000ff" class="Apple-style-span">"If something's hard to do, then it's not worth doing." - Homer Simpson</span></div> </div>
 
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aphh

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<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>I understand the dilemma you're having, but you are overthinking it.&nbsp; We measure the solar day as 24 hours.&nbsp; In other words, it takes 24 hours for the sun to reach it's apex in the sky.&nbsp; 24 hours isn't really how long it takes for the Earth to make a complete rotation about it's axis.&nbsp; It's rotation is slightly less than 24 hours (23 hours, 56 minutes).&nbsp; If you used that time to define our clocks, then it would eventually become dark at 12 noon. <br /> Posted by derekmcd</DIV></p><p>He's not overthinking it. He noticed the same thing that the ancient scientists determined regarding the relationship of the sun, stars and the earth. </p><p>That's how the 24 hr clock and a almanac got started. Plus the slight tweaking of the clock and the almanac each year and each 4 year period that is required as we move along around the Sun.</p>
 
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derekmcd

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<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>He's not overthinking it. He noticed the same thing that the ancient scientists determined regarding the relationship of the sun, stars and the earth. That's how the 24 hr clock and a almanac got started. Plus the slight tweaking each year and each 4 year period that is required as we move along around the Sun. <br /> Posted by aphh</DIV></p><p>Overthinking it might have been a wrong assessment... no argument there.&nbsp; He's using the sidereal motion to define the 24 hours clock... that's where the crux of his dilemma lies.&nbsp;</p> <div class="Discussion_UserSignature"> <div> </div><br /><div><span style="color:#0000ff" class="Apple-style-span">"If something's hard to do, then it's not worth doing." - Homer Simpson</span></div> </div>
 
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jimg44

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<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>That's also correct. It takes 24 hours for the a spot on the earth to point back at the sun, it takes the earth 23 hours and 56 minutes for the spot to point back at the same spot in the sky. In other words, the sky rises 4 minutes earlier each day. The clock time doesn't, because it is referenced to the sun. <br /> Posted by MeteorWayne</DIV></p><p><font size="2">Thank you! That's what was throwing me off.&nbsp;</font> </p>
 
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CalliArcale

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<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>Thank you! That's what was throwing me off.&nbsp; <br /> Posted by jimg44</DIV></p><p>In addition, I'd just like to elaborate on something derekmcd alluded to -- sidereal rotation.</p><p>There are actually two kinds of day -- the kind we're all used to, and a sidereal day.&nbsp; The sidereal day is the amount of time it takes for one of the so-called "fixed stars" to go from one spot in the sky to the same spot again.&nbsp; It takes slightly longer for the Sun to return to the same apparent spot in the sky, and that's what gives us our normal 24-hour day.</p><p>The ancients described sidereal time by the rate at which the Sun seemed to slide past the background stars.&nbsp; They were very careful observers, and had figured out that the stars were actually present even though you can't see them.&nbsp; They mostly used the sunrise as their reference point for this, and as a consequence, many astrological events revolve around the heliacal rising of a particular constellation -- that is to say, when it rises with the Sun, and therefore the Sun is in that particular "house".&nbsp; (Some argue that this is what is actually meant in the Bible by seeing the star in the East; after all, the account is clearly informed by ancient Babylonian astrology, whether the tale is true or not.) </p><p>http://en.wikipedia.org/wiki/Sidereal_day </p><p>To get a much more visual idea of the Sun's apparent motion against the background of stars, check out the SOHO website.&nbsp; The SOHO LASCO instrument is a coronograph -- it creates an artificial eclipse by means of an opaque mask over the Sun.&nbsp; This lets it see the faint corona at all times -- and also to see all kinds of background stars, planets, and even hoardes of sungrazing comets.&nbsp; (SOHO turned out to be the most productive comet-hunter of all time.) You'll see occasionally streaks -- thee are cosmic ray hits on the CCD imager.&nbsp; Right now, there's a really bright object to the left of the Sun.&nbsp; That's Venus. </p><p>SOHO movies, near-real-time data; pick LASCO C3 for the best view of stars behind the Sun </p><p>EDIT: For extra cool-factor, if you look at the video soon (i.e. before they put up a new one), near the beginning you can see a sungrazing comet diving to its death.&nbsp; I think that's the historic 1500th comet discoverd by SOHO.&nbsp; The team has been especially proud of that one, becuase it officially makes SOHO a more prolific comet discoverer than all of the other comet discoverers in history combined! </p> <div class="Discussion_UserSignature"> <p> </p><p><font color="#666699"><em>"People assume that time is a strict progression of cause to effect, but actually from a non-linear, non-subjective viewpoint it's more like a big ball of wibbly wobbly . . . timey wimey . . . stuff."</em>  -- The Tenth Doctor, "Blink"</font></p> </div>
 
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jimg44

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Nearly two years later, I have a follow-up question:

There are 525,600 minutes in a Calendar year (365 days x 24 hours x 60 minutes).

The Earth rotates once every 1,436 minutes (23 hours x 60 minutes + 56 minutes)

Dividing 525,600 by 1,436 gives you 366.01671 rotations per calendar year.

If the Earth rotates 366 times per year, why are there only 365 sunrises?
 
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MeteorWayne

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Two corrections. First of all a year is ~ 365.25 days (remember leap years? The actual number is a tiny bit less, but it's close enough.. So there are 525960 minutes in an "average" year. And this is a case where you should use the 24:00 hour solar day, not the 23:56 sidereal day. So there are 1440 minutes in a solar day.

525960/1440= 365.25

So for a regular year 525600/1440 = 365
For a leap year 527040/1440 = 366
 
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Gravity_Ray

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jimg44":1bd0zyaa said:
Nearly two years later, I have a follow-up question:

There are 525,600 minutes in a Calendar year (365 days x 24 hours x 60 minutes).

The Earth rotates once every 1,436 minutes (23 hours x 60 minutes + 56 minutes)

Dividing 525,600 by 1,436 gives you 366.01671 rotations per calendar year.

If the Earth rotates 366 times per year, why are there only 365 sunrises?
Woah Jim, you remind me of "deep thought". 2 years for a follow up question? Dude you are so Zen.
 
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jimg44

Guest
MeteorWayne":2u4pqaf3 said:
Two corrections. First of all a year is ~ 365.25 days (remember leap years? The actual number is a tiny bit less, but it's close enough.. So there are 525960 minutes in an "average" year. And this is a case where you should use the 24:00 hour solar day, not the 23:56 sidereal day. So there are 1440 minutes in a solar day.

525960/1440= 365.25

So for a regular year 525600/1440 = 365
For a leap year 527040/1440 = 366

Thanks a lot for the info, but it doesn't quite answer the question, because 365.25 is not a calendar year, and the 24 hour day is not the period of rotation.

For instance, in 2010, 525600 minutes will elapse from 12:00AM on Jan. 1 to 11:59PM on Dec. 31. During that time, the Earth will rotate 366.01671 times (525600/1436), yet there will only be 365 sunrises.

What am I missing?
 
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MeteorWayne

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You are missing what I stated. Relative to the sun, the earth rotation period is exactly 24 h, not the 23:56 sidereal day period.

And a year has either 365 days (3 out of 4 years) or 366 days (1 out of 4 years)
 
3

3488

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Remember, further to Wayne told you, the Sun appears from Earth to move from west to east along the Ecliptic. Wayne clearly explained to you that the SOLAR day is 24 hours, but the SIDEREAL day is 23 hours, 56 minutes & 4 seconds.

Therefore if for example the star Sirius / Alpha Canis Majoris, rises at Midnight, then tomorrow will be 23:56, following day 23:52, day after 23:48, etc. However the Sun will still be transiting the southern horizon at NOON / Midday / 12:00 PM. This is due to the Earth's orbital motion around the sun.

During the intervening 24 hours the Sun has appeared to move 3 minutes & 56 seconds of Right Ascension along the Ecliptic.

I think I have that right.

Andrew Brown.
 
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CalliArcale

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jimg44":1xo0g144 said:
If the Earth rotates 366 times per year, why are there only 365 sunrises?
Well, actually, there's a much simpler answer.

One year is defined as having 365 sunrises. ;-) (Exception: leap years have 366.) However many times the Earth has to rotate in order to cause that to happen is unimportant.

But one can't help but wondering -- why are there fewer sunrises than rotations? It's because the Earth takes 23 hours and and 56 minutes to rotate, but it takes 24 hours for somebody on the surface to see the Sun go from apex to apex in the sky. A day is four minutes longer than a rotation, and over the course of a year, those minutes add up.

In fact, if you think about it, they should add up to about one extra rotation, because at the end of the year, you're back to looking at the same stars at midnight as you did a year ago. (Well, almost; the year is slightly longer than 365 days, so they'll be ever so slightly off. Leap years correct every now and again so we don't wind up eventually celebrating Fourth of July with a snowball fight.)
 
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Fallingstar1971

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This wouldnt have anything to do with Keplers law would it? As the Earth moves closer to the sun, it spins faster then it does when its farther away. This is why the same point on the Earth will point to the sun at noon in any point of its orbit. As the Earth swings in at its closest approach (northern hemisphere winter) it must spin faster to bring the sun overhead (or at its highest point) at noon. In the summer (Northern hemisphere) Earth is at its farthest point and not as much correction is needed to bring the sun overhead and so it rotates slower.

Draw an oval on a piece of paper. This will be earths orbit. Place the sun near one end or the other of the oval.

Now take a coin and move it along the path you just drew. Rotate the penny 360 degrees at different points along the orbit to simulate day/night.

You will quickly see that one 360 degree rotation is not enough to bring the sun overhead as you rotate the penny when its closest to the sun. The Earth must rotate much MORE than 360 degrees (sometimes almost an extra quarter turn) in one "day" to bring the Sun directly overhead.

And at the other end of the oval where Earth is farthest away, you will see that the extra rotation needed to bring it back in line with the noon day Sun is very small, hense the Earth does not rotate the same amount, ergo the Earth is rotating SLOWER in the same 24 hr. "day"

Granted these changes are small. Our orbit is nearly circular, but these laws still apply. They apply to any orbit that is not a perfect circle.

So, in a nutshell, the Earths rotational speed changes while it goes around the sun. These changes keep the noon day sun at its highest point.

Star
 
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MeteorWayne

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Not really. The solar day is defined by when the sun is overhead at noon, and average of 24:00 hours per day. Sure some are a little shorter or longer, but only by a second or so, and over a year they average out. There's really no reason to make it any more complicated than that.

Yes the distance along the orbit is faster in the northern hemisphere winter (when the earth is closer to the sun) than it is in the summer when it is further away (that's whay meteor folks use solar longitude, rather than date and time for shower peaks), but the rotation rate does not change. It's 24 hours a day.
 
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MrUniverse

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It's the revolution (the movement of the Earth along it's orbit) that changes speed as the Earth moves closer or farther from the sun in it's elliptical orbit.
Also, about half the time it seems like the Sun is about an hour away from it's apex in the sky at noon ;) ;)
 
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trumptor

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jimg44":1i5hbm4i said:
If the Earth rotates 366 times per year, why are there only 365 sunrises?

Because there is one revolution around the sun also occuring during that year which adds a rotation, that goes unnoticed when just counting the sunrises.

Imagine being on the moon and seeing how many Earthrises there are during one revolution. You would see the Earth as fixed in the sky and would count zero because the same side always faces us.

Now if you were looking from a point in space outside of moons orbit, you would see the back of the moon when it is between you and the Earth, its side when it is at a right angle between you and the Earth, the side we see from Earth when it is on the other side of the Earth, and when it gets back to its original point, you will have seen it rotate one time.

The Earth and sun are the same. You have to add this one rotation that isn't obvious by looking at sunrises. Every sunrise the same side of Earth is facing the sun, just as the moon was facing the Earth, so you have to add that little bit to the rotation to get the same side facing the sun. And in a year, that little extra, as with the moon, adds up to a full rotation.



Sorry, my explanation is clear in my head but I'm having trouble conveying it, lol.
 
K

kg

Guest
MeteorWayne":3j9s52g1 said:
Not really. The solar day is defined by when the sun is overhead at noon, and average of 24:00 hours per day. Sure some are a little shorter or longer, but only by a second or so, and over a year they average out. There's really no reason to make it any more complicated than that.....
Of course it is quite common for it to be dark at noon above the artic circle...
I think it is a bit funny when people are alarmed to find that the earth doesn't run like a clock! What with it's elipticle orbit, a tilt in it's axis, and a moon tugging it around, who in their right mind would build a timepiece like this and expect it to run properly! The sun can run "fast" or "slow" by as much as 15 minutes or so. I think the equation of time is somewhat relevent here.
http://en.wikipedia.org/wiki/Equation_of_time
...and here is an explanation of the Analemma, that funny figure eight thing you see somewhere in the Pacific Ocean on old globes.
http://en.wikipedia.org/wiki/Analemma
 
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bdewoody

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I think the OP's question is like asking why isn't white paint ever black. duh, the definition of noon is when the sun is directly overhead ( at latitudes between the arctic circle and the antarctic circle).
 
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