# Position of constellations

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1. Oct 29, 2014

### Pjpic

If the Big Dipper goes around once every 24 hours, why is it in a different position at midnight in December contrasted with midnight in June?

2. Oct 29, 2014

### Staff: Mentor

The earth has a tilt so that as we orbit the sun we see the Big Dipper in a different location. The apparent movement of 24 hours is due to the Earths spin and on top of that is the apparent movement due to the earths orbit around the sun.

You can see a similar effect with the moon where the earths spin causes the moon to traverse the sky but each night at the same time it's moved by a 10+ degrees or so from the previous night due to its 29.5 day orbit about the earth.

Last edited: Oct 29, 2014
3. Oct 29, 2014

### Pjpic

So, the orbit of the earth around the sun adds some time to the 24 hour motion of the Big Dipper around the pole?

In a year's time how great is the change (does it add one rotation per year or does it plus for a time then minus for a time)?

The position of the sun seems to be always the same at noon, why doesn't the orbit affect that?

4. Oct 29, 2014

### Staff: Mentor

http://curious.astro.cornell.edu/question.php?number=300 [Broken]

Remember the sun changes its height in the sky with the seasons due to the earth's tilted axis.

Last edited by a moderator: May 7, 2017
5. Oct 29, 2014

### Pjpic

Would it be true for all the planets, that have a day/night cycle, that the sidereal day about lines up with the solar day after one orbit?

6. Oct 29, 2014

### Staff: Mentor

I think that could happen for a while but if you think about the moon its spin has tidally locked with the earth ie gravity has an effect on the spin:

http://en.wikipedia.org/wiki/Tidal_locking

for the planets I think they all have different spin rates and its not necessary tied to the orbit. In fact the earth year is 365.25 days right?

Someone more knowledgeable than I should comment on this though.

7. Oct 30, 2014

### Staff: Mentor

So, the term you are looking for is sidereal time. The sidereal day is 23 hours, 56 minutes and 4 seconds. Multiply the difference out and you see that the earth's revolution is indeed responsible for adding an extra day to the apparent motion of the stars.

Also, the tilt of Earth's axis is why the sun's path moves up and down throughout the year, but has no impact on the stars: the north star, for example, is always in exactly the same place. The other stars just rotate around it.