Are the stars always in the same position relative to earth

bibibobo

when u look up at the sky at night are the stars always in the same position? did the early sea faring navigators use the position of stars to navigate? e.g. north star.... how come if the earth is rotating on is own axis can u navigate with stars which are surely changing position as the earth rotates?

albertw

bibibobo wrote:

when u look up at the sky at night are the stars always in the same position? did the early sea faring navigators use the position of stars to navigate? e.g. north star.... how come if the earth is rotating on is own axis can u navigate with stars which are surely changing position as the earth rotates?

The north star is always in the same position, north (give or take a couple of degrees). All the other stars will appear to rotate around this point over the course of a night. You use the other stars as a guide to find the pole star, usually by following the two pointer stars at the end of the plough.

The height of the north star above the ground can tell your your latitude. At the equator it would be on the horizon, at the north pole it would be directly above.

From this one observation you know the direction you are travelling and your latitude.

Longitude is another matter... and well described in "Longitude" by Dava Sobel.

hth,

Cheers,
~Al

Capt'n Midnight

No.

precession means the pole star changes every few thousand years.
3000 B.C. it was Alpha Draconis and in about 13,000 years Vega


http://starchild.gsfc.nasa.gov/docs/StarChild/shadow/questions/question64.html

It was the ancient Greek astronomer and mathematician Hipparchus who first estimated the precession of the Earth's axis around 130 B.C

also the sun and other stars all move relative to each other so over longer times the constellations will drift apart

and in several billion years our galaxy will collide with andromoda so anyone's guess as to what the night sky will look like then.

Flukey

As people have said things are changing. We won't notice much difference in our lifetime, but everything out there is moving, so it is all changing all the time.

Seanie M

Everything in the galaxy is moving around. In actual fact, this is what the Plough will look like in 100,000 years.

Flukey

I was in New Orleans over the past week. While we were busy at other things, I did get to see the night sky a little. What struck me was the difference in the height of the constellations in the sky. New Orleans is about 90° West and more importantly about 30° North. Orion and Sirius for example, were a lot higher in the sky than they look from here in Ireland. Has anyone had any similar experiences?

Seanie M

Flukey wrote:

What struck me was the difference in the height of the constellations in the sky. New Orleans is about 90° West and more importantly about 30° North. Orion and Sirius for example, were a lot higher in the sky than they look from here in Ireland. Has anyone had any similar experiences?

New Oreans also being 32 +/- 2 degrees further south of Ireland, hence why they will appear higher. At the North Pole (90 degrees), I think you can barely see Betelgese in Orion.

Seanie.

Flukey

I know all that Seanie M, but it was still strange to see them that way. You get used to seeing constellations in the same general areas of the sky, so it is a little unusual to see them at different positions. This was the furthest I have been from Ireland, so the most pronounced example of it I have seen.

ISAW

Flukey wrote:

I know all that Seanie M, but it was still strange to see them that way. You get used to seeing constellations in the same general areas of the sky, so it is a little unusual to see them at different positions. This was the furthest I have been from Ireland, so the most pronounced example of it I have seen.

At the equator ( well between the tropics) you will get to see the whole sky. But only one half (north and south) at any time of the year as the other half will be in daylight. The north star will be on or near the horizon (below it fi you go south of the equator).

But it it probably the southern stars which look so odd as you move south. Sirius becomes very bright -the brightest in the sky ( indeed it is fairly bright from Ireland but not as bright as some stars that get overhead e.g. capella since the atmosphere dimms it - a process known as "extinction")

As regards things looking different, you may be used to Betelgeux being the top left star in Orion and Rigel being the bottom right one. If you are in the southern hemisphere it is the other way around with Rigel on the top left. The Moon also is "upside down". I would think stars close to the celestial equator should be more prevalent for this effect on personal perception. But as stated the stars do move over time and have a measurable "stellar parallax"

ISAW

bibibobo wrote:

when u look up at the sky at night are the stars always in the same position? did the early sea faring navigators use the position of stars to navigate? e.g. north star.... how come if the earth is rotating on is own axis can u navigate with stars which are surely changing position as the earth rotates?

Two more points.
stars have proper motion over time. i.e. they change position in relation to each other. you have to remember that the shape of a constellation is only according to our viewpoint. some of the stars are much further away so the shape would change depending on where you moved to in the Galaxy.
so stars do change position in relation to each other and precession also occurs but these are only noticable to the naked eye over at least a century I would guess.

Second as regards the navigation idea. If the north pole is under the north star then whereever you are in the Northern hemisphere you need only look for that star to work out latitude. If you are at the North pole it is overhead and if you are at the Equator it would be on the northern horizon. so the angle it is at tells you latitude. In dublin for example it is about 53 degrees above the horizon. We can do the same for southern stars of for the Sun and Moon by making tables of where they will be over the year.

Problem come with the daily rotation. The Earth does about 365.25 rotations for the sun to come back to the same place in the sky i.e. the Earth goes once around the Sun. I statedabout It is important to know exactly how many and to set your yearly calendar by it. Then you can say that
at a certain time on a certain day if you are at a certain place then the Sun (or another star should be in a certain position). You only have to measure the position of the Sun or of a Star when it is at its highest in the sky i.e. the Sun at noon and then look at an accurate clock. Or you could and then look up you tables to see where it should be say at Greenwitch at a certain time and then look at the time you have. the time difference between your clock and Greenwich will tell you how far East or West you are. for example Dublin is about 6 degrees West. We are on Summer time so 1 pm is really noon. Now go out an look up at noon. You will find the Sun is not at its highest point. matter of fact we are about 22 minutes slower (i.e. 15 degrees equals one hour and 24 hours = 1 day = 360 degrees). so the Sun wont get directly south (its highest point) until 20 plus minutes later. Now you could just come out and wait till the Sun is south and look at the watch and see it reads 1:20 (which is really 12:20) and deduce 20 mins is a third of an hour and therefore you are a third of 15 degrees or 5 degrees west of the Greenwich.

ISAW

ISAW wrote:

the time difference between your clock and Greenwich will tell you how far East or West you are. for example Dublin is about 6 degrees West. We are on Summer time so 1 pm is really noon. Now go out an look up at noon. You will find the Sun is not at its highest point. matter of fact we are about 22 minutes slower (i.e. 15 degrees equals one hour and 24 hours = 1 day = 360 degrees). so the Sun wont get directly south (its highest point) until 20 plus minutes later. Now you could just come out and wait till the Sun is south and look at the watch and see it reads 1:20 (which is really 12:20) and deduce 20 mins is a third of an hour and therefore you are a third of 15 degrees or 5 degrees west of the Greenwich.

I should have added that your clocks are on Universal Time i.e. GMT the time it is in London. Of course every time you travel 15 degrees West you turn your clock back an hour. This is a local adjustment for the effect I described above. I believe, in some countries north of India there are half hour adjustments. There are also adjustments of one day over the international date line. This line is not a straight north south line so that it takes in island groups and the like without splitting them up and keeping them with other nearby countries. so one could be further east than someone else but almost a whole day behind instead of a few minutes ahead.