How to find the center of star trails circle?

[yjiang]

I took my first star trails recently. My question is how to find the center of star trails circle like this:

http://www.danheller.com/images/FAQ/...img1.html#img5

My photo is also 15 mins exposure but does not form the circle at all.

Any advice is highly appreciated.

Yi

[Colin Southern]

I suspect that if you want to capture an image showing the centre of rotation then you'll probably need to be in the right part of the world. I think you're going to need more than a 15 minute exposure too; I suspect several hours minimum with a wide angle lens.

[timo2]

In the northern hemisphere, you want to find Polaris, the pole star, or centre of rotation:

[Colin Southern]

In the northern hemisphere, you want to find Polaris, the pole star, or centre of rotation:

http://www.youtube.com/watch?v=0TFZDO8KcTw

Do you know of anything for the Southern hemisphere Tim?

[revi]

I suspect that if you want to capture an image showing the centre of rotation then you'll probably need to be in the right part of the world. I think you're going to need more than a 15 minute exposure too; I suspect several hours minimum with a wide angle lens.

As there are two centres of rotation on opposite sides (N and S), one should always be visible (you might have trouble close to the equator, better find a an uncluttered view N or S )

[timo2]

Do you know of anything for the Southern hemisphere Tim?

There isn’t a central bright star (like Polaris) in the southern hemisphere but you need to look out for the Southern Cross as an approximation of the centre of rotation.

[yjiang]

Thank you very much for this knowledge! I will try next time.

[PopsPhotos]

For you Southerners:

http://www.windows2universe.org/the_universe/crux.html

Pops

[Colin Southern]

For you Southerners:

http://www.windows2universe.org/the_universe/crux.html

Pops

Crux my "rear end" - that's the Southern Cross (named after me, of course!).

[PopsPhotos]

Don't feel badly. Up here they spell "dipper" "bear."

Pops

[Rodders]

As said before, there are two points (North & South) that the stars rotate around and these are known as the Celestial Poles. The North one is marked by a fairly bright star in the constellation of Ursa Minor known as Polaris.

The Southern Celestial Pole is located in the Constellation of Octans and is much harder to find as there are no bright stars in the close vicinity. The picture below shows one way of finding the SCP.



As you can see, its a matter of using the Southern Cross and the Pointer stars (Alpha & Beta Centauri) to locate the SCP. This method will work any time of night at any time of the year and is handy to know in case you ever get lost.

[Colin Southern]

I'm feeling a star-trails competition thread coming on

[Rodders]

I'm feeling a star-trails competition thread coming on

That sounds like fun

[Donald]

That sounds like fun

Rodney
Are you well known for your use of irony?

In fact, as an exercise in learning about long exposure shooting through trying it, it's not that daft an idea. Just need to get a night without clouds which hasn't happened here for a while.

[Rodders]

Rodney
Are you well known for your use of irony?

In fact, as an exercise in learning about long exposure shooting through trying it, it's not that daft an idea. Just need to get a night without clouds which hasn't happened here for a while.

Hello Donald,

No irony here , long exposure star trail shots are a true art in themselves and one very hard to master. I first ventured into general photography just over a year ago through Astronomy & Astrophotography. This entrance into phototography has helped me to understand ISO, focus, light, low light & balance of light.

A star-trail competition will be fun, however like you said, trying to get a good night can be a problem at times.

[mastamak]

Here is a brief lesson in physical astronomy or, how to find the centre of a star trail circle. The stars circle around the celestial pole so you have to work out where that is. In the northern hemisphere you have a handy visible star called Polaris that is fairly close to the celestial north pole. If you can recognise Polaris then all you have to do is point your camera in that direction. The direction from your standpoint to Polaris is true north and the height of the star above the horizon is the same as your latitude (see later).

Here in the southern hemisphere we are not so lucky. Our circumpolar star is call sigma octantis and it is not visible to the naked eye. You should be able to pick it up in a photo taken with a good telephoto lens. The declination of sigma octantis is 89 degrees so if you manage to capture it in a photo it will describe a circle 1 degree in radius about the south pole. As a young surveyor I used to observe sigma octantis to obtain my position but nowadays it is all done with GPS.

Without the advantage of knowing where Polaris is to find the celestial pole you firstly need to know your latitude. You can look it up on Google Earth. If you are at latitude 30 degrees then to find the pole you need to look due north (or south) (use a compass) at a point 30 degrees above the horizon. That is, above the virtual horizon not the top those 3000m mountains that obscure your local horizon. If you don't have a sextant, theodolite, alidade or astrolabe then you will have to figure out some other way of measuring the height above the horizon. You can use your arm and fingers. Hold you right arm straight out with fingers and thumb fully extended. The angle between the tip of the thumb and tip of the little finger subtended at your eye is about 15 degrees. Two hand spans will give you the 30 degrees. To fine tune, the angle subtended by the distance between any two fingers is 4-5 degrees. I have used this method to find sigma octantis frequently. It gets you close to the spot and if you have a star chart and are familiar with the stars in the area you can soon hone in on the little blighter.

Grant

[Dave Humphries]

Thanks Grant,

You have a knack of explaining these things in terms I can relate to.

If I lived in the southern hemisphere and had a telescope, I'm sure I'd find the little blighter

Cheers,