How accurately can you tell the time using a photo of the stars?

Question

If I were to go out on a clear night, in an area with low light pollution, and take a photo of the stars, could an expert analyze the photo and figure out what date the photo is from? We can assume a powerful camera, but not reaching up to the strength of a telescope.

This question stems from a larger project. It would be very convenient for me if it were possible to take a photo of the stars to tell the time. Even more convenient if you could narrow it down to within 10 minutes, but if that's not possible I'm wondering how close we can get. Can you tell the year? The millennium? Do you need to know which location on Earth the photo was taken from? Does it help if you already know the year in which the photo was taken?

You can assume access to modern computing, existing databases, state-of-the-art software, etc. (no supercomputers)

Thanks!

Answer 1

To make a clock you need something that changes predictably with time, and stars are nearly constant, making them poor clocks.

The stars do move across the sky each night, but since the morning stars of winter become the evening stars of spring, and the sky looks different in different locations you need to know the date of the image, the location of the photo, and the exact direction of the photo. That could give sub 10 min accuracy. Unfortunately while date and location may often be recorded with a photo, exact direction is not, unless the horizon is visible in the image.

Without that information, and it is possible to 'get lucky'. For example, if there are two or more planets in the shot, their locations could be measured to give at least the date of the photo. Similarly if a rare event like a nova is visible that can offer dating information. If the shot happened to catch a bright meteor, and the date was known well enough, you could get almost 1 second accuracy. Variable stars might also give the chance of at least dating the image.

With no planets you can use the very slow proper motion of the stars. With a good enough telescopic shot of the night sky you can be quite accurate, but with a wide view of a camera you wont be able to get enough accuracy to give any useful date.

The exposure time of a typical camera is to short to capture any stars. If you just point and shoot at tye sky, you just get black. To capture images of stars you would need a timed exposure. A telescope would help too.

So if you photographer wanted to create an image which could be timed accurately, and they had time to prepare for it, or it was a lucky shot this might be possible, otherwise not.