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

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!

• Are we expected to presume neither the moon nor any of the planets are in view? Commented Apr 10, 2016 at 5:06
• Getting the horizon from just a photo will be difficult, as local topography varies a lot. Commented Apr 10, 2016 at 5:43
• Requiring an event that's going to be available at least once a week is acceptable. So, having the moon in-frame is totally reasonable. 1 planet should also be easy enough to capture, but I'm not sure if requiring 2 planets is reasonable. Commented Apr 10, 2016 at 23:22

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.

• Thanks, this answer is helpful. Long exposures are okay. How likely is it that 2 planets would be in the shot? Was brainstorming with an astronomer, and it seemed that using parallax you could tell the season. A planet would give you information about the year, and the moon would be your best bet at telling the current minute. Requiring the moon to be in-frame seems completely reasonable. Requiring a rare event like a nova is not reasonable. Commented Apr 10, 2016 at 23:14
• Knowing the date of the photo might be too much. Knowing the month + country/state is probably okay, but direction is probably not. Commented Apr 10, 2016 at 23:25
• "using parallax you could tell the season". Do you mean the parallax between nearby stars and stars further away? If so, that would require extremely high resolution on the photograph. Also, if you can get Saturn (or even Jupiter) in frame, you can do a pretty good job of finding the approximate date, which may help find the approximate time. Do you have sample photos you're willing to share? Finally, I assume you're familiar with astrometry.net ?
– user21
Commented Apr 12, 2016 at 22:23