Astrotags explained

What are astrotags, how do you add them to photos and why should you bother?

Astrotags are a new way to label your astronomy photos with their celestial subject and its location. This short film shows you how. So have a watch, then read on for extra info. If everyone joins in we can make a beautiful and accurate map of the night sky... so pass the word on.

Introducing astrotags from Royal Observatory Greenwich on Vimeo.
Film by Jim Le Fevre and Mike Paterson. Thanks to Blur and EMI Records for permission to use 'Far Out'.

Get astrotagging

Want to astrotag your photos? Thanks to, astrotagging is super easy.

To create most of the astrotags we’re after, you just need to add your pictures to the Astronomy Photographer of the Year group on Flickr. There, a specially built robot is busy going through all the photos and using the geometry of the stars to work out and add astrotags automatically.

But there are two astrotags that we’d be really grateful if you could create yourself.

Date and time

The first astrotag we'd like you to add describes the exact date and time your photo was taken, in Greenwich Mean Time. This tag needs to take this format:

  • astro:gmt=yyyy-mm-ddThh:mm

yyyy is the year, mm is the month, dd is the day, hh is the hour (using the 24 hour clock), and mm is minutes. T separates the date from the time. So if, for example, your photo was taken in London at 9.30pm on 14 January 2009, your astrotag would be:

  • astro:gmt=2009-01-14T21:30

If your photo is a long exposure please use the time you started taking the picture.


The second astrotag we’d like you to add describes the main astronomical subject of your photo, using its English name or letter and number combination. Don’t worry if you don’t know what your photo shows. The robot will return a list of all known astronomical objects in your picture. All you need to do is rollover your picture in Flickr to see what’s what, then create your astrotag in the following format:

  • astro:subject=

Here’s an example.

M45 redone (Astrotags explained)By johnny9s
Some rights reserved

The astrotag for this photo would be:

  • astro:subject="NGC 1432"


  • astro:subject="Maia nebula"

Subjects with spaces should be enclosed with "..."

Because the Astrometry robot uses the stars to calculate what's in your picture, it isn't able to label planets, moons, or other moving subjects such as comets. If your photo is of one of these astronomical subjects, but you're not sure which, post a message to the group. We'll help if we can.

More info

That’s the basics covered. But if you’d like to find out more about the science of astrotags, including what each astrotag means, you can. We've also got the inside story on how the robot works. And if you’re a developer we’d love you to use astrotags to build your own astro-photo stuff. If you do, tell us and we’ll try to showcase it on this website.