I've always wondered how someone looking at the sky into the enormous amount of stars and other things out there figures out that whether it is an already identified object or is he looking at something new? Is there some sort of tools like latitude and longitude to pin point something in the sky or some catalog where you can see if this is an identified object and what it is called, how it looks like and where it is located and all?

As you might have guessed, I don't have any science background, just asking out of a common man's curiosity, so please be gentle with the jargon you use.

  • $\begingroup$ Just to note, for a long time you didn't. For instance, Messier published his catalog not intending to document galaxies, nebulas and such, but to help identify things which weren't comets. And dying stars were mistakenly identified as being "new". $\endgroup$
    – Michael
    Feb 23, 2020 at 0:55

1 Answer 1


There exist many astronomical catalogs, reaching from rather simple catalogs like the Bayer catalog for stars, or the Messier catalog for galaxies, nebulae, etc., to huge catalogs with millions of entries. A catalog with about a billion objects is scheduled for 2022.

There exist also printed editions of some catalogs.

Objects in the sky have several properties, by which they can be described and mostly identified.

One of the most frequently used properties is the position in the sky. From the local geographic position and the time, the observed position of the object can be translated into a celestial coordinate system, e.g. the equatorial coordinate system. The position of an object is described in this system by declination and right ascension.

Two other frequently used properties are magnitude and color.

Also frequently used are patterns of stars, meaning their relative position to each other, beginning with star constellations.

The analysis of steller spectra allows a more detailed identification.

Longer term observations may uncover variability and proper motion as additional properties.

Here is a description of parameters (including properties) provided by the BSC5p catalog.

  • $\begingroup$ What all details will this catalog contain? Can you give a little more details about the procedure one may go through to locate and identify an object he just saw in the sky in this catalog? I can assume it might require some skill and knowledge about this but could you give a general idea? $\endgroup$
    – Mat J
    Feb 8, 2014 at 18:44
  • $\begingroup$ To use a simple example - one could go out at this time of year and (in the Northern Hemisphere) see Sirius, which is the brightest star in the sky, or Polaris, which is essentially due North and unmoving as a result. From that beginning we can look at the pattern in the atlas/catalogue and recognise the patterns from there. $\endgroup$ Feb 9, 2014 at 23:37
  • $\begingroup$ Yes, that's one of the usual ways to it. First look at the patterns of brighter stars for an overview, then go down into detail for fainter stars. Not far from Sirius you'll see e.g. Jupiter (moves over time), the brightest most object (besides Sun and Moon) at the moment. You'll also find the constellation Orion not far away from Sirius. At Orion you may e.g. go down into detail. $\endgroup$
    – Gerald
    Feb 10, 2014 at 11:46
  • $\begingroup$ Here a nice introduction to february night sky, northern hemisphere: youtube.com/watch?v=AvCK4dJQrSw $\endgroup$
    – Gerald
    Feb 10, 2014 at 12:10

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