How are space phenomena named?

Question

More specifically, how does a name like SDSS J010013.02+280225.8 (one of the most massive black holes) come about. What do the numbers mean?

In: Other

Answer 1

SDSS stands for Sloan Digital Sky Survey, a detailed map of celestial objects made over the past 15 years.

The numbers describe the position of the object in the sky. We imagine celestial objects are projected onto a [spherical bubble](https://en.wikipedia.org/wiki/Celestial_sphere) surrounding the Earth, and describe their coordinates in terms of “latitude” and “longitude” on this celestial sphere.

The celestial equivalent of latitude, called “declination”, is simply measured in degrees minutes and seconds, as on Earth’s surface. So for example the north star, Polaris, is near +90 declination.

But the celestial equivalent of longitude, called “right ascension”, is measured in hours, minutes, and seconds, with 24 hours corresponding to one full circle: this is handy because the Earth rotates with respect to the celestial sphere once every 24 hours.

So “SDSS J010013.02+280225.8” means “The object listed in the Sloan Digital Sky Survey at right ascension 01 hours, 00 minutes, 13.02 seconds, declination 28 degrees, 02 minutes, 25.8 seconds.” Plug that into a [digital sky map](http://www.avastronomyclub.org/skymap/d/skymap.php) and you’ll find that that’s in the constellation Pisces. If you go outside tonight and see a bright orange “star” high in the sky, that’s Mars, and these coordinates are a bit north of that.

The last question to answer is what’s the “J” about. That’s related to the question, where is zero “longitude”? It’s an arbitrary choice. On Earth, we choose Greenwich, UK as the zero point for historical reasons. In the sky, we choose the location of the Sun in our sky on the spring equinox. However, the Earth wobbles on its axis every [26,000](https://en.wikipedia.org/wiki/Axial_precession) years, so the position of the spring equinox changes from year to year. The “J” indicates that we’re using the J2000 system, measuring with respect to where the spring equinox was in the year 2000.