Question of Convention for Astrometric Angular Units

[andand]

Hello,

Couple of quick questions here. In the Hipparcos and Tycho catalogs, RA is listed in HMS and DEC is in DMS (a point the supporting documentation recognizes as being computationally inconvenient). The obvious conversion from HMS to DMS involves converting HMS to "decimal hours" and then multiplying by 360/24 = 15 to get the decimal degrees. From there, the conversion to DMS is fairly straightforward.

So the first question, is this approach correct?

Now these catalogs also use milliarcseconds (mas) for some measurements, most notably relative motion and parallax.

The second question is which "arcsecond" or "milliarcsecond" is being used here, the one from the DMS convention or the one from the HMS convention (the latter being 15 times larger than the former)?

Any help answering this is greatly appreciated. Thanks.

-&&

[selden]

Unfortunately, Celestia uses fractional degrees for the RA of Star locations in STC catalogs and fractional hours in DSC catalogs. The conversion you describe is appropriate for STC catalogs.

Astronomical catalogs specify parallax and proper motion in degrees.

A brief discussion of proper motion and parallax is available at http://csep10.phys.utk.edu/astr162/lect ... roper.html

[andand]

Selden,

Thanks. This is very helpful.

-&&