What is the right ascension of a star that is on the meridian at midnight at the time of the autumnal equinox?

Answer 1

We can figure this out using a little knowledge and a little logic. The Sun is at 0 hours RA on the vernal equinox (that's the definition of 0h RA) so it must be at 12 hours RA on the autumnal equinox. At midnight, the Sun is below you, so anything on your meridian must be 12 hours away from the Sun. Therefore a star on the meridian must be about 0 hours RA.

(Of course, this does not account for daylight savings time, or where you are in your time zone).

Answer 2

Both good answers above.
You also need to know, however, how many hours between the actual equinox and your local midnight -- since the equinox won't necessarily be at your local midnight :)

But the first answer gave a good approximation.

Answer 3

You'll need to know what year as the autumnal equinox varies in time and day each year.