# Sidereal Time

1. Oct 7, 2009

### Starproj

Help!

I need some clarification on definitions, because it seems like I am getting contradictory information.

My textbook defines sidereal time as simply the right ascension that is on the local meridian. It further defines sidereal time as being the RA of a star + the hour angle of the star, which makes sense, as if the hour angle was zero, the star is on the meridian, and it's RA would be the sidereal time. So once again, sidereal time equals the RA on the local meridian.

I get that and am ready to accept that definition.

But then I am presented with a specific: the vernal equinox. At noon on the vernal equinox the sun is on the meridian. From my understanding based on the definition above, the sidereal time would be 0 hours, as RA = 0 is on my meridian. But the book claims that I have to rotate earth 180 degrees and that sidereal time is equal to 12 hours - RA on meridian.

So it seems I have two contradictory definitions of sidereal time:

1) Sidereal time = RA on local meridian
2) Sidereal time = 12 hours - RA on local meridian.

They can't both be right. What am I missing? What is the book trying to differentiate?

2. Oct 7, 2009

### mgb_phys

Isn't sidereal time from midnight