How do I find out when a constellation reached its zenith? I found this website to look at the sky at the specific time and location: http://www.fourmilab.ch/cgi-bin/Yoursky - you can insert latitude and longitude and time to see how the skies looked at any location/time. What I need to find out is when a particular constellation reached its zenith. I see that if I change the Universal time the location of the constellation in the sky changes, but where is the zenith of a constellation? The constellation in the question is Bootes. How do I know its zenith time let's say for today? preferably using the website above.
Picture example from the website:
The key number you need to find is the &quot;right ascension&quot; This tell you the &quot;longitude&quot; of the constellation, relative to the other stars in the sky.  It is measured in hours and minutes
Bootes has a right ascention of about 14:30  This means that on the day of the spring equinox (March 21) it will culminate at 02:30  (in local solar time, you'll need to make adjustments for daylight saving time or if your local noon isn't at 12:00) Note that 14:30 is 2:30 pm
Every month it will culminate two hours earlier So on April 21 it will culminate at about 00:30  (conveniently there are 12 months and 24 hours)
Working the other way, On January 21st it would culminate four hours later, or at about 06:30, and now, in Early February it culminates at about 06:00
This kind of calculation is good enough for a rough estimate.  More accurate calculations can be done, but are more convenient to get a computer to do them.