# 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:

• Terminology. "The zenith" is the point directly above you. It's probable that Bootes will never reach the zenith where you live. You probably mean "When does Bootes culminate?" That means "reach the highest point that it can reach". Jan 31, 2021 at 9:17
• Another useful piece of terminology in this context is "meridian". Jan 31, 2021 at 10:02
• @JamesK Yes, thanks for the clarification. That is indeed what I meant. Jan 31, 2021 at 18:56

The key number you need to find is the "right ascension" This tell you the "longitude" 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.

• And the constellations are pretty big, so there is no need to calculate it accurately, let's say +- 0.5 h. Jan 31, 2021 at 19:39
• Very interesting, thanks for your answer. Does the location of the observer affect the time of the culmination? For example if one person is in Los Angeles and the other is in Moscow, Russia, for both of them the constellation will culminate at 2:30 PM local time on the day of spring equinox? Also, is the right ascent the only thing we need to know to determine the culmination? So if the right ascent of a constellation is 16 hours, I understand that it will culminate at 04:00 local solar time on the day of the spring equinox? Jan 31, 2021 at 20:42
• No, it won't culminate at the same time. Take for example star Sun. Does it culminate (= noon) at the same time? Of course not. And yes, the only thing we need is right ascension (= RA). Jan 31, 2021 at 20:46
• @User123 So assuming +- 30 minutes difference, will it then culminate at about the same time? What I'm interested in is if in the example above the culmination time of the constellation for both Los Angeles and Moscow will be about the same. The reason I'm asking is that I found a culmination time for the adjacent constellation Corona Borealis and it says that on the day of the spring equinox it is at 3:30 AM Los Angeles time. But according to the calculations above if the right ascent is about 15 hours 30 minutes, that makes the culmination at 3:30 PM so I'm confused. Jan 31, 2021 at 21:34
• @User123 I think you're slightly confused on this point. Eg, if a star transits the meridian at Greenwich at 14:30 UTC then it will transit the meridian at 45°E at (almost) 14:30 Moscow time. Feb 1, 2021 at 1:13