<p>Zenith right ascension = sidereal time.</p>

<p>Zenith declination = geographic latitude.</p>

CC BY-SA 3.0
2016-08-15T21:49:20.623Z
Could you develop your answer more? Right now this could fit as a comment.
Sir Cumference
CC BY-SA 3.0
2016-08-16T03:41:15.923Z
Following site instruction "Avoid answering questions in comments." OP has connected the other dots. What's missing?
Mike G
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2016-08-15T15:43:01.283Z
2016-08-15T15:43:01.283Z
Mike G
<p>I'm writing a script to generate a star chart for a given observation point (latitude and longitude) at a given time.  I've found a database of stars with their right ascension and declination from J2000, and with that written a program that displays a star chart with the center being any right ascension and declination that I feed into it.  I've also written a program that will convert local time into local sidereal time. Now I just need to figure out how to determine the zenith for an observer in J2000 right ascension and declination given their longitude, latitude, and local sidereal time.</p>

<p>I've found a wikipedia page that gives formulas for converting between celestial coordinate systems, but the formula for converting equatorial and horizontal coordinates doesn't seem to depend on the sidereal time, which I imagine it should for my case.  </p>

<p>I've tried finding a vector that represents the observer's zenith at the given time, then rotating it about the earth's rotation axis by the sidereal time, but that's giving incorrect results.</p>

CC BY-SA 3.0
2016-08-15T13:55:48.710Z
Im curious why your writing your own script when there are loads of software packages out there that can do it for you, is it out of your own curiosity?
Dean
CC BY-SA 3.0
2016-08-15T14:12:29.440Z
@Dean: Yeah, as you can probably tell from my question, I'm more of a programmer dabbling in astronomy than an astronomer dabbling in programming.  I found the star database I mentioned online and I thought it would be neat to do something with it.
sgfw
CC BY-SA 3.0
2016-08-15T14:21:04.553Z
Ah okay cool, well I think what your essentially talking about is converting Right Ascension & Declination (RA/Dec) coordinates into Altitude & Azimuth (Alt/Az) coordinates; this might help with your online searches in the future. I was doing something similar myself until I found a software package that did what I needed, but before I found it I came across [this](http://www.slideserve.com/griffith-fields/phys216-practical-astrophysics-lecture-2-coordinate-systems-1) lecture on coordinates which was really useful.
Dean
<star><coordinate>
How can I map the zenith for an observer at a given time to equatorial coordinates (as would be measured in J2000)?
sgfw
# How can I map the zenith for an observer at a given time to equatorial coordinates (as would be measured in J2000)?

(Asked by: username_0 on 2016-08-15)

I'm writing a script to generate a star chart for a given observation point (latitude and longitude) at a given time. I've found a database of stars with their right ascension and declination from J2000, and with that written a program that displays a star chart with the center being any right ascension and declination that I feed into it. I've also written a program that will convert local time into local sidereal time. Now I just need to figure out how to determine the zenith for an observer in J2000 right ascension and declination given their longitude, latitude, and local sidereal time.

I've found a wikipedia page that gives formulas for converting between celestial coordinate systems, but the formula for converting equatorial and horizontal coordinates doesn't seem to depend on the sidereal time, which I imagine it should for my case.

I've tried finding a vector that represents the observer's zenith at the given time, then rotating it about the earth's rotation axis by the sidereal time, but that's giving incorrect results.

**Tags:** star, coordinate
> **Comments:**
> **username_1**: Im curious why your writing your own script when there are loads of software packages out there that can do it for you, is it out of your own curiosity?  
> **username_0**: @Dean: Yeah, as you can probably tell from my question, I'm more of a programmer dabbling in astronomy than an astronomer dabbling in programming.  I found the star database I mentioned online and I thought it would be neat to do something with it.  
> **username_1**: Ah okay cool, well I think what your essentially talking about is converting Right Ascension & Declination (RA/Dec) coordinates into Altitude & Azimuth (Alt/Az) coordinates; this might help with your online searches in the future. I was doing something similar myself until I found a software package that did what I needed, but before I found it I came across [this](http://www.slideserve.com/griffith-fields/phys216-practical-astrophysics-lecture-2-coordinate-systems-1) lecture on coordinates which was really useful.  

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## 1 answers

### Answer by username_2 on 2016-08-15. Score: 3

Zenith right ascension = sidereal time.

Zenith declination = geographic latitude.

> **Comments:**
> **username_3**: Could you develop your answer more? Right now this could fit as a comment.  
> **username_2**: Following site instruction "Avoid answering questions in comments." OP has connected the other dots. What's missing?  

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