How do you calculate declination and Right Ascension from Earth Coordinates?

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Discussion Overview

The discussion centers on the calculation of declination and Right Ascension (RA) from specific Earth coordinates (40.23°N and 15.89°E) while examining the night sky. It explores the relationship between Earth-based coordinates and celestial coordinates, as well as the implications for stargazing.

Discussion Character

  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant questions the premise of the original inquiry, stating that declination and RA are coordinates of the sky and depend on the observer's line of sight rather than their Earth location.
  • Another participant agrees, emphasizing that starmaps showing RA and Dec can be used universally, and the only variation is whether specific coordinates are above or below the local horizon at a given time.
  • A different perspective introduces a formula for calculating RA and Dec, suggesting that RA can be derived from Local Mean Sidereal Time (LMST) and that declination can be equated to latitude, although this approach may imply a specific context or condition.

Areas of Agreement / Disagreement

Participants express differing views on the relationship between Earth coordinates and celestial coordinates. While some agree that declination and RA are independent of the observer's location, others propose a method for calculation that suggests a connection. The discussion remains unresolved regarding the implications of these differing perspectives.

Contextual Notes

There are limitations in the assumptions made about the relationship between Earth coordinates and celestial coordinates, particularly regarding the conditions under which RA and Dec can be calculated from a specific location.

Brynjolf
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Say a person is positioned here: 40.23°N and 15.89°E and was examining the night sky.

How do you calculate the declination and Right Ascension from that location's coordinates?

I know the RA is measured in hours up to 24 and Declination in degrees.

Any ideas?
 
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Your question doesn't make much sense. Declination and Right Ascension are coordinates of a point on the sky. So it depends not on where you are on Earth, but on where in the sky you are looking. At any time, you can see half the sky, so you could be looking at a wide range of RA and Dec coordinates.
 
Hi Brynjolf

welcome to PF :)

yes I have to agree with physguy on that one. Starmaps/atlases are produced showing RA and Dec co-ordinates of the sky. You are able to use that star map anywhere in the world without having to do any conversion.

The only thing that varies is whether a particular RA and Dec co-ordinate is above or below your local horizon at a given time of the day/year from you current location

cheers
Dave
 
Straight up Azimith = 0 and Altitude = 90 at":

40.23°N and 15.89°E

RA = LMST

DEC = Latitude = 15.89°E

LMST (Local Mean Sidereal Time = GMST0 + time * 15 + Longitude)
GMST0 (Greenwich Mean Sidereal Time at Midnight)

https://www.physicsforums.com/showthread.php?t=636728
 
Last edited:

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