Celestia Forums

IN my attempt to understand RA and DEC coordinates and the strange way they are plotted according to the earth's location has made it difficult to do anything more than eyeball objects and plot them as they appear...Now knowing how many stars there are and how many galaxies and nebulae...This will take 100 trillion years to complete....not that I plan to add everything in...

OK what I want to know is the math behind converting RA and DEC into degrees that I can use to plot them in my program properly...or close enough as it may be...I have supplied some simple diagrams showing how I have set up my points of reference...

http://celestialvisions.net/pics/milkyc1.jpg

http://celestialvisions.net/pics/milkyc2.jpg

The commands you see below is what I use in my scripting interface to plot these coordinates...As you can see its very simple and straight forward and works rather well...But of course is tedious when trying to match existiong data by eye alone...What I want is a kind of conversion system based on these figues...if at all possible...

Ras',

RA (Right Ascension) and Dec (Declination) are exactly the same as longitude and latitude respectively, except that they are projected onto the sky.

Instead of Greenwitch, though, RA is measured from the location of the Vernal Equinox. Also, the units are different for RA: it's measured in "Hours". 1 hour is the same as 15 degrees.

Does this help at all?

Hi Rassilon,

here is a quote from my STC guide, for the RA and Dec cordinates.


[quote]RA = Right Ascension, the angle in degrees in the equatorial plane of the earth, measured from the Vernal Equinox where the star is located. The angle runs counterclockwise. A bit further down is a figure with an explanation of the Vernal Equinox.
Celestia uses degrees with decimals for the RA, instead of the in astronomy normally used format of hours (h), minutes (m) and seconds (s). Therefore perhaps a conversion of the angles has to be undertaken, if a star is placed.
1h = 60m and 1m = 60s, a full circle of 360 degrees represents 24h, so 1 hour is the equivalent of 15 degrees.
To convert any given RA from time to degrees use the following equations:
RA (Time) = hours + minutes/(60 min/h) + seconds/(3600 sec/h)

RA (Degree) = RA (Time) * 15 degrees/hour
Example: RA = 11h 27m 35.9s

RA (Time) = 11+27/60+35.9/3600 hours = 11.45997222 hours

RA (Degree) = 11.45997222 hours * 15 degrees/hour = 171.8995833 degrees

Dec = the Declination is the angle between the connecting line between earth and star and the celestial equator. It defines how far above or below the equatorial plane the star is positioned. Celestia uses degrees with decimals for this purpose. In astronomy on the other hand the declination is given in the format of degrees, arc minutes and arc seconds. So the conversion to degrees works a bit different from that of the RA.

1 degree = 60?€™ (arc minutes) and 1?€™ = 60?€

You will find the STC-guide on the Celestia Motherlode:

http://www.celestiamotherlode.net/catalog/documentation.html

Adirondack