#1
Jun2811, 11:34 PM

P: n/a

I am seeking help calculating the Right Ascension.
What I would like to know specifically, is if the Right Ascension can be calculated knowing the Zodiacal Longitude or Celestial Longitude of a plant, body or star. For example, for a body at 11° Pisces 57', which is 341°57' of Celestial Longitude, how can I determine the Right Ascension? Other variables that I know are the Sidereal Time and the Local Sidereal Time. I also know the terrestrial geographic Latitude. Variables that I can calculate (which perhaps might be helpful) are the Declination (the Arcsine[Sine(Obliquity) * Sine(Celestial Longitude)]). I can also calculate the Ascensional Difference (the Arcsine[Tan(Declination) * Tan(Latitude)]). Would it be possible to calculate the Oblique Ascension based on any the variables I know? If I could do that, then I could use the Oblique Ascension and Ascensional Difference to find Right Ascension. I am familiar with: Hour Angle = Local Sidereal Time  Right Ascension. However, I don't really understand what the Hour Angle is, other than I have seen it repeatedly defined as "the amount of time since an object has crossed the meridian." Which meridian? The location of the observer, or is the meridian a fixed point? Anyway, it appears that if the Right Ascension is 0° then the Hour Angle = Local Sidereal Time. For shats and gaggles, I set up a spread sheet and using the Local Sidereal Time, just calculated the Hour Angle using the Right Ascension in 15° increments to 360°. What I saw was the Hour Angle decreasing as the Right Ascension increases, but I don't understand the relationship between the two. Does the Hour Angle equate to Oblique Ascension or Celestial Longitude? Am I not approaching this right? Am I missing something here? Thanks in advance for anyone's help. 



#2
Jun2911, 12:25 AM

HW Helper
P: 2,324

What's zodiacal longitude? What's celestial longitude? They're not standard astronomy terms, and a Google search doesn't give many clues. I suspect "zodiacal longitude" is the same thing as right ascension, just expressed in a different format.




#3
Jun2911, 02:08 AM

P: 6,863




#4
Jul611, 05:57 PM

P: n/a

Help with Right Ascension and Hour Angle
Thanks for your replies.
I am good at "plugging in" the variables and constants, if I know what the equation is. I understand that certain Zodiac Signs cross the Meridian (or maybe it's the Horizon) faster than others. I have continued to search and did find a fairly good treatise on the subject called Modern Almagest. (It's in Adobe format here if anyone is interested http://farside.ph.utexas.edu/syntaxis/Almagest.pdf) I have attempted to read through it, and as best as I can tell, I think the Right Ascension is expressed as RA = Inverse Tangent (Cosine Obliquity * Tangent Longitude) I have no idea what that means, but my Excel 2002 spreadsheet has those functions, so I'll plug in some numbers and see what happens. 



#5
Jul611, 06:47 PM

P: 365

This formula among others can be found in Wikipedia under "Ecliptical Longitude". 


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