The most common coordinates in astronomy are ‘Right Ascension & Declination’ (RA/DEC) and ‘Altitude & Azimuth’ (ALT/AZ) both are useful and converting RA/DEC to Alt/Az is important if you want to answer questions like ‘What altitude is the moon at the moment?’.
Right Ascension is normally represented in the units Hours and Minutes but in AstroExcel I have chosen to use degrees, it is easy to convert from RA hours to degrees – just multiply by 15 and for degrees to RA just divide by 15. All the functions in AstroExcel that ask for RA will ask for it in degrees and all functions returning RA will return them in degrees. I have included in AstroExcel the Messier catalogue with a column showing the RA degrees so you can get used to this.
To calculate Alt/Az you need to know your location on Earth (longitude and Latitude) – North and East are positive, South and West negative, this is a standard convention – plus the date and time (UT) represented in days after 12:00 on the 1st Januaray 2000 (see here for help)
Let’s take a closer look at AltAz conversion.
I want to know the Altitude and Azimuth of Messier object M1 for my location at 22:00 on the 1st January 2015.
- date and time in J2000 format – this is the date standard for functions in AstroExcel, we use the days2000() function for this.
- Declination of object of interest
- Right Ascension of object of interest
- Longitude of observer (Sounth -ve)
- Latitude of observer (West -ve)
- an index to say if the result should be an altitude (index= 1) or an azimuth (index=2)
The rectangular() function will convert spherical co=ordinates into rectangular (Cartesian) co-ordinates
- Index = 1 = Return x co-ordinate
- Index = 2 = Return y co-ordinate
- Index = 3 = Return z co-ordinate
The spherical() function will convert rectangular (Cartesian) co-ordinates into spherical co-ordinates.
- Index = 1 = Return r co-ordinate
- Index = 2 = Return theta co-ordinate
- Index = 3 = Return phi co-ordinate
The rLength() function is used if you have the Cartesian co-ordinates and want to find the distance to the origin (0,0,0)
From equatorial or ecliptic coordinates of two objects find their angular separation, all in degrees, i.e. RA is in degrees not hours, This function is useful for finding the distance between two objects such as the planets after you have calculated their positions.
User RADEC() for converting from altitude and azimuth to RA and Dec coordinates.
|RADec(J2000, Altitude, Azimuth, latitude, Longitude, index)||J2000 is the date in J2000 format
Altitude in degrees
Azimuth in degrees
Latitude in degrees East is negative
Longitude in degrees South is Negative
index is 1 for RA and 2 for Dec. in degrees