date and time in J2000 format – this is the date standard for functions in AstroExcel, we use the days2000() function for this.

I want to know the Altitude and Azimuth of Messier object M1 for my location at 22:00 on the 1st January 2015. 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 have included in AstroExcel the Messier catalogue with a column showing the RA degrees so you can get used to this. 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.

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. 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?’.

Stellarium web find alt and az