For the Astronomy Calculator, it was recommended to use double precision (8-byte precision). I researched this is my ASP.NET book and found that data type. It was described as having the ability to hold large fractional numbers but not as accurate as Decimal. Decimal was described as having the ability to store extremely large fractional numbers.
So, I don’t know exactly which one to use but decided to go with Decimal until I notice a performance or memory issue.