Macrobius
Megaphoron
The interval of time required for the right ascension of the fictitious mean sun to increase by 24h. It is defined as beginning when the mean sun's longitude is precisely 280° (or RA 8h 40m); this value was chosen as it corresponds roughly to January 1. The Besselian year is virtually identical to the tropical year and is 365.2422 days long. It was the basic unit of the now-superseded Besselian epoch.
Calculating Besselian Epoch - Boyce Astro
This lesson will show how to convert image dates to Besselian, which is the recognized standard for Double Star research.
boyce-astro.org