Friedrich Wilhelm Bessel (1784-1846) was a
German mathematician and
astonomer. He observed a great number of stars from the
Konigbergs observatory and is known for computing the first precise
constant for the
precession constant.
Because of the way the earth moves (wobbles in a way once every 26'000 years, see precession) the positions of stars and other objects in space will apear to move 0,01 degrees from west to east each year. When calculating with positions in space, it is therefor necesary to relate positions to a certain epoch in time. For example, the positions of certain stars today are different from the positions they had 100 years ago.
Until 1984 the Besselian year was used, and the last Bessilian Epoch is B1950, which relates to the mean equator and equinox of 1949 December 31st 22:09 UT.
The Besselian year was for a long time a more convenient system to use
in astonomy. A Besselian year is one complete revolution in right ascension of the fictitious mean sun as defined by Newcomb. A year starts the moment when the ecliptic longitude of the mean sun is 280 degrees (280 degrees was chosen because this is near the start of the calendar year), and is about 365.2422 days long. The beginning of the year is denoted by the suffix '.0', for example B1923.0. (The B is used for destinction from the epochs defined by the Julian epochs which have been used since 1984.)
Today, we are in the epoch known as J2000. It's referencepoint is the mean equator and equinox of year 2000 January 1st 12:00 hours Universal Time (UT).
For more information on space and mesurements and maths concerning space, see: http://www.jpl.nasa.gov/basics/