Although the speed of light is very fast, it is not infinite,
and the incredible astronomical distances we are dealing with
can lead to easily measurable timing effects due to light's finite speed.
One such light time effect occurs because of the Earth's orbit around the Sun.
Although the Earth's semimajor axis is only 8.3 light minutes in size, this moving
platform of an Earth can therefore be several light minutes closer to or further
away from stars, especially those near the ecliptic. To account for this variation,
astronomers recalculate Julian Days as if we were observing the star from the center
of the Sun, i.e., a Heliocentric Julian Day. This is usually calculated by computer.
For example, 10/09/1995 12:00 UT gives Julian Date 2450000.0.
Use the "view source" option on your browser to view the script used to
perform these calculations.
See also Heliocentric Julian Day Using Excel.