There are no less than three different kinds of “day”: i) sidereal, ii) astronomical or solar and iii) solar or mean.
- The Sidereal Day is the time of one complete revolution of the earth – e.g., from one transit of any given fixed star across the meridian (mid-heaven) till its next transit thereof.
- The Astronomical Day is similarly the time between successive transits of the Sun’s semi-diameter across the meridian and is there about 1 degree (or 4m.) longer.
- The Mean Solar Day is twenty-four hours of “mean time,” as ordinarily used for all civil purposes and is measured by a second’s pendulum of 39’13929 inches in length for the latitude of Greenwich.
Off these three kinds of “Day” we are only concerned with
the first and third, which furnish us with our “Sidereal” and “Mean” time by
which our calculations are made.
The reason why the astronomical day is not used, is that
owing to the Sun’s irregular motion, sometimes more and sometimes less than 1
degree, the true astronomical day is not a convenient standard, being sometimes
less and sometimes more than 24h. 4m. of sidereal time. This discrepancy it is
which gives rise to the “Equation of Time” found in certain old Ephemerides
calculated for ‘apparent noon.’ Astronomers therefore adopt as their standard
the mean solar day.
The mean solar day is simply the average value of the
varying solar days throughout the entire year and therefore actually consists
of 1/365¼ part of the Equinoctial year – which is the time between two
successive passages of the Sun across the equator from S. to N., or in other
words its entry into Aries.
