Published December 2008,May 2008,December 2011,February 2011.
Up to about 1,900 BCE the Celestial Pole was Thuban a star in
the 'tail' of the constellation Draco. By 1,000 BCE it was Thuban
in the constellation Ursa Minor. Today Polaris is the last star in
the 'tail' of Ursa Minor
'Sun time' and 'clock time' are different. Sun time is based on
the fact that the sun reaches its highest point (the meridian), in
the middle of the day, and on the next day at its highest point, it
will have completed a full cycle. However, the time between the sun
reaching successive meridians is often different from clock time.
According to clock time, from May to August, the day is close to 24
hours, but in late October the days are about 15 minutes shorter,
while in mid February the days are about 14 minutes longer. For our
daily routines, it is important to have a constant 'clock time' of
24 hours. This variation is called the 'Equation of Time' and shows
the relationship between sun time and clock time. The variation has
two causes; the plane of the Earth's equator is inclined to the
Earth's orbit around the Sun, and the orbit of the Earth around the
sun is an ellipse and not a circle. The National Maritime Museum
website shows two separate graphs for these causes, and a third
graph where they are combined to give the full correction.
See National Maritime Museum - link below.