| Table
of Contents
View the Stars
Measuring Position
Important Points
The Earth's Tilt
Effects of Tilt
Sun's Motion
Stars' Motion
The North Star
Precession
Timekeeping
Sun as Timekeeper
Sideral Time
Ceasar's Calender
Ceasar's Solution
Does Earth Spin?
External Links
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It was quickly
observed that the Sun does not make a good time keeper. That is,
the lenght of an apparent solar day is different depending on the time
of the year. Why do you think this problem exists?
There are two main reasons why the Sun makes a poor timekeeer.
- The Earth's orbit is not a perfect circle.
- The Earth's orbit is really an ellipse. As the Sun moves around the sun on it's
elliptical path the Sun's speed changes. The Sun travels faster when it is near the
Sun than when it is farther away from the Sun.
- The ecliptic is titled by 23.5o to the celestial equator.
- A good portion of the Sun's motion near the equinoxes is north to south. This
causes the net daily eastward progress in the sky to be somewhat shortened. However,
near the solstices, the Sun's motion is nearly parallel to the celestial equator, so there
is very little shortening near the beginning of summer and winter.
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