The Eclipse Year

If the lunar orbit were fixed in space, such that the nodes occurred always in the same locations, then the Sun would pass through those nodes once per solar year. This is not the case, though; in fact the nodes are moving backwards around the lunar orbit, a motion that is termed precession. (This is similar to the way in which a toy gyroscope twists around.) In consequence the Sun gets to the nodes (where eclipses may occur) progressively earlier, producing a type of year that is somewhat shorter. This is called an eclipse year, lasting about 346.6 days, and it is a crucial cycle of time because it will affect the frequencies and characteristics of eclipses. In any calendar year one will usually find pairs of solar eclipses separated in time by close to half an eclipse year (173 days). An example is June 10 and December 4, 2002, a separation of 176 days.

Why is this slightly more than half an eclipse year? There are two contributing factors. First, the Sun does not move across the sky at a constant rate throughout the year, because our orbit is not precisely circular. The Sun's apparent speed is slowest around June/ July, when it is furthest from Earth (astronomers call this aphelion), and quickest around December/January, when we are closest to that orb (perihelion). Second, and more significant, eclipses do not necessarily occur precisely on the node, but rather there is a range of possible positions called the ecliptic limits. These limits are defined in the Appendix.

During each eclipse year, eclipses can take place only while the Sun and Moon are within the ecliptic limits, defining periods known as the eclipse seasons. The lengths of such seasons depend upon the eclipse type in question: Solar or lunar? Are they partial or total? We will discuss this matter shortly and show how the eclipse seasons come about in the Appendix.

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