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Frequency of a Lunar Eclipse
Historically, lunar eclipses occur twice a year. Usually, one occurs in a warm month and one in a cold month. Of course, there are exceptions to this. 2005 saw eclipses in March and October, while in 2008, only one total eclipse occurred -- in February. Since 2009, both partial and total eclipses have occurred in both June and December each year. NASA uses a complex calculation called the Saros Series to predict lunar eclipses well into the future.
Total Lunar Eclipse
To understand eclipses, it is important to learn about the types of shadows that can occur during an eclipse. An umbra is described as the full shadow of the Earth on the moon, while the penumbra is the partially shaded outer region of a shadow. For a total lunar eclipse to occur, the moon must past directly through the Earth's umbra, or darkest shadow. The longest total eclipse ever recorded was in June 2011 and lasted a little more than 100 minutes. The actual progression of an eclipse can last about four hours.
Partial Lunar Eclipse
A partial lunar eclipse occurs when only a small piece of the moon enters into the Earth's umbra. According to NASA, partial eclipses have only occurred six times in the past 10 years. More frequent than a partial lunar eclipse is the penumbral eclipse. A penumbral eclipse appears as a slight darkening of the moon rather than the complete darkness or red appearance of a total lunar eclipse. This occurs when the moon enters the Earth's penumbra.
Winter Solstice Eclipse
In 2010, a total lunar eclipse coincided with the winter solstice on December 21. This rare occurrence happened once before 372 years earlier. Scientifically, the coincidence of both of these events occurring on the same day has no significance. However, Wiccans, astrologists and many others believe that these two events occurring on the same day is quite significant to their beliefs, signifying a time of renewal. The next time these two events are set to occur on the same date is December 21, 2094.