Tonight, July 31st, a rare occurrence will happen. You can say that it is something that happens once in a blue moon, because it is a blue moon.
What do we mean by a blue moon? There are actually two definitions of what a blue moon is.
- A blue moon is the third full moon in a season where four full moons occur. Most times, there are only three full moons in one season, but on the rare occasion when a full moon is at the beginning of the season, the season may have four.
- A blue moon may also mean the second full moon in a calendar month. The full moon of tonight is this type of blue moon.
Why do blue moons occur and why are they rare? If you go back to when we discussed the
lunar cycle and the
synodic period of the moon, we know that the time from one full moon to the successive full moon is approximately 29.5 days. Calendar months last anywhere from 28 days to 31 days. Therefore, if a full moon is on the first of a month, that month will have a blue moon (second definition). The seasons are, on average, a little over 90 days. As stated above, if the full moon occurs on the solstice or equinox (or a day after), it is possible to have four full moons in the season.
If you miss the blue moon tonight, don't worry. There will be another blue moon on May 21, 2016. This will be the third blue moon in the northern spring (southern fall) of 2016.
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