Blue Moon

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.

1. 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.
2. 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.

The Vernal Equinox

On March 20th, 2015, the northern hemisphere's spring season officially began. This time of year is called the vernal equinox, or spring equinox. We've discussed equinox before and we learned that equinox comes from the Latin for "equal night". It just means that there are the same number of hours when the Sun is above the horizon (day) as there are when the Sun is below (night).

We have discussed the autumnal equinox before, and we learned that this is when the Sun appears to travel from north to south along the ecliptic across the celestial equator. In this case, the vernal equinox occurs when the Sun travels across the celestial equator from south to north along the ecliptic. You should note that when the spring starts in the northern hemisphere, fall or autumn begins in the southern hemisphere.

Another fun thing to note is that on either equinox, the Sun rises due east and sets due west. At the equator, the Sun is directly overhead at noon. As the Sun travels farther north on the ecliptic and we get deeper into the northern spring, the Sun will be farther north in the sky at noon until it reaches its farthest point north at the summer solstice (which will be discussed as we approach that time of year).