Blue moons
Something relatively rare will be occurring in the first quarter of 2018 involving full moons.
A blue moon is the second full moon in a calendar month. Full moons come 29.5306 days (approximately 29 days, 12 hours, 44 minutes, and 3 seconds) apart - the so-called synodic month, to distinguish it from the shorter sidereal month of one earth orbit. If a full moon occurs early enough in the first day of a 30-day month, or the first 36 or so hours of a 31-day month, another will follow on the 30th or 31st of the same month.
That by itself isn’t so unusual.
But if the month is January, and the second full moon occurs late enough, the next one may not be until early March, meaning that February would have no full moon. This is the only month that can possibly be without a full moon.
Moreover, when this happens, because a full moon occurs early in March, that month might also have a second (blue) full moon. That’s exactly what is happening in 2018. These are its full moons.
The Metonic cycle
But there’s more. This also occurred in the same three months in 1999.
Why? Because 19 (tropical) years of 365.2422 days each is only 118 minutes shorter than 235 synodic months of 29.5306 days, that is (19 x 365.2422) / (235 x 29.5306) = (6939.6018) / (6939.6910) = 0.9999871464
Eclipse
But even that's not all. The second full moon in January will be eclipsed, when the earth is between the sun and moon.
Why doesn't this not happen every month as the moon orbits from closer to the sun than the earth to further?
Because the plane of the lunar orbit is tilted relative to the plane of the earth's orbit, meaning that the moon is above that plane half the month, below it half the month, and passes through it twice a month while going from above to below and then back above again. Eclipses only occur at these nodes where the moon's orbit intersects the plane of the earth's orbit, and only if they lie on the line connecting the sun and earth.
Something relatively rare will be occurring in the first quarter of 2018 involving full moons.
A blue moon is the second full moon in a calendar month. Full moons come 29.5306 days (approximately 29 days, 12 hours, 44 minutes, and 3 seconds) apart - the so-called synodic month, to distinguish it from the shorter sidereal month of one earth orbit. If a full moon occurs early enough in the first day of a 30-day month, or the first 36 or so hours of a 31-day month, another will follow on the 30th or 31st of the same month.
That by itself isn’t so unusual.
But if the month is January, and the second full moon occurs late enough, the next one may not be until early March, meaning that February would have no full moon. This is the only month that can possibly be without a full moon.
Moreover, when this happens, because a full moon occurs early in March, that month might also have a second (blue) full moon. That’s exactly what is happening in 2018. These are its full moons.
2018 Jan 01 20:26 Mon (Central Standard Time = UTC-6)
2018 Jan 31 07:28 Wed
2018 Mar 01 18:52 Thu
2018 Mar 31 06:38 Sat
2018 Jan 31 07:28 Wed
2018 Mar 01 18:52 Thu
2018 Mar 31 06:38 Sat
The Metonic cycle
But there’s more. This also occurred in the same three months in 1999.
1999 Jan 01 22:49 Fri (Central Standard Time)
1999 Jan 31 10:06 Sun
1999 Mar 01 12:58 Tue
1999 Mar 31 05:49 Wed
1999 Jan 31 10:06 Sun
1999 Mar 01 12:58 Tue
1999 Mar 31 05:49 Wed
Why? Because 19 (tropical) years of 365.2422 days each is only 118 minutes shorter than 235 synodic months of 29.5306 days, that is (19 x 365.2422) / (235 x 29.5306) = (6939.6018) / (6939.6910) = 0.9999871464
Eclipse
But even that's not all. The second full moon in January will be eclipsed, when the earth is between the sun and moon.
Why doesn't this not happen every month as the moon orbits from closer to the sun than the earth to further?
Because the plane of the lunar orbit is tilted relative to the plane of the earth's orbit, meaning that the moon is above that plane half the month, below it half the month, and passes through it twice a month while going from above to below and then back above again. Eclipses only occur at these nodes where the moon's orbit intersects the plane of the earth's orbit, and only if they lie on the line connecting the sun and earth.