Tyrel L. answered • 12/17/19
11 year teacher with a passion for learning and teaching!
It takes about 173.31 days for the Earth to go from the point in its orbit when the sun lines up with one of the lunar nodes. The nodes are the points in the lunar orbit when the moon's orbit intersects the plane in which the Earth orbits the sun. For just over 30 days around the time when the orbits perfectly intersect eclipses can occur and this is known as an eclipse season. The moon can go through all phases in just under 30 days and lunar eclipses occur when the moon is full (on the opposite side of the Earth from the sun). This gives the moon plenty of time to reach the full moon phase at least once per eclipse season and possibly twice if it happens right at the beginning and end. Being more centered in the eclipse season and thus closer to the node, along with other mechanics of the lunar orbit can affect how deep the moon goes into Earth's shadow to produce a total, partial, or penumbral eclipse.
To continually calculate lunar eclipse occurrence it is better to count from the best alignment of the nodes and not just whichever eclipse occurred last which could have been at any point in the eclipse season. To do this it is possible to get data on when the moon was actually in the node crossing from below the plane of Earth's orbit or from above. To estimate this point in the lunar orbit without outside data, it could be closely approximated when 2 lunar eclipses occur in one eclipse season and taking a date roughly in the middle as the starting point. Then simply counting 173.31 days from that time in regular intervals and observing any full moon that occurs within about 15 days on either side you will likely catch any lunar eclipse that can be viewed. In practice this will end up being about every 6th full moon but it can happen after just 5 and then 2 in a row can be eclipsed.
This brings up the difference between calculating the occurrence vs actually observing these eclipses since you also have to have the timing of the spin of the Earth right for your location to be facing toward the moon during the hours that the eclipse is in progress. An eclipse can occur on the other side of the Earth when an observer is experiencing day time, and by the time the Earth turns and the full moon rises for that observer, the the moon could be well out of the Earth's shadow and the eclipse could be over.
