# Lunar cycle time

You’d think the amount of time from one full moon to the next would always be the same, but I notice on my calendar that it varies. For example, from the next full moon on April 5 to the following one is 29 days 9 hrs 30 mins. But the cycle after that is 29 days 7 hrs 47 mins. Why are the cycles different lengths, and what’s the min and max? I don’t think the moon is speeding up / slowing down all the time.

Yes the Moon really is speeding up and slowing down. Its orbit, like any other, has a degree of eccentricity, which means that it is an ellipse. Although its eccentricity is small and the orbit looks circular, those slight differences in speed cause the exact times of full and new moons to vary.

The Moon orbits the Earth in a little over 27 days. At one point in that orbit, it reaches its closest approach to Earth (perigee) where it also has its fastest orbital velocity. Opposite that point is apogee, where it is farthest from the Earth and has its slowest velocity. But the Earth/Moon system also orbits the Sun, so that by the time the Moon has completed its 27 day orbit, the sun appears to have moved and the Moon must continue another 2 days or so to catch up. But that’s not all. The angles at which perigee and apogee precess around the Earth in something like a 6 year cycle. So the exact times when the Moon is full or new are different every month.

And there must be some effect from the eccentricity of the Earth’s orbit, which is about a third that of the moon’s. Because of it, the Sun appears to move faster or slower in the sky, and causes the analemma, the equation of time. It would take the moon longer, or shorter, to “catch” up.

Just FYI, the precession period of the moon’s orbit is around eighteen years. But everything else is dead on in the previous posts.

Translation for those of us that don’t speak astronomer:

A full moon occurs when Sun, Earth and Moon are in a straight line. One month this happens at a particular point on the Moon’s orbit around the Earth. But the Earth is itself moving around the Sun, so a month later the straight line occurs at a diiferent point on the Moon’s orbit.

You’re right; the precession of the Moon’s ascending node is 18.6 years. I was referring to the precession of the argument of periapsis. Cite. But since the longitude of periapsis is dependent on the ascending node anyway, they add up and since they’re in different directions, I don’t feel like doing the math right now.

Translation for non-scientists: the Moon’s orbit is even more complex than I said it was.

Thank you, cityboy. I’m enough of a geek to know what the argument of periapsis is but insufficiently geeky to know that it doesn’t change at the same rate as the line of nodes.