Minor Lunar Standstill

¡SkyCaramba! Weekly astronomy blog for the week of October 11 to 17, 2015

Over the next month or so, look at the moon whenever it’s up, especially whenever it’s rising or setting. Notice how far north or south it goes in the sky. In about a month’s time, it goes from its northermost point, across the equator, to its southernmost point, across the equator, and back to its northermost point again.

Last month, the moon reached 18.1° from the equator at its northernmost and southernmost points. This month, it’s getting a little farther at 18.2°. It will be 18.3° south on November 15 and 18.4° north on November 27. Those measurements will keep increasing month after month until February and March 2025 when the moon reaches 28.7° from the equator. Then, the moon’s reach north and south shrinks just a little each month until after another nine years it’s down to 18.1° again.

Even though the moon isn’t really holding still during any of these motions, the extremes in this 18.6 year cycle have come to be called lunar standstills. The part of the cycle the moon just went through is called a minor lunar standstill. Likewise, the part the moon will go through in 2025 is called a major lunar standstill. This 18.6 year cycle happens because of the gravitational interaction between the earth, the sun, and the moon.

For simplicity’s sake, we often say the moon goes around the earth. Actually, the earth and moon tug on each other. They orbit a center of mass that is on a line between the centers of the two bodies. Because the earth has more mass than the moon, their center of mass (also called the barycenter) is closer to the earth’s center than the moon’s center. In fact, the earth has so much more mass than the moon, the earth-moon barycenter is about 1,707 kilometers below earth’s surface wherever the moon is overhead.

Add to this the fact that the earth goes around the sun in a slightly elliptical orbit. While the earth and moon are affecting each other’s positions, the gravitational pull they experience from the sun changes slightly too as their distance from it changes.

Things get even more complicated than that. All the planets tug just a little on the sun and on each other. You might imagine the mathematics involved in calculating the exact positions of the sun, the moon, and any of the planets is quite involved. The modern astronomer has the luxury of making a computer figure it all out in as much time as it takes to press a button on a keyboard. Astronomers of a century ago hired mathematicians to calculate object positions using pen and paper. Their formulae were less rigorous, but it could still take hours to anticipate a few nights’ observations.

You don’t need to worry about any of the math if you don’t want to. Just enjoy the view of the moon as it goes farther north and south every month over the next decade.