09 March 2020

The Moon: where is it going to rise tonight?

Moonrise in San Francisco
An old friend, Scott in San Francisco, posted a question on Facebook and it caught my attention. He illustrated a quandary that he faced when observing the moonrise in San Francisco on two consecutive nights and wondered why the two locations were so different. Here is what he wrote: "I am puzzled ... Can someone explain how the moon changes its position so radically in one day? I created a hand drawing of the moon, its location, and time as it rose over the San Francisco skyline the last two nights." There are two (well actually three) factors at work here.

Before we dive into the specific factors, let's review the basics. The moon, like the sun, rises in the east and sets in the west, but of course the precise point of sunrise varies throughout the year. The sun rises either north of east in the summer when the days are longer, or south of east in the winter when the days are shorter. The full moon, which Scott was observing when he sketched the images last month, has an opposite behavior compared to the sun ... that is, the moon is in the opposite part of the sky compared to the sun and as such, in the winter the moon rises north of east (when the full moon nights are longer) and south of east in the summer (when the full moon nights are shorter).

In addition to that macro change in the position of the moonrise compared to sunrise throughout a year, there are daily changes to the position of the moonrise. The moon, being much closer to Earth than the sun, moves rapidly in its 29 day orbit around the Earth and from one day to the next is approximately 12 degrees further eastward in comparison to its location the night before. That is the main reason that a rising moon rises approximately one hour later each evening, and the primary factor that is illustrated in the drawing above. The Earth has to turn more to get to the place where the moon has moved, in this case from 8:00 pm to 9:00 pm/.

Moonrise position changing
But there is a subtle secondary effect that Scott captured in his sketch. That is that the location of moonrise is quite a bit shifted on the horizon. In this case, it is to the south from one day to the next, not just a later moonrise time but also a point on the horizon that is south of the previous day. What is happening here is that the moon's path around the Earth is quite similar to the sun's path around the sky and that is a path that follows the zodiac band around the sky, something also called the ecliptic. Because the moon follows this path, and in this special circumstance in which a full moon is observed on successive nights, the winter full moon appears to rise further and further south on consecutive nights until it finally moves to last quarter moon phase and then it will resume its travels northward (north of east) as it rises late each night. In fact, the moon is simply moving across the known Zodiac constellations of Virgo, then Libra (see image on left), and eventually Scorpius and Sagittarius, all of which are low on the horizon and low on the ecliptic, and therefore are part of the reason why the moon rises further and further south of east on consecutive nights in the winter. The image shows how the moon moves across the Zodiac from night to night, but it will take some creativity to juxtapose this on the eastern horizon to fully understand why the moon is further south of east on consecutive nights. Think about it. It requires some big picture thinking.

There is a third factor for the change in the location of moonrise on consecutive nights which is based on the moon's orbital inclination, but that requires fairly expert knowledge of celestial mechanics to fully comprehend so we will save that for another time ... or a star party.

Today is the full moon, March 9th, 2020. You can try to reproduce Scott's experiment now, looking closely at the eastern horizon and locating landmarks where you can spot the moonrise on consecutive evenings. For the next 3 or 4 evenings the moon will rise about an hour later each evening, and also will rise at a point further to the south of east compared to the night before.

I wish you clear skies and happy viewing.

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