The waxing moon is shining in my window at night, now, but it is lower on the horizon every day. I don’t mean it’s later when I go to bed; I mean that the moon follows a lower arc in the sky every night, even as it becomes fuller. What’s going on?

Waxing gibbous moon, looking south before sunset April 14.

I’ve done a good deal of talking about the sun being lower each day in the sky until the winter solstice. Since then the sun has been higher in the sky each day, though it’s still only 35 degrees above the horizon at noon, and will never go above 48.6 degrees here at 64 degrees 50 minutes north. But what about the moon?

The moon goes around the earth  approximately in the same plane the earth goes around the sun. (We’ll get to the “approximately” later.) That means that when the moon is full, it is opposite the sun in the sky. If the sun is way below the horizon at solar midnight, as it is around the winter solstice, the full moon will be high in the sky. In fact, it will follow nearly the same path on the winter solstice as the sun follows on the summer solstice. On the summer solstice, the full moon will barely peek above the southern horizon.

The new moon, on the other hand, is in nearly the same place in the sky as the sun. The new moon will follow approximately the same path as the sun. The new moon will be very low in the sky in winter, and very high in the summer.

What about other phases? Right now the moon is waxing gibbous — that is, it is between first quarter (when it looks like a half circle in the sky) and full. On April 14 its highest altitude was 26.5 degrees; tomorrow it will be 20.3 degrees. Effectively, it is following the same path as the sun did in mid-March. I won’t even try to go through the mathematics and geometry involved, but for practical purposes the waxing moon is highest in spring, with the first quarter being highest near the vernal equinox (northward equinox, on the Confederation calendar) and the third quarter being lowest. In fall, this is reversed, with the waning moon being higher in the sky.

Up until now, we’ve been assuming that the moon’s orbit is in the same plane as the Earth’s. If that were really true, we’d have two eclipses — one lunar and one solar –each month. In fact, the moon’s orbit is slightly inclined to the ecliptic — about 5 degrees, to be exact. The direction of the inclination changes due to the pull of the earth’s equatorial bulge, which causes the plane of the orbit to precess with a period of 18.6 years. The inclination of the orbit makes the moon appear to move slightly above the sun’s path in the sky for half a lunar cycle, and slightly below it for the other half. Which part of the cycle depends on time of year and where the moon is in its precession cycle, but it is possible for the moon to be circumpolar or to remain entirely below the horizon here in the Fairbanks area, even though we are south of the Arctic Circle.

Right now, eclipses are near the solstices, so we won’t see the full moon stay below the horizon all night in summer, or be above the horizon at solar noon in the winter — though I’ve seen it in other years.

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