Moon Questions

A week or two ago, one of my friends in the San Antonio Astronomical Association asked some interesting questions about the moon. He wanted to know how the moon would appear from the poles, from the Arctic and Antarctic circles, from the Tropic lines of Cancer and Capricorn and from the equator. Here is my slightly edited answer. I am interested in comments, as this is just the result of a thought experiment on my part. I have no experience seeing the moon from those areas.

Hunter,

Not many have worked on your puzzle. However:

In Australia, which is about the southern hemisphere opposite of us here in Texas, I have posted previously that the moon (and the sun) behave pretty much oppositely to what we experience.

The moon still rises in the east and sets in the west, but as one looks toward the horizon under the moon, one is looking north. So the moon rises from the right and sets to the left. Here in Texas, it rises from the left and sets to the right.

When the moon is young, the left side of the moon is lit up down there, instead of the right side like we are used to seeing. And when looking at the moon with binoculars when it is at its highest (north), the South Pole of the moon is at the top of the field of view and the North Pole is at the bottom.

The next questions depend on a fact that the moon's plane of revolution is near the plane of the ecliptic, not the plane of the earth's equator. That is to say that the moon's plane of rotation is pretty much in the same plane as the earth's (and venus and Mercury and Mars...) plane of rotation. (It is actually about 5 degrees out of that plane, which makes all that follows an approximation)

Those circles of latitude you talked about are mainly used to describe the motion of the sun across the seasons. Because of the tilt of the earth, the noon sun is directly overhead at the equator on only two days a year. They are the equinox days in March and September. On all other days, a point either north (late March to early September) or south (late September through early March) of the equator has the sun directly overhead at local noon.

In fact, to a pretty close approximation, the entire equator has its local noon directly overhead on the equinoxes. A circle of points north (or south) of the equator has its local noon directly overhead on every other day. On June 21st or so (depending on how close to a leap year we are), that circle which has its local noon directly overhead is at the Tropic of Cancer - about 23-1/2 degrees of north latitude -- just a little south of us in San Antonio. On Dec 21st or so, local noon is experienced directly overhead along the Tropic of Capricorn. (Remember that Capricorn is south of Cancer in the sky).

Starting from the spring equinox, when the North Pole is tipped toward the sun, the sun never sets at the north pole until the fall equinox. As spring progresses, there is an ever larger circle of earth surrounding the north pole in which the sun never sets at any time during the day. It rises higher and lower, but never sets until about an equivalent number of days the other side of the equinox. The furthest south this ring works its way outward from the North Pole, on the day of the Summer Solstice is a ring of points that are called the Arctic Circle at about 66-1/2 degrees. The same sort of thing happens the other half of the year inside the Antarctic Circle around the South Pole and surrounding what we call the winter solstice.

The other half of the year, there is an equivalent area in which the sun never rises around one of the poles. It has the same area as the area in which the sun never sets on the other pole of the globe.

What does the moon do at the far north and far south? Assuming that the moon is really in the plane of the ecliptic (and that is off by a little bit) Then there is a point near the north and south poles for each day of the year where the moon does not set for the first time that year. That day will be the day that the sun first stays up all day and the day that the sun first stays down all day.

For the north pole and the south pole, that day occurs only twice a year, on the equinoxes. For points on the Arctic Circle and the Antarctic Circle, it will occur only on the solstices. For the rest of the points on the pole side of the Arctic and Antarctic circles, that will occur 4 times a year. The places on which it occurs each day will describe two circles which are parallel to the Arctic and Antarctic Circles.

On the equator, something akin to the sun occurs regarding the moon. For an observer on the equator, and near the summer solstice, when the North Pole is tipped toward the sun, a full moon will appear to be nearer the north horizon than at any other time. During the rest of that lunar month, if one looks toward the north, the moon will rise in the east to the right. The left portion of the moon will be lit in the early evening when the moon is young and the moon's south pole will be up in a pair of binoculars. (In my original post I had some of this backwards.)

Near the day of the winter solstice, a full moon will appear to be nearer the south horizon than at any other time. During the rest of that lunar month, if one looks toward the south, the moon will rise in the east to the left. The right portion of the moon will be lit in the early evening when the moon is young and the north pole of the moon will be up in a pair of binoculars.

Near the day of the equinoxes, the moon will pass nearly overhead. If it is a full moon, at these times the moon may pass through the earth's shadow and a lunar eclipse may occur. If it is a new moon, and the geometry is just right, a solar eclipse may occur.

On other days, there is a circle of points on which a full moon will pass directly through the zenith, directly overhead. The furthest north this circle will move is near the tropic of Cancer, as the summer solstice approaches. The opposite is true as the circle approaches the tropic of Capricorn as the winter solstice approaches.

All of this is complicated further because the moon is not really in the ecliptic, it is in a plane slightly off set from it. That really messes up the eclipses, because the moon can be anywhere in a ten degree arc north or south of the positions I have described above as the “ideal positions” The movement of the moon up and down with regard to the ecliptic can be described in longer cycles called saros cycles. And that is beyond the scope of what I can do in my head.

Dark Skies,

Rick

Thanks to Matt Rottman for the photograph of the moon.