High Moon

No, the title isn't a typo. If I'd meant to talk about the election again I would have called this post High Noon; it only means that I'm prone to making bad puns. I really do want to talk about today's full moon and how high it is in the sky.

I find that one of the pleasant compensations of the colder temperatures and loss of daylight this time of year is that it is also a time of brilliant full moons. Whether you call today's full moon the Harvest Moon or, more correctly, the Hunter's Moon, its appearance is the same. The full moons surrounding winter solstice in late December are higher, brighter and up in the sky longer than at other times of year. We may lose the sun earlier in the day, but the moon takes its place when its phase is near full.

This assumes, of course, that the sky isn't clouded over. More importantly you need to live at mid to high latitudes to get this effect. Since Ottawa is midway between the equator and the north pole, it qualifies. And the further north you go, the longer the moon is in the sky although it doesn't get quite to high above the horizon. This high full moon phenomenon is no coincidence; it is always true. The reason is that at full moon the moon is almost, but not quite, positioned directly opposite the sun from our vantage point.

I won't draw any fancy charts myself, though you can find those here on Wikipedia and in other referenced articles. The basic mechanics are as follows. The Moon orbits the Earth in nearly the same plane that the Earth orbits the sun; the planes are inclined to each other by 5 degrees. If the orbits were in the same plane there would be both a lunar eclipse and a solar eclipse every month. Instead the moon usually misses the Earth's shadow at full moon.

Now because of the opposite positions of sun and moon there are a couple of interesting geometrical facts. One is that at full moon, moonrise and sunset occur at the same time (again, not exactly but close enough), as do moonset and sunrise. This means that when the sun isn't in the sky the moon is (and vice versa), so that the full moon is visible throughout the long winter nights. Second, when the sun is low in the sky during the day it dips far below the horizon during the night. Since the moon is opposite to it, the moon must therefore rise high in the sky, reaching it's peak at local midnight (when the moon is due south, and not according to the clock). The same effect occurs in the southern hemisphere, offset with the seasons by 6 months.

The result is that for the next several months of cold weather for the days surrounding full moon each month we will have a brightly lit landscape throughout the night. Since the cold often brings clear skies we are likely to get the full benefit of this effect.

Is it of any use other than being pretty? Perhaps not in the city where there is artificial lighting. It is in the great outdoors where it is very noticable and useful. That's why the Harvest Moon and Hunter's Moon have the names they do; these are activities taking place outside of cities. For urban dwellers it does however have a recreational benefit. Think of midnight skiing and snowmobiling. With only a small amount of care and avoidance of deep shadows among the trees it is possible to frolic all night long, if you remember to dress warmly.

Think about that and look up the next few nights, which will hopefully clear from time to time. Winter does have its perquisites. Make your night-long party plans now.