If you've been watching the Moon after sunset this week, you may have noticed it lies lower in the sky than usual (at least if you live at mid-northern latitudes). It's not your imagination — the Moon is figuratively scraping the treetops this week. Although this is no great mystery, it never hurts to remind yourself about the Moon's monthly and yearly cycles.

The Moon's maximum altitude from any given location depends on its declination — how far north or south of the celestial equator it lies. And because the Moon orbits Earth in nearly the same plane as Earth orbits the Sun, our satellite's declination hinges mainly on its phase and the time of year.

This shows up most noticeably at Full Moon. In December, for example, when the Sun lies near the winter solstice, the Full Moon lies near the summer solstice. So, just like the Sun in June, December's Full Moon passes closer to the zenith than any other Full Moon. The reverse also holds true: June's Full Moon lies near the winter solstice, so it takes a low arc across the sky.

The same thing holds true at other lunar phases. This week, we experienced a First Quarter Moon. By definition, that means the Moon has moved one-quarter of the way around its orbit relative to the Sun. And because the Sun reaches the autumnal equinox this weekend, the First Quarter Moon lies near the winter solstice. The result: a low Moon after sunset. The effect is more pronounced this year because the Moon currently lies 5° south of the ecliptic, about as far south as it gets.

How much difference does this make? This week's First Quarter Moon appeared barely 20° above the horizon at sunset for an observer at 40° north latitude. Last March, the First Quarter Moon appeared nearly 80° above the horizon — just over 10° from the zenith.