The Motions of the Moon

The Moon is unique among planetary bodies in that even from high-temperate latitudes, such as the northern USA or northern Europe, it can appear high in the sky every year. With planets that are further away from the Sun than the Earth, high nighttime altitudes are only achieved when the planet is reaching opposition in the winter sky, because then the Earth's polar axis is tilted toward the planet. Frustratingly, the best seeing conditions tend to occur in the summer, when the polar jet stream is far away and high-pressure weather systems are clear, not cloudy. With Jupiter taking 12 years and Saturn taking 29 years to orbit the Sun, waiting for a planet to be high in the sky again can be a frustrating business, unless you live near the equator, or are prepared to travel abroad. The Moon orbits the Earth every month and so it is always at a high northerly declination once a month and a high southerly declination two weeks later. This declination shift is largely due to the Earth's axial tilt of 23.5 degrees, but there is an extra 5 degree tilt (the angle the Moon's orbit makes with the ecliptic) superimposed on this 23.5 degrees. Thus, in the most extreme cases the Moon can actually range between +28.5 and -28.5 declination during a month or, nine years later, between +18.5 and -18.5 degrees. Actually, I am oversimplifying here. If you take into account the absolute extremes, caused by the Earth-Moon distance variations, the Moon can range between +28.7 and -28.7 Dec, as it does in 2006, 2025, and 2043. Regardless of which hemisphere you live in, the full Moon is highest in winter, the first-quarter Moon is highest in spring, and the last-quarter Moon is highest in autumn. The new or old crescent moons, being so close to the Sun, are at their highest from late spring to early autumn.

There are two additional orbital factors that the keen lunar imager will learn to appreciate: the variation in lunar distance and the effects of libration.

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