Lunar Standstill - Informal Explanation

Informal Explanation

As the Earth spins on its axis, the stars above us appear to move in circles. It appears to us as if all the stars are fixed in a great sphere surrounding us. In the same way that we measure positions on the Earth using latitude and longitude, we measure positions of stars on this sphere in right ascension (equivalent to longitude) and declination (equivalent to latitude). Viewed from a place on the Earth which has latitude 50°, then the stars directly above have a declination of 50°.

Unlike the stars, the Sun and Moon do not have a fixed declination. As the Earth travels its annual orbit around the Sun, with its rotational axis tilted at about 23.5° from the "vertical" (a line perpendicular to the orbit), the Sun's declination changes from +23.5° at the Summer Solstice to −23.5° at the Winter Solstice. Thus, in the northern hemisphere, the Sun is higher in the sky and visible for a longer period of time in June than it is in December. This is the cause of the Earth's seasons.

The Moon also changes in declination, but it does so in only a month. So it might go from a declination of +25° to −25° in just two weeks, returning to +25° two weeks later. Thus, in just one month the Moon's altitude at its culmination (when it is due south on the meridian) can move from being high in the sky, to low over the horizon, and back again.

Unlike the Sun, the Moon's maximum and minimum declination also varies. This is because the plane of the Moon's orbit around the Earth is inclined by about 5° to the plane of the Earth's orbit around the Sun, and the direction of this inclination gradually changes over an 18.6-year cycle, alternately working "with" and "against" the 23.5° tilt of the Earth's axis. As a consequence, the maximum declination of the Moon varies from (23.5° − 5°) = 18.5° to (23.5° + 5°) = 28.5°. The effect of this is that at one particular time (the minor lunar standstill), the Moon will change its declination during the month from +18.5° to −18.5°, which is a total movement of 37°. This is not a particularly big change, and may not be very noticeable in the sky. However, 9.3 years later, during the major lunar standstill, the Moon will change its declination during the month from +28.5° to −28.5°, which is a total movement of 57°, which is enough to take its culmination from high in the sky to low on the horizon in just two weeks (half an orbit).

Strictly speaking, the lunar standstill is an instant in time: it does not persist over the two weeks that the Moon takes to move from its maximum (positive) declination to minimum (negative) declination, and it most likely will not exactly coincide with either extreme. However, because the 18.6-year cycle of standstills is so much longer than the Moon's orbital period, the change in the declination range over periods as short as half an orbit is very small.