Inconstant Moon"

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07/06/2002 "Inconstant Moon"

EQUINOX: All the planets and most of the moons in the Solar System revolve around the Sun in a single plane. Seen from the Earth, the Sun and planets appear to travel within a few degrees of an imaginary line named the Ecliptic. The Earth’s axis and equator are tilted relative to the Ecliptic, so for half of the year the Sun appears to be north of the equator (so the northern hemisphere enjoys longer days) and for the other half the opposite is true. Twice a year, the Sun’s path along the Ecliptic crosses the equator, and days and nights are equal in length at all latitudes - these are the vernal (spring) and autumnal (fall) equinoxes. They fall on about the 21st of March and September.

The Moon’s orbit also broadly follows the Ecliptic, so at the time of the equinoxes the new and full moons will be essentially over the equator, the quarter moons will be at their furthest from it. Consequently, at sunrise and sunset the angle between the horizon and the Moon’s path is either at it’s greatest or smallest.

The equinoxes mark the beginnings of spring and autumn. The corresponding start dates for summer and winter are the solstices (from the Latin sol, sun, sistere, make stand). These are the days when the Sun is stationary at its greatest separation from the equator before gradually converging with it again. On these days the noon Sun is at its lowest or highest point in the sky for the year.

Inconstant Moon


Libration in latitude is due to the Moon's axis being slightly inclined relative to the Earth's. Each of the lunar poles will appear to be alternately tipped slightly toward the terrestrial observer over a roughly four week cycle.

Diurnal libration is due to the observer being on the surface of the Earth, up to four thousand miles to one side of the Earth-Moon axis, a significant proportion of the centre-to-centre distance. The difference in perspective between the rising and setting of the Moon appears as a slight turning of the Moon first to west and then to east.

Libration of longitude is an effect of the Moon's varying rate of travel along its slightly elliptical orbit. Its rotation on its own axis is more regular, the difference appearing again as a slight east-west oscillation.

Although the Moon always presents the same face towards the Earth, due to its rotation and revolution being locked to the same period, the combined effect of these different librations allows us over time to see some 59% of its surface.

António Cidadão has created a spectacular 133K animation illustrating libration


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