Astronomy: Zodiac

From day to day the Sun, Moon and all the other planets except Pluto (inclination ≈17.1º) appear to move anticlockwise across the sky within the zodiac, a strip ≈8º either side of the ecliptic. The Moon overtakes the Sun about once a month.

The Greeks divided the zodiac into twelve areas and named them after the constellations that occupied the positions at the time. Due to precession the constellations are now 30º east of the constellations for which they were named. The astrological dates are therefore about a month out from those of the constellations, the astronomy dates advancing by approximately one day every seventy-one years.

Claudius Ptolemaeus (c.100-c.170) listed forty-eight constellations, all of them, of course, visible from the northern hemisphere. More were added in the sixteenth, seventeenth and eighteenth centuries, especially when explorers began to visit the southern hemisphere.

In 1930 the sky was divided up into eighty-eight areas. This was primarily to help make the work of the astronomers more efficient, and so the revised boundaries of the constellations do not therefore in any meaningful sense equate to those of the zodiac signs. Along with the twelve original constellations a thirteenth constellation, Ophiuchus, was included within the bounds of the zodiac. The direction of any celestial object can be indicated by saying that it ‘lies in’ a certain constellation.