Sun and Calendar · 9 of 9

Ophiuchus, the Thirteenth

Which constellation on the Sun’s path do the horoscopes leave out?

The Sun crosses thirteen constellations, not twelve — and spends longer in the Serpent-Bearer than in the Scorpion next door.

You’ll be able to find Ophiuchus, the constellation the Sun crosses between Scorpius and Sagittarius.

9 steps · about 7 min

Before this: The Ecliptic (8 min), The Wandering Equinox (9 min)

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In 2016 a story ran that NASA had added a thirteenth sign. NASA changed nothing. The dashed line is the ecliptic, the Sun's path: north of Antares it crosses the feet of Ophiuchus, the Serpent-Bearer, on borders drawn in 1930.

Ophiuchus is a man holding a snake: head at the top, the close pair of his hand, his knee below. Serpens, the snake, is the only constellation in two pieces, head to the west and tail to the east, cut in two by the man holding it.

Closer in. The line runs between Dschubba and Acrab, the Scorpion's head, then along the Serpent-Bearer's feet. Antares, the heart, is the measured 4.6° south of it: near enough to seem part of the Sun's road, and not on it.

The outline is the boundary between Scorpius and Ophiuchus, drawn for the IAU in 1930 along the sky's grid lines: straight borders, like the US states'. They give Scorpius only the measured 6.6° of the ecliptic.

The Sun moves about a degree a day, so lengths become dates: under a week in Scorpius, then 18 days across the Serpent-Bearer's feet, north of Antares about 2 December.

The signs are twelve equal 30° slices, pinned to the stars about 2,000 years ago; the slow wobble of Earth's axis has slid them nearly a sign west since (The Wandering Equinox). So the sign of Sagittarius begins with the Sun still in Libra.

Signs and stars slid apart, from The Wandering Equinox

No two of the thirteen stretches match. Virgo holds the measured 44° of the ecliptic, 44½ days of Sun; Scorpius, the shortest, six and a half. Cancer gets 21, Libra 23, Sagittarius 33. Figures this uneven were never going to share twelve equal slices.

Neither count is wrong: a sign is a season, a constellation a patch of sky (The Ecliptic). To see the thirteenth, find red Antares on a June-to-August evening, then go east to the feet where the Sun stands in December.

The thirteenth stretch of the Sun's road.

The Sun crosses his feet each December
Signs are equal slices; constellations aren't
Borders drawn in 1930, not by NASA

Ophiuchus, the Thirteenth

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