Constellations 06

Ophiuchus, the Thirteenth

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

Working out when this is best placed from where you are…

Latitude

Longitude

Which night

Date

Hour

Used for this lesson only — your map settings, and every other lesson, stay as they are.

Sources

HYG stellar database v4.2 — star positions and every separation quoted here
IAU constellation boundaries (Delporte 1930), VizieR VI/49 — the Sun's dates in each constellation are computed against these
Meeus, Astronomical Algorithms, 2nd ed. (1998), ch. 25 — the Sun's position through the year
Rogers, "Origins of the ancient constellations: I. The Mesopotamian traditions", JBAA 108, 9 (1998) — the twelve equal signs

← GUIDES

The dashed line is the ecliptic, the Sun's path through the year. Past the Scorpion's head, just north of Antares, it leaves Scorpius and crosses the feet of Ophiuchus, the Serpent-Bearer: a big, faint figure whose head, Rasalhague, stands 36° north of the line. The Sun passes through thirteen constellations, and this is the one left out of the twelve.

Ophiuchus is a man holding a snake. Trace a big lopsided loop: Rasalhague at the top, Cebalrai down one side, the close pair Yed Prior and Yed Posterior on the other, Sabik at the knee.

And the snake he holds. Serpens is the only constellation in two pieces: the head to the west, with Unukalhai at its heart, and the tail trailing east to Alya. He grips it at Yed Prior and at Sabik, and his body is the gap between the halves — one snake, 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 — θ Ophiuchi is 1.8° south of it. Antares, the Scorpion's heart, is 4.6° south of the line: near enough to look like part of the Sun's road, not on it.

The outline is the boundary between Scorpius and Ophiuchus, drawn for the IAU by Eugène Delporte in 1930 along lines of right ascension and declination — administrative lines, like a surveyor's. They give Scorpius only 6.6° of the ecliptic, the stretch past its head. Even Antares is inside by just 1.6°: the border runs along a parallel north of it.

The Sun moves about a degree a day along the line, so lengths become dates. It enters Scorpius about 23 November and leaves six and a half days later, on the 29th or 30th. Then come 18.6° of Ophiuchus: 18 days, to about 18 December. On about 2 December it passes north of Antares, already in Ophiuchus.

The signs are something else: twelve equal 30° slices of the ecliptic, counted from the March equinox — a calendar of the seasons, not a map of the stars. The sign of Sagittarius begins at the marked point, about 22 November, with the Sun still in Libra. It ends about 21 December, and for nearly 25 of those days the Sun is in Scorpius or Ophiuchus.

None of the thirteen stretches match. Virgo holds 44° of the ecliptic, and the Sun spends 44½ days in it, about 16 or 17 September to 31 October. Scorpius, the shortest, gets six and a half. Between them: Cancer 21 days, Libra 23, Sagittarius 33, Taurus 38. Figures of such different sizes were never going to share out twelve equal slices.

Neither count is wrong: a sign is a season, a constellation is a patch of sky, and they have been sliding apart since the signs were set — precession, another lesson. To see the thirteenth yourself, find red Antares on an evening between June and August. The faint stars about 12° east of it are the Serpent-Bearer's feet, and in December the Sun stands among them.