The zodiac

Every month the Moon passes Aldebaran, Regulus, Spica and Antares in turn, moving eastward among the stars by about 13° a day. The bright planets are found in the same band of sky, and so is the Sun, although its glare hides the stars around it. The middle line of this band is the ecliptic, the path along which the Sun moves among the stars in the course of a year (Section 1.6). Knowing the constellations along it tells us where to look for the Moon and the planets, and in the planetarium round of the olympiad the ecliptic is one of the lines we may be asked to draw on a sky. We met most of these constellations in Sections O1.2 to O1.5. In this section we put them in order, compare them with the signs of the zodiac and learn to trace the ecliptic among the stars, and then we add the three faint ones that remain, Libra, Capricornus and Aquarius.

O1.7.1The ecliptic and the zodiac

The Moon's orbit is tilted by 5.1° to the plane of Earth's orbit, so the Moon never strays more than about 5.3° from the ecliptic. The orbits of the planets are tilted by a few degrees too. Seen from Earth, Jupiter and Saturn keep within 3° of the ecliptic, Mercury within 5°, Mars within 7°, and Venus, which strays furthest, within 9°. The band about 9° either side of the ecliptic is called the zodiac, and the Moon and the bright planets are always found in it.

When the IAU drew the borders of the constellations in 1930 (Section O1.1), the ecliptic was cut into thirteen stretches of very different lengths. The table lists them in order eastward from Pisces, which holds the March equinox point, with the date on which the Sun entered each in 2026. The dates are in Universal Time, and in other years they can differ by a day.

Virgo holds the longest stretch, 44.0°, and the Sun spends 44 days in it. Scorpius holds only 6.6°, and the Sun crosses it in 7 days, from 23 to 30 November. It then spends 18 days in Ophiuchus, the Serpent Bearer, before it reaches Sagittarius. Ophiuchus is a large constellation north of Scorpius (Section O1.4), and its southern part reaches down between Scorpius and Sagittarius, where the ecliptic crosses it.

O1.7.2Signs and constellations

Astrology divides the ecliptic in a different way. Its twelve signs are equal stretches of 30°, counted eastward from the March equinox point. The sign of Aries begins on the day of the March equinox, 20 March in 2026, the sign of Taurus begins when the Sun has moved 30° further, and so on round the ecliptic. When the signs were named, more than two thousand years ago, they roughly matched the constellations. In the time of Hipparchus, about 130 BC, the March equinox lay just inside the constellation Aries, about 1° from its western edge by the modern borders, and the sign of Aries began there. This is why the March equinox point is still called the first point of Aries.

Hipparchus also discovered precession (Section 2.7), which moves the equinoxes westward along the ecliptic by 1.40° a century. Since his time the March equinox has moved about 30°, and the signs have moved with it, so each sign now lies mostly in the constellation west of the one it is named after. In 2026 the sign of Sagittarius runs from 22 November to 21 December, but for most of that time the Sun stands among the stars of Scorpius and Ophiuchus. The March equinox itself now lies in Pisces, 8.0° east of the border with Aquarius. The zodiac figure shows both divisions at once, with the signs along its bottom edge and the constellations above them.

O1.7.3Tracing the ecliptic

A few points fix the position of the ecliptic among the stars. It crosses the celestial equator at the two equinox points, the March equinox point in Pisces and the September equinox point in Virgo. Halfway between them it reaches 23.4° north of the equator at the June solstice point, which lies in Taurus, 0.5° from the border of Gemini, and 23.4° south at the December solstice point in Sagittarius. Seen from the northern hemisphere, the part of the ecliptic in Taurus and Gemini therefore rides high, and the part in Sagittarius stays low. Because the full Moon lies opposite the Sun, the full Moon of December is high in the sky and the full Moon of June is low, give or take the 5° by which the Moon can stray from the ecliptic. In the southern hemisphere it is the other way round.

Between these points the bright stars near the ecliptic let us draw it in. Six stars brighter than magnitude 3.6 lie within 1° of it. They are Regulus in Leo, Zubenelgenubi in Libra, Zavijava in Virgo, and Propus, Tejat and Wasat in Gemini. The ecliptic passes about 10° south of Hamal in Aries, then between the Pleiades, which lie 4° north of it, and Aldebaran, 5.5° south of it. It runs through the feet of the Twins, 6.7° south of Pollux, and through the middle of Cancer, 1.3° south of the Beehive. It passes within 0.5° of Regulus, so the Sun passes close to Regulus each year, around 23 August. It then passes 2.1° north of Spica and within 0.3° of Zubenelgenubi. In Scorpius it runs through the head of the Scorpion, between Acrab, 1.0° north of it, and Dschubba, 2.0° south, with Antares 4.6° south. In Sagittarius it passes just north of the lid of the Teapot, 2.1° from Kaus Borealis, and in Capricornus it runs between Dabih, 4.6° north of it, and Deneb Algedi, 2.6° south. No bright star marks it in Aquarius, where it runs midway between Sadalsuud, 8.6° north, and Skat, 8.2° south. In Pisces it passes just south of the Circlet, 3.4° from its nearest star, λ Psc.

O1.7.4Libra, Capricornus and Aquarius

Libra, the Scales, lies between Virgo and Scorpius. The Arabic names of its two brightest stars mean the southern claw and the northern claw, because the Greeks drew these stars as the claws of the Scorpion. Zubeneschamali (β Lib), magnitude 2.61, is a little brighter than Zubenelgenubi (α² Lib), magnitude 2.75, and lies 9° north-east of it. Zubenelgenubi is 21° from Spica, a little less than halfway along the 46° from Spica to Antares and about 4° north of the line between them.

Capricornus, the Sea Goat, is a large triangle of faint stars south of Altair, with one corner pointing south. A line from Vega through Altair, continued about three quarters as far again, reaches Dabih (β Cap), magnitude 3.05, and Algedi (α² Cap) at the western corner. Algedi has a fainter neighbour, α¹ Cap, only 6′ away, and the two stars, of magnitudes 3.6 and 4.3, can be seen separately with the naked eye. The eastern corner is Deneb Algedi (δ Cap), magnitude 2.85, the brightest star of Capricornus, 21° from Dabih. It lies on the line from Altair to Fomalhaut, about two thirds of the way along.

Aquarius, the Water Bearer, lies east of Capricornus and south-west of the Great Square of Pegasus (Section O1.5). Its two brightest stars, Sadalsuud (β Aqr), magnitude 2.90, and Sadalmelik (α Aqr), magnitude 2.95, are almost equal. Sadalmelik is 11.5° south-east of Enif, about a quarter of the way along the line from Enif to Fomalhaut, and it lies only 0.3° south of the celestial equator. About 6° east of it, a small Y of four faint stars called the Water Jar sits within 1.4° of the equator. Skat (δ Aqr), magnitude 3.27, lies 14° north of Fomalhaut.