The sky — astronomy

Zodiac Constellations: What Lies Along the Sun’s Path

The ecliptic, the Sun’s apparent yearly path, crosses 13 constellations under the boundaries set by the International Astronomical Union (IAU): the 12 familiar ones plus Ophiuchus. They differ greatly in size because the borders follow sky coordinates, not the shapes of the star patterns. A constellation is a region of sky, which is not the same thing as an astrological sign.

Over a year, the Sun appears to travel against the background stars along a single line called the ecliptic. The constellations it passes through are known as the zodiac constellations. Here is what they are, and why they vary so much in size.

Constellations as regions of the sky

Ancient cultures saw constellations as pictures drawn around bright stars: heroes, animals, tools. In modern astronomy the word means something more precise: a region of the celestial sphere with exact boundaries. Every point in the sky belongs to exactly one constellation, much as every spot on a map belongs to one country.

That makes communication easy. If an astronomer reports a comet in Leo or a nova in Sagittarius, everyone knows which patch of sky is meant. The full set of regions numbers 88, as described in 88 constellations.

The boundaries adopted by the IAU

The International Astronomical Union (IAU) is the body that standardises constellation names and outlines. The modern borders were drawn up by the Belgian astronomer Eugène Delporte and adopted by the IAU in the late 1920s. They follow lines of right ascension and declination (the sky’s equivalents of longitude and latitude), which is why the outlines look like staircases of straight segments rather than smooth curves around the star figures.

These borders settle every dispute about where one constellation ends and the next begins. The stick figures are an aid to the eye; the official meaning belongs to the region.

Which constellations does the ecliptic cross?

Under IAU boundaries the ecliptic passes through 13 constellations. In the order the Sun meets them, starting near the March equinox point in Pisces:

• Pisces
• Aries
• Taurus
• Gemini
• Cancer
• Leo
• Virgo
• Libra
• Scorpius
• Ophiuchus
• Sagittarius
• Capricornus
• Aquarius

The thirteenth, Ophiuchus (the Serpent Bearer), lies between Scorpius and Sagittarius. It is a full constellation, though not part of the traditional set of twelve signs. For more on that popular question, see Is there a 13th zodiac sign?.

The Moon and planets also stay close to the ecliptic, because their orbits lie nearly in the same plane as Earth’s. So we usually find them in front of these same constellations. The Moon’s orbit is tilted by about 5° to the ecliptic, so it can stray slightly outside the band.

Why the sizes differ

The zodiac constellations are far from equal. Virgo is one of the largest constellations in the whole sky, while Cancer and Aries are fairly small and faint. There are two main reasons:

1. Historical inheritance. The figures were invented by different cultures at different times; nobody set out to slice the sky into equal parts.
2. Coordinate-based borders. The official regions were drawn to cover the entire sky without gaps, not to have equal areas.

As a result, the Sun spends very different lengths of time in each one: more than a month crossing Virgo, but only about a week in Scorpius. This follows from the geometry of the borders and the pace of Earth’s orbit around the Sun.

There is one more effect. Because of precession, the slow wobble of Earth’s axis with a period of roughly 26,000 years, the dates when the Sun enters a given constellation drift gradually over the centuries.

A constellation is not a sign

Here is the key distinction. A constellation is a region of sky, measured and catalogued by astronomers. A zodiac sign in Western astrology is an equal 30° division of the ecliptic, counted from the March equinox point. Signs carry the constellations’ names because, many centuries ago, the two roughly overlapped. Today they no longer match: the sign is a convenient division of the yearly circle, while the constellation is a real group of stars with uneven borders.

Astrology is a cultural tradition with its own logic and is not part of astronomy. So when someone says “the Sun is in Leo”, it helps to ask which meaning is intended: its position among the stars, or the astrological division. You can read more in Why zodiac signs don’t match the constellations and Astronomy vs astrology.

Both ways of looking at the sky have their place, as long as we keep them apart. And the good news is that the zodiac constellations are visible to the naked eye: all you need is a dark sky and a little patience.

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