What is the zodiac?

What is a sun sign?

Seen from Earth, the Sun creeps eastward against the background stars by about a degree a day, and in a year it goes all the way round. That yearly path is called the ecliptic, and the zodiac is the band of sky along it. Astrology cuts the band into twelve equal signs of 30°, counted from the point where the Sun stands at the March equinox. Your sun sign is the sign the Sun was in when you were born. Nothing more mysterious than that. ^1,2

Ptolemy, writing in Alexandria in the second century, admits that a circle has no natural starting point and then starts at the spring equinox anyway. That choice ties the twelve signs to the seasons rather than to the stars: Aries always opens the northern spring, Cancer always begins at the June solstice, Libra at the September equinox and Capricorn at the December solstice. ^2,3

A sun sign is only the first line of a birth chart. The Moon, the rising sign and every planet sit in signs of their own, and astrologers read them together. The sun sign became the famous one for a practical reason: it is the only placement you can work out from a birthday alone, with no clock and no almanac. That is also why newspapers could print one column for everyone born in the same month. ⁴

How the twelve signs are made

The structure every sign page on this site builds on.

The signs are named after constellations, but they are not the same thing. Constellations are star pictures of all sizes; signs are equal slices of 30°. When the Babylonians fixed the twelve, the slices and the pictures roughly lined up. Earth’s axis wobbles slowly, like a spinning top, so the equinox point drifts backwards against the stars, and over two thousand years signs and constellations have slid most of a sign apart. NASA adds that the Sun actually crosses thirteen constellations: the Babylonians left out Ophiuchus, the serpent-bearer, to fit a twelve-month calendar. ^1,5

That drift is why there are two zodiacs. Western astrology uses the tropical zodiac, measured from the equinox and so tied to the seasons; it is the one used by newspaper horoscopes, by this site and by the machine. The sidereal zodiac, used in Indian astrology, measures from the stars instead, which puts most people’s sidereal sun sign one sign earlier than their tropical one. Neither is a measurement error. They answer different questions: where the Sun was against the seasons, or against the stars. ¹

The dates move a little from year to year. A year is about 365¼ days, not 365, so the moment the Sun enters a sign slides by roughly six hours each year and jumps back with every leap day. The change happens at one instant for the whole world, which can be the evening of one date in Los Angeles and the next morning in Tokyo. That is why one website starts Leo on July 22 and another on July 23, and why the machine’s own tables give spans such as “July 22, 23 or 24”. Born on one of those days, you are on the cusp, and the exact minute decides.

Two old groupings give the twelve their shape. Ptolemy set the signs in four triangles of three, each sign 120° from the next, and later astrologers named the triangles after the four elements: fire, earth, air and water. He also sorted them by the seasons, which is where the three modalities come from: a sign that opens a season (cardinal), one in the middle of it (fixed) and one at its end, already changing (mutable). Four elements times three modes makes twelve, and no two signs share both. ²

Each sign also has a ruling planet, its “house” in Ptolemy’s word. He gave Leo to the Sun and Cancer to the Moon, then shared the five planets he could see out in mirror pairs on either side. Uranus, Neptune and Pluto, found in 1781, 1846 and 1930, were later handed Aquarius, Pisces and Scorpio by modern astrologers, while the old rulers kept their claim. ^2,6,7,8

Usual dates; the first and last day of each sign can shift by a day (see above). Rulers are the traditional ones, with the modern co-ruler in brackets.

Three things most people don’t know

Aries began as a farmhand

Before it was a ram, the first sign was a man and a sheep. Babylonian astronomers saw a farm worker they called the Hired Man in these stars, standing with a sheep, and late Babylonian texts name the sign Agru after him. The Ram came later. ^9,10

The Tropic of Cancer is in Taurus

The line on the map is named for the sign the Sun stood in at the June solstice when the name was coined. The slow wobble of Earth’s axis has since carried that point into Taurus, as NASA’s Astronomy Picture of the Day explains, but the line kept the Crab’s name. ²²

The newspaper horoscope began with a royal birth

In August 1930 the Sunday Express asked R. H. Naylor to cast a horoscope for the newborn Princess Margaret. Readers liked it so much that it became a weekly column, and by 1937 it was sorted by the twelve sun signs. Nearly every horoscope column since has followed that pattern. ⁴