Sun sign, in full
Western astrology divides the ecliptic, the apparent yearly path of the Sun against the stars, into twelve equal 30-degree arcs. The count starts at the March equinox, the point where the Sun crosses the celestial equator heading north. That point is 0 degrees Aries by definition, and the twelve signs follow in fixed order from there. A Sun sign is whichever of those arcs the Sun was crossing on the day someone was born.
Because the scheme is anchored to the equinox rather than to the stars, it is called the tropical zodiac, and it has drifted away from the constellations that gave the signs their names. Earth's axis wobbles on a cycle of about 25,800 years, which moves the equinox point one degree roughly every 72 years. The accumulated offset now runs to something like 24 degrees, close to a whole sign, so a late-March birth counts as tropical Aries while the Sun itself stands in front of the constellation Pisces.
A Sun sign is also one placement out of many. A full birth chart positions the Moon, the planets and the ascendant as well, and astrologers generally treat the Sun sign on its own as the thinnest reading available. Sign boundaries shift by a day between years, so a birth within about 24 hours of a boundary needs a chart cast for the actual birth time and place before anyone can say which side it falls on.
- Astrology
- Star sign, Birth sign
A worked example
Leo runs from about 23 July to 22 August on the tropical zodiac, which is the stretch where the Sun sits between 120 and 150 degrees of ecliptic longitude measured from the March equinox point. The constellation Leo, marked out by the boundaries astronomers use, currently has the Sun behind it from roughly mid-August to mid-September. Same name, two different measurements, about three weeks apart.
Sources
Earth's axial precession completes a cycle in roughly 25,800 years, moving the equinox point about one degree every 72 years.
Wikipedia — Axial precession (rate: 50.3 arcseconds per year, 1 degree per 71.6 years, period 25,772 years)
See also
Where it comes up on this site
Sun sign: frequently asked
Yes, in everyday use. Astrologers prefer Sun sign because a chart holds eleven other placements the casual word hides.
The Sun enters each sign at an exact moment that drifts through the four-year leap cycle, quoted in universal time. Published date ranges round that off, so sources disagree by a day at the boundaries. A chart cast for your birth time and place settles it, and nothing else will.
It means the tropical zodiac is measuring the seasons rather than the stars, and 0 degrees Aries is the March equinox whatever sits behind it. Sidereal astrology, the tradition used across India, keeps the signs pinned to the constellations instead, which is why the two systems give different answers for the same birth and both consider themselves correct.