Zodiac Sign and Compatibility Calculator

Your sun sign is the stretch of the zodiac the Sun was crossing when you were born. Enter a birth date (and time, if you were born on a cusp) to find it, then compare it with someone else's.

For entertainment and self-reflection: The Sun's position is computed astronomically; astrological meanings and compatibility are traditional symbolism with no scientific support. Nothing on this page is a prediction, and it is not financial, medical, psychological or legal advice. Use it as a prompt for thought, not as a basis for decisions.

How your sun sign is found

Over a year the Sun appears to travel once around the sky against the background stars, along a path called the ecliptic. Western astrology divides that circle into twelve equal 30-degree segments, starting from the point where the Sun crosses the celestial equator in March (the March equinox). Those segments are the signs, from Aries at 0 to 30 degrees through to Pisces at 330 to 360 degrees. Your sun sign is simply the segment the Sun was in at the moment you were born.

Because the calendar year and the solar year do not line up perfectly, the date when the Sun moves into each sign shifts by a day or so from year to year. That is why newspaper horoscopes give slightly different dates, and why people born on the edge between two signs (a "cusp") are sometimes unsure which they are. This calculator does not use a fixed date table. It computes the Sun's actual position for your birth date and time using the open-source Astronomy Engine library, which is based on the VSOP87 planetary theory and checked against NASA's JPL ephemeris. In our tests its solar positions matched JPL Horizons to within a fraction of an arcminute. It then finds the exact moments the Sun entered and left your sign in that year and shows them in your device's time zone.

If you leave the time blank the calculator uses noon. That is fine for almost everyone. If the result panel says you were born within a day of a sign change, enter your birth time to be sure, and make sure your device is set to the time zone where you were born, or adjust the time accordingly.

The twelve signs

Typical dates shift by about a day from year to year. The calculator uses the exact dates for your birth year.

Worked example

Someone born on June 15, 1990 enters their date. The Sun was at about 24 degrees of Gemini, so their sun sign is Gemini, an air sign of mutable mode ruled by Mercury. In 1990 the Sun entered Gemini on May 21 and left it on June 21. Their partner, born on October 2, 1992, is a Libra, another air sign. Gemini and Libra are four signs apart, a "trine" in astrological language, the aspect traditionally considered the easiest and most harmonious, partly because both signs share an element.

Now try a cusp. In 2008 the Sun entered Aries at about 10:48 PM Pacific Daylight Time on March 19 (05:48 UTC on March 20). Someone born in Los Angeles at 11:00 PM on March 19, 2008 is therefore an Aries, even though most fixed charts say Pisces runs until March 20; a baby born two hours earlier that evening is a Pisces. Enter the date with the time, on a device set to Pacific time, and the calculator gets both right. This is exactly the case the calculator is designed for.

How compatibility is judged

Sun-sign compatibility in popular astrology rests on two simple ideas. The first is the element: fire (Aries, Leo, Sagittarius), earth (Taurus, Virgo, Capricorn), air (Gemini, Libra, Aquarius) and water (Cancer, Scorpio, Pisces). Signs of the same element are said to understand each other naturally; fire and air are said to feed each other, and earth and water to support each other. The second is the angle between the signs, borrowed from the aspects astrologers use between planets: signs two apart form a sextile (friendly), three apart a square (challenging but dynamic), four apart a trine (easy), five apart a quincunx (awkward adjustment), and six apart an opposition (attraction of opposites). The calculator names the aspect and the elements and gives a short traditional reading.

Astrologers themselves point out that a sun sign alone is a very small part of a chart: the Moon, rising sign and planets all play their parts. Statistical studies, including a large analysis of marriage and divorce records, have found no tendency for couples with "compatible" signs to marry more often or stay together longer. Read the compatibility result as a playful prompt, not a verdict.

Where the zodiac comes from

The zodiac began in Babylonia. Astronomer-priests there divided the ecliptic into twelve equal parts of 30 degrees by about the fifth century BC, using constellations the Sun passed through. Greek astronomers adopted the system after Alexander's conquests, and Claudius Ptolemy's Tetrabiblos, written in Alexandria in the second century AD, became the foundational text of Western astrology. The idea that the sun sign alone describes personality is much more recent: it grew out of newspaper horoscope columns in the 1930s, beginning with R. H. Naylor's columns in the British Sunday Express.

One more point of genuine astronomy. Because Earth's axis slowly wobbles (precession), the signs used in Western astrology, which are tied to the equinox, have drifted about 24 degrees from the constellations that share their names. On your birthday the Sun was probably in front of the constellation before your sign. Western "tropical" astrology deliberately follows the seasons rather than the stars; Indian "sidereal" astrology follows the stars. Neither is a mistake, they are different conventions. The calculator uses the tropical zodiac, the standard in Western horoscopes.

Using it for self-reflection

Sign descriptions are broad by design. You will probably recognise yourself in parts of several signs, which psychologists call the Barnum or Forer effect. Instead of asking whether the description is "true", ask which parts you would like to be true, or which you resist. An Aries reading "bold and eager to start" might ask what they are hesitating over. A Virgo reading "practical and analytical" might ask where they could let something be good enough. Compatibility notes can open a gentle conversation with a partner about how you each handle conflict, plans and free time.

Frequently asked questions

Sources and further reading

• Astronomy Engine by Don Cross (MIT licence), github.com/cosinekitty/astronomy; positions checked against NASA JPL Horizons.
• Wikipedia: "Zodiac", "Astrological sign", "Tetrabiblos", "Axial precession", "Horoscope".
• Rogers, J. H. "Origins of the ancient constellations", Journal of the British Astronomical Association 108 (1998).
• Carlson, S. "A double-blind test of astrology", Nature 318 (1985).
• Silverman, B. "Studies of astrology", Journal of Psychology 77 (1971): sun signs of 2,978 married and 478 divorced couples.

By Stanley King · About 8calc · Last reviewed October 2026

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