Your Sun sign, cusp days and the 13th sign

How Signology works it out

1. Signology computes the Sun's position with Astronomy Engine, an open-source library.
2. Each sign covers 30 degrees of the tropical zodiac, starting with Aries at the March equinox.
3. Without a birth time, it reads your whole birth day, in your device's time zone or a birthplace zone you choose.
4. If the Sun changed sign that day, you see both signs and the clock time of the change, and you choose. Until then, the reading uses the midday sign.

Why another site may show something different

Many horoscope columns use fixed dates, such as Aries from March 21 to April 19, but the real change can come a day earlier or later. On the 12 change days a year, about 3 in 100 birthdays, a table and the sky can disagree.

Vedic astrology, the main tradition in India, measures signs against the stars. Its zodiac sits about 24 degrees from the tropical one, so for nearly 4 in 5 people born from 1966 to 1991 it gives the previous sign.

The 13th sign, Ophiuchus, is a constellation behind the Sun from about November 29 to December 18. The twelve signs are equal slices of the year, so Signology does not use it.

Check it yourself

The US Naval Observatory lists the equinoxes and solstices, when the Sun enters Aries, Cancer, Libra and Capricorn. For any sign, use JPL Horizons with the Sun as target, the observer Geocentric and observer ecliptic longitude (quantity 31): each multiple of 30 starts a sign, with Aries at 0.

What tradition links with it

Astrologers link the Sun with the center of who you are, what gives you energy and direction. One small step this week: read your sign's description, or both if you were born on a change day, and mark one line that sounds like you.