Big three calculator

The big three are the three placements astrologers read first. Your Sun sign is where the Sun was when you were born: the sign you give when someone asks yours, read as what you are growing into. Your Moon sign is where the Moon was, read as your needs and moods. Your rising sign is the sign that was coming up on the eastern horizon, read as the way you come across. Together they make a rough sketch that the rest of the chart fills in.

People say “Leo Sun, Pisces Moon, Virgo rising” as shorthand for a whole personality, and the combination often says more than any one of the three. When the Sun and the rising sign match, people tend to read you as exactly what you are. A Moon in a very different element from the Sun often describes a private self that surprises people who only know the public one.

The Sun moves about a degree a day, so your birth date is usually enough. Only on the twelve or so days a year when it changes sign do the year and the time matter, and the date the change falls on shifts from year to year, which is why a table of sign dates can be wrong for a birthday near the edge. The Moon moves about 13 degrees a day and changes sign every two to two and a half days, so on roughly four days in ten the time decides. The rising sign turns through all twelve signs in a day and cannot be found at all without a time.

The calculator treats each one on its own terms. With a birth time you get three signs and their degrees. Without one you still get your Sun sign, your Moon sign if the Moon stayed in one sign all day (or both possibilities and the time it moved if it did not), and a table of the sign rising at every time of your birth date.

Your birth date, time and place are converted to Universal Time with the time-zone history of your birthplace. The Sun and Moon come from astronomy-engine, an open-source ephemeris, and the rising sign from the sidereal time and the tilt of the Earth's axis. All three are placed in the tropical zodiac, measured from the March equinox, and agree with the Swiss Ephemeris to within about an arcminute.

Read the three descriptions under your result as a conversation rather than three verdicts. Where do they agree? Where do they pull against each other? Then open your whole chart: it adds the other planets, the houses and the angles between them, and names well-known people who share your Sun, Moon or rising sign. For one of the three on its own, with more detail, there is a Moon sign calculator and a rising sign calculator.