Birth Chart Calculator

What does this tool actually compute?

Three placements and nothing else. The Sun sign comes from fixed calendar date ranges, the Moon sign from an approximate formula for the Moon’s ecliptic longitude, and the Rising sign from a rough estimate built on the Sun’s position and the birth time. It does not calculate houses, the other planets, aspects or a chart wheel, so it is not a full natal chart.

How is the Sun sign decided?

By a lookup of month and day only, for example Gemini from 21 May to 20 June and Capricorn from 22 December to 19 January. The year, hour and place are ignored. The Sun actually changes sign on a slightly different day and hour each year, so someone born on a boundary day may belong to the neighbouring sign, and the tool does not check.

How is the Moon sign worked out?

The tool converts the birth date and time to days since 1 January 2000, 12:00 UT. It then takes the Moon’s mean longitude (218.316° plus 13.176396° per day) and adds one correction of 6.289° times the sine of the mean anomaly (134.963° plus 13.064993° per day). The result is divided by 30° to pick the sign. Smaller perturbation terms are left out, so the longitude can be off by a degree or two and a Moon near a sign boundary may land in the wrong sign.

Which time zone does the birth time use?

None. The hour and minute are treated as local mean solar time at the longitude you enter and converted to universal time by subtracting longitude ÷ 15 hours. Time zones and daylight saving are not looked up. Entering a clock time that is an hour off moves the Moon by about half a degree but moves the estimated Rising sign by about half a sign.

How is the Rising sign estimated, and does latitude matter?

The tool assumes the Sun is on the eastern horizon at 06:00 and that the ascendant advances 15° per hour, so Rising longitude equals the Sun’s longitude plus 15° times (hour minus 6). Latitude is accepted but not used, and longitude only affects the Moon. A real ascendant moves faster or slower depending on the sign and the latitude, so this estimate can miss by a sign or more.

Where do the element, modality and keyword come from?

Only from the Sun sign, through fixed tables in the code. Element groups the signs into Fire, Earth, Air and Water; modality groups them into Cardinal, Fixed and Mutable. The keyword is a short phrase from the tool’s own list of traditional associations for that sign, such as “Curious and adaptable” for Gemini.

What happens if I leave the time blank or enter 0?

Clearing the hour box makes it 0, which is a valid midnight and is used as such (it is not replaced by the default of 12), and clearing the minute box makes it 0 minutes. Hours are held between 0 and 23, minutes between 0 and 59, and longitude between −180 and 180. If the date field is empty the tool uses 15 June 1990.

Is astrology scientifically supported?

No. Astrology is a traditional, belief-based practice, and its claims have not been scientifically validated. This tool predicts nothing about a person; it reports where an approximation puts three points and attaches the traditional label for the Sun sign.