How to use it
- Enter the Date of birth and the Time on the clock where you were born. If you do not know the time, tick I do not know the time.
- Type your birth city in Place and pick it from the list. For a place that is not listed, tick Enter coordinates instead and type the latitude, longitude and time zone.
- Choose a House system. Placidus is the usual choice; Whole Sign and Equal are simpler.
- Read the wheel and the tables. Use Download the wheel as PNG to keep the wheel, which is made on your device.
How the positions are calculated
Your local time is turned into a UTC instant with the time zone rules of the place for that date, including the old clock changes. The Sun, Moon and planets are shown as geocentric, apparent ecliptic longitudes of the date, calculated by astronomy-engine, an MIT-licensed library that matches the NASA ephemeris to a few arc seconds.
A planet is retrograde when its longitude is decreasing that day, as seen from the Earth. The Moon's North Node is the mean node, as given in Jean Meeus, Astronomical Algorithms (1998), chapter 47.
- Sidereal time, the Ascendant and the Midheaven follow the formulas of Jean Meeus, Astronomical Algorithms, chapters 12, 14 and 22.
- The houses depend on the time and the place: four minutes of time move the Ascendant by about one degree.
Houses, aspects and the balance
Placidus divides the day into twelve parts of equal time, so houses differ in size. Whole Sign makes each house one full sign, starting with the sign of the Ascendant. Equal makes twelve houses of 30 degrees from the Ascendant.
Placidus cannot be calculated above about 66 degrees of latitude, because the ecliptic does not set there. In that case the chart uses the Equal system and says so.
- Aspects are the five major ones: conjunction and opposition within 8 degrees, trine and square within 7 degrees, sextile within 5 degrees. This site chose these orbs.
- The balance counts the ten bodies from the Sun to Pluto, the Moon included, by element (fire, earth, air, water) and by mode (cardinal, fixed, mutable).
When the time is not known
Without a birth time there is no Ascendant and no houses. The planets are shown for 12:00 local time, and the Moon is shown as a range: its sign at the start of the day and every change of sign during it. The Moon changes sign about every two and a half days, so the range often covers one change.
Worked example: New York, 4 July 1990, 14:30
The local time is 14:30 daylight time in New York, UTC-4, so the chart uses 18:30 UTC. At that moment the Sun is at 12°31′ Cancer, in the ninth house, and the Moon at 5°54′ Sagittarius, in the second house.
The Ascendant is 28°27′ Libra and the Midheaven 3°41′ Leo. Saturn, Uranus, Neptune and Pluto are retrograde that day. Mercury, Jupiter and the Sun share the ninth house, and Mercury forms a trine with Pluto within 0°05′.
Frequently asked questions
Do I need the exact birth time?
For the houses and the Ascendant, yes: they change about every four minutes, so a wrong time moves them. Without a time the planets are still correct for the day, and the Moon is shown as a range.
What if my birth certificate shows a different time?
Certificates may record the clock time of a time zone that differed then, or local mean time. Use the local clock time of that place on that date; the chart converts it with the zone rules of that year.
What is the difference between Placidus and Whole Sign?
Placidus cuts the sky by time, so house sizes vary and a sign can hold two house cusps or none. Whole Sign makes every house one full sign, which is simpler and common in older traditions.
Is a birth chart scientifically valid?
No. The positions are real astronomy, but the link between the sky and character or events comes from tradition and has not been shown by science. Use the chart for reflection and entertainment.
What does R next to a planet mean?
R marks a retrograde planet. Seen from the Earth it appears to move backwards for a few weeks or months. Astrologers read this as a time for review, but the motion is an effect of the viewpoint.
Does the calculator keep my birth data?
No. The date, time and place are used in your browser, and nothing is stored or sent to us. The only file is the wheel you choose to save as a PNG on your own device.
Sources
- astronomy-engine, the MIT-licensed library that calculates the positions
- JPL Horizons, the NASA ephemeris used to check the positions
- IANA Time Zone Database, the historical clock rules of each zone
- GeoNames, city coordinates and time zones (CC BY 4.0)
Figures and text last checked on Tuesday, November 10, 2026.