Why Time and Location Change the Result
The sun-sign calculation uses your birth date and time, so people born near a sign change can receive different signs from tools using a more precise ephemeris. The moon moves much faster, changing signs every two to two-and-a-half days, so its position depends on the specific date and, near a boundary, the time. The rising sign depends on which zodiac point crosses the local eastern horizon at that moment, so it also needs a time, UTC offset, and location. The calculations here use simplified position formulas adapted from Paul Schlyter's planetary-position guide. They provide sign-level estimates, not a professionally verified full natal chart.
Frequently Asked Questions
How accurate is this birth chart calculator?
This calculator uses compact position formulas to estimate tropical zodiac positions. Checked against a full astronomical ephemeris across 1900-2099, its sun, moon, planet, and ascendant positions agreed to within about a tenth of a degree - so the sign shown only differs from a professional chart when a placement sits right on a sign boundary. The bigger source of error is usually the input: the rising sign and houses are particularly sensitive to errors in birth time, UTC offset, or coordinates. Treat the results as estimates, not as a precision-certified chart.
Do I need to know my exact birth time?
Only for the rising sign, which changes roughly every two hours and is highly sensitive to birth time - even a few minutes of error can shift it into a neighboring sign. Sun and moon signs are far less time-sensitive; a birth date alone is enough for the sun sign, and a date plus a rough time is enough for the moon sign in most cases.
Why does my rising sign need a birth location but my moon sign doesn't?
The moon and sun's zodiac positions at a given moment are the same no matter where on Earth you're standing. The rising sign is different: it's defined by which point of the zodiac is crossing your local eastern horizon at your birth moment, which depends entirely on your latitude and longitude as well as the time.
Is my birth information stored or sent anywhere?
No. Every calculation on this page runs in your browser using JavaScript - nothing about your birth date, time, or location is transmitted, logged, or stored anywhere, including by this site.
What if I don't know my exact birth time?
Check "I don't know my exact birth time" and the calculator will show an approximate moon sign for midday on your birth date - local midday if you choose a UTC offset, otherwise 12:00 UTC. Near a sign boundary this can differ from your actual moon sign. The rising sign can't be estimated without the birth time, UTC offset, and location.