How it’s calculated
The math behind your chart — and how to trust it.
A birth chart is only as good as the astronomy under it. Science of Light is free, but the engine isn’t a shortcut — it’s built to land on the same numbers a professional pays for. Here’s exactly how, in plain terms.
Where the planets are
Planetary positions come from a high-precision astronomical model and are validated to the arc-minute against the Swiss Ephemeris — the same gold standard used by JHora and the paid professional suites. That’s less than the width of the Moon in the sky: close enough that your nakshatra, sign, and house placements match the pros’.
Sidereal, not tropical
Western astrology uses the tropical zodiac, tied to the seasons. Vedic astrology uses the sidereal zodiac, tied to the actual stars. The gap between them — the ayanamsa — grows about 50 arc-seconds a year as Earth slowly wobbles (precession), and today sits near 24°. Subtract it correctly and your chart reflects the real sky; get it wrong and placements drift by nearly a whole sign.
Your time zone, as it actually was
A chart is exquisitely sensitive to the birth minute, so the time zone has to be the one in force at your birth place and date — not today’s. The engine resolves historical offsets, including daylight-saving shifts and the half-hour and 45-minute zones (India, Nepal), so a 1980 birth in a zone that has since changed still computes correctly.
Choose your ayanamsa
Lahiri is the standard and the default — leave it unless you’ve been reading your chart in another tradition. If your astrologer uses Raman or KP, switch here and every screen recalibrates to match. The setting is saved on your device.
Active calibration: Lahiri — today the offset is 24° 13′ 49″. Cast a chart and it’ll use this system.
No black box, no account
Everything runs in your browser. There’s no server doing the math, no database storing your birth data, and no account to make. Your chart is yours. See how your data is handled and the story behind the project.