The observatory’s main instrument

Your birth chart, computed.

Sun through Pluto by tropical sign and degree, plus your rising sign — from real ephemeris math, in your browser. Free, no email required.

The instrument

Enter your birth data

Local clock time at the birthplace. Unknown time → computed for 12:00 local, no rising sign.

41.8781° N · 87.6298° W

The offset is only a guess from longitude — check what the local clock actually read at your birth (daylight saving shifted it in many places and years). We don’t fake historical time-zone lookups.

No observation yet — enter birth data above

01 — Method

How this calculator works

This is an astronomical instrument first: the positions are the same ones an ephemeris table would print, mapped onto the zodiac frame astrology uses. Five facts define every number on this page:

  1. 01

    Apparent geocentric positions

    For the instant of birth (converted to Universal Time), we compute each body's geocentric position with light-time and aberration corrections — the sky as it actually appeared from Earth, not idealized orbits.

  2. 02

    Ecliptic longitude of date

    Each position is expressed as ecliptic longitude measured along the true ecliptic and equinox of date — the coordinate astrology has always used. 0° is the March equinox point; the full circle is 360°.

  3. 03

    Tropical signs, 30° each

    Longitude maps directly to sign: 0°–30° is Aries, 30°–60° Taurus, and so on through Pisces. A reading of “Capricorn 10°22′” means the body sat 10 degrees 22 arcminutes into Capricorn's segment.

  4. 04

    The ascendant from sidereal time

    With a birth time and place, the rising degree comes from the standard horizon formula: Asc = atan2(cos RAMC, −(sin RAMC · cos ε + tan φ · sin ε)), where RAMC is local apparent sidereal time in degrees, φ the latitude, and ε the obliquity of the ecliptic.

  5. 05

    The source of truth

    All ephemeris math runs on astronomy-engine (MIT license) — truncated VSOP87 for the planets, an ELP2000-derived model for the Moon — accurate to roughly one arcminute across 1700–2100. Your data never leaves the browser; the computation happens on your device.

One honest limit: converting a historical local birth time to Universal Time needs the full time-zone database, which this page does not ship. You confirm the UTC offset your birthplace clock used; everything after that is deterministic astronomy.

Frequently asked questions

What is a birth chart (natal chart)?

A birth chart is a map of where the Sun, Moon, and planets stood — measured as ecliptic longitude, their position along the zodiac — at the exact moment and place you were born. Each body falls in one of the 12 tropical zodiac signs (30° each), and astrologers read the pattern as a whole rather than the sun sign alone.

What is a rising sign (ascendant)?

The rising sign, or ascendant, is the zodiac degree that was coming up over the eastern horizon at your moment of birth. It depends on the exact time and the latitude/longitude of the birthplace, and it changes roughly every two hours — which is why astrologers ask for a precise birth time. It is computed from local sidereal time, your latitude, and the tilt of Earth's axis.

Why do I need my exact birth time?

The ascendant moves through the full zodiac once a day, so even 30 minutes shifts it several degrees and can change the rising sign entirely. The Moon also moves about 13° per day, so a birth time can decide its sign on days when it crosses a boundary. Without a time, this calculator uses 12:00 local, skips the rising sign, and flags the Moon as approximate.

What's the difference between the tropical and sidereal zodiac?

This calculator uses the tropical zodiac of Western astrology: 0° Aries is fixed to the March equinox, and each sign spans exactly 30° of ecliptic longitude from there. The sidereal zodiac used in Vedic astrology anchors signs to the constellations instead, which currently places every position about 24° earlier. Same sky, different reference frame.

How are the planet positions calculated?

In your browser, with astronomy-engine — an MIT-licensed ephemeris library built on truncated VSOP87 planetary theory and an ELP2000-derived lunar model, accurate to about one arcminute. We take each body's apparent geocentric position (light-time and aberration corrected) and convert it to ecliptic longitude of date, the standard measure astrologers use. Nothing is estimated from date ranges.

Why do I have to pick a UTC offset?

Your birth certificate shows local clock time, but the math needs Universal Time. Converting between them for historical dates requires the full time-zone database — daylight-saving rules changed constantly through the 20th century — so instead of silently guessing wrong, we suggest an offset from your longitude and let you confirm what the local clock actually used.

Got your placements? Read what they mean — start with your sun sign’s full guide, or find your sign the quick way with the setting circle.