Rising Sign (Ascendant) Calculator
Your Rising Sign (Ascendant) is the zodiac sign that was rising on the eastern horizon at the exact moment of your birth. It shapes your outward personality and first impressions.
Formula
1. Convert to Universal Time (UT):
UT = Local Time − UTC Offset
2. Julian Date (JD):
JD = JDN + (UT − 12) / 24
where JDN uses the proleptic Gregorian calendar formula.
3. Greenwich Mean Sidereal Time (GMST):
T = (JD − 2451545.0) / 36525
GMST = 280.46061837 + 360.98564736629 × (JD − 2451545.0) + 0.000387933T² − T³/38710000 (degrees)
4. Local Sidereal Time (LST):
LST = GMST + Longitude (degrees)
5. Obliquity of the Ecliptic (ε):
ε = 23.439291111 − 0.013004167T − 0.000000164T² + 0.000000504T³
6. Ascendant Ecliptic Longitude:
ASC = atan2(−cos(RAMC), sin(RAMC)·cos(ε) + tan(φ)·sin(ε))
where RAMC = LST (in degrees), φ = geographic latitude.
Quadrant correction: if cos(RAMC) > 0, add 180°.
7. Zodiac Sign:
Sign = floor(ASC / 30°) → maps to one of 12 signs (Aries = 0°–30°, etc.)
Assumptions & References
- Uses the standard Western tropical zodiac (Aries begins at 0° ecliptic longitude).
- GMST formula from the IAU 1982 standard (accurate to ~0.1 arc-second for dates 1800–2200).
- Obliquity formula from Meeus, Astronomical Algorithms, 2nd ed., Chapter 22.
- Ascendant formula is the classical Placidus/standard formula used in Western astrology.
- Birth time must be the local clock time; enter the correct UTC offset for that date (accounting for DST manually).
- Accuracy degrades for extreme latitudes (above ~65°N/S) where some signs may not rise.
- Sun longitude is computed for obliquity refinement only; a mean obliquity is sufficient for sign-level accuracy.
- Results are for entertainment/astrological purposes; not a substitute for professional chart software.