Castor is one of the named Gemini pair. Its Bayer designation begins with Alpha, yet Pollux is visibly brighter in the IAU table. This makes the pair a useful reminder that a catalogue designation is not a brightness ranking or a horoscope interpretation.
| IAU name | Castor |
|---|---|
| Designation | Alpha Geminorum · HR 2891 |
| Constellation | Gemini |
| Visual magnitude in IAU table | 1.98 |
| J2000 right ascension | 113.649428° |
| J2000 declination | 31.888276° |
| Tropical longitude on 1 Jan 2026 | 20.61° Cancer |
| Ecliptic latitude on 1 Jan 2026 | 10.10° |
Reading the coordinates
At the stated 2026 reference instant, the projected tropical position is 20.61° Cancer, with ecliptic latitude 10.10°. Its latitude is substantial. A planet at the same ecliptic longitude may still be separated by many degrees in the sky. The locator reports the full angle for that reason.
Right ascension and declination locate Castor on an equatorial sky map; the tropical position rotates and projects that J2000 direction into a dated ecliptic frame. Precession changes the printed zodiac longitude over centuries. It does not move the star from Gemini to a different astronomical constellation. This is why an undated “fixed degree” should be checked against its source.
A useful research question
When a text says “the Twins,” does it distinguish Castor from Pollux? A clear answer identifies the named star, the date, and the method of comparison. Ptolemy’s surviving astrological discussion groups stars by constellation figure and assigns symbolic qualities; it is a historical text, not a measured guide to personal outcomes.
Test a recorded birth moment →
Position calculation
The IAU table identifies the named star and gives its right ascension and declination at J2000. Astronomy Engine rotates that direction to the true ecliptic and equinox of a chosen date. The displayed tropical longitude is the angle around the ecliptic; latitude records how far above or below that plane the star lies. The reference date on this page is 1 January 2026 at 12:00 UTC.
The calculation does not propagate individual stellar proper motion, parallax, or perspective effects. It is intended for modern reference dates, not ancient reconstructions. A planet sharing a projected longitude with a star can still be far away on the sky. The chart locator shows that full angular separation and limits automatic matches to stars within 40° of the ecliptic.