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A Complete Guide to the Solar System and the Night Sky
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public Observing fromLocation: Greenwich, United Kingdom edit_location_alt
SunriseRise: 6:40  
clear_day Transit: 12:10  
SunsetSet: 17:41
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All Constellations » Gemini » 

Castor - α Geminorum (alpha Geminorum)

Altitude: 64.4°   Azimuth: 228.7°   Direction: South-West

Castor, also designated as α Geminorum (alpha Geminorum), is a variable and multiple main-sequence star in the constellation of Gemini.

Castor visual magnitude is 1.98, making it the 46th brightest star in the sky. Thanks to its high brightness, Castor is clearly visible when observed from locations with dark skies, and should be also quite easily visible from light polluted areas.

The proper motion of Castor is -0.171 arcsec per year in Right Ascension and -0.098 arcsec per year in Declination and the associated displacement for the next 1000 years is represented with the red arrow.

Image of Castor - α Geminorum (alpha Geminorum) star
DSS2 image of Castor. The red arrow shows the motion of Castor during the next 1000 years (-2.85 arcmin in Right Ascension and -1.63 arcmin in declination);

Key facts about Castor


Castor - α Geminorum (alpha Geminorum) is above the horizon from Greenwich, United Kingdom edit_location_alt.

Altitude: 64.4°
Azimuth: 228.7°
Direction: South-West

Celestial coordinates and finder chart of Castor

Castor is situated north of the celestial equator, as such, it is more easily visible from the northern hemisphere. Celestial coordinates for the J2000 equinox as well as galactic coordinates of Castor are provided in the following table:

07h 34m 36s
+31° 53’ 17”

The simplified sky map below shows the position of Castor in the sky:

Finder chart Castor - α Geminorum (alpha Geminorum) star
Simplified finder chart for Castor - α Geminorum (alpha Geminorum).

Visibility of Castor from your location

Location: Greenwich, United Kingdom edit_location_alt
Latitude: 51° 28’ 47” N
Longitude: 0° 00’ 00” E
Timezone: Europe/London

Today's Castor - α Geminorum (alpha Geminorum) rise, transit and set times from Greenwich, United Kingdom edit_location_alt are the following (all times relative to the local timezone Europe/London):

Azimuth: 58.1°
Max altitude: 70.4°
Azimuth: 301.9°

Distance of Castor from the Sun and relative movement

Castor is distant 51.53 light years from the Sun and it is moving far from the Sun at the speed of 6 kilometers per second.

15.81 pc
51.53 ly
6 km/s
-0.171 arcsec/year
-0.098 arcsec/year

Spectral properties of Castor

Castor belongs to spectral class A1 and has a luminosity class of V corresponding to a main-sequence star.


The red dot in the diagram below shows where Castor is situated in the Hertzsprung–Russell diagram.

HR Diagram
H-R diagram showing the position of Castor. Attribution: Richard Powell / CC BY-SA

Castor star system properties

Castor is a visual double star which can be observed with the help of medium or large telescopes. The table below shows key information about the Castor double sysyem:

2 arcsec

Alternative designations of Castor

α Geminorum
(alpha Geminorum)
66 Geminorum
Gl 278 A