December 20
The Camel's Brand Mark
Sun Position
The Sun is in Sagittarius, at roughly 23.4 degrees south declination, the day before the solstice. Northern Hemisphere days have reached their minimum length; the turning point is at hand.
Sky Highlight
Gemini is near the zenith for mid-northern observers by midnight, making this a fine week to examine the constellation's full breadth, from Propus and Tejat at the feet all the way up to Castor and Pollux at the heads, spanning about 20 degrees of sky. Tonight Alhena marks the foot of the twins.
Deep Sky Object
NGC 2392, the Eskimo Nebula in Gemini, about 2,870 light-years away, is best seen in December when Gemini transits high; a 150mm telescope shows its disk shape clearly, and larger apertures reveal the textured inner shell surrounded by the fainter outer halo.
Featured Star
Alhena (γ Gem) is an A0IV white subgiant about 109 light-years away, a spectroscopic binary with a closely orbiting companion detected only through Doppler shifts. Its Arabic name refers to a brand mark made on a camel's neck, and it sits at the foot of the twins' shared border, a relatively near and straightforward white star in a region full of distant supergiants.
Around This Date
- December 21, 1968Apollo 8 launched from Kennedy Space Center, beginning humanity's first crewed journey to another world's gravitational sphere of influence, with Frank Borman, Jim Lovell, and William Anders bound for the Moon.
- December 15, 1970Venera 7 confirmed the Venusian surface temperature of around 465°C, a reading that validated models of the greenhouse effect and permanently altered how planetary scientists thought about atmospheric dynamics.
Alhena is the foot of the twins and the edge of the pattern, the solstice is one day away, and the year is about to pivot.