December 23
The Brightest in the Sky
Sun Position
The Sun is in Capricorn, at about 23.4 degrees south declination, just past the solstice. Northern Hemisphere days are beginning their imperceptible lengthening; southern days are beginning their slow shortening.
Sky Highlight
Sirius reaches its highest point in the south around midnight for mid-northern observers in late December, and transits near the zenith for observers at about 17°S latitude. Its blue-white scintillation near the horizon is often mistaken for a UFO; the color play is produced by differential refraction in Earth's atmosphere.
Deep Sky Object
M41, the open cluster in Canis Major about 2,300 light-years away, is directly south of Sirius by about 4 degrees, a naked-eye object from a dark site and a showpiece in binoculars. It contains about 100 stars and an orange giant near its center that stands out clearly in binoculars.
Featured Star
Sirius (α CMa) is an A1Vm main-sequence star just 8.6 light-years away (one of the Sun's nearest neighbors) and the brightest star in the night sky by a comfortable margin, shining at magnitude -1.46. The Egyptians timed the Nile flood to its heliacal rising; today it anchors every southern winter and northern winter sky as reliably as any star can.
Around This Date
- December 23, 1672Cassini discovered Rhea, the fifth moon of Saturn to be identified, using a refracting telescope at the Paris Observatory, confirming that the outer planets had complex systems of their own.
- December 24, 1968Apollo 8 entered lunar orbit, and on Christmas Eve Frank Borman, Jim Lovell, and William Anders became the first humans to see the far side of the Moon and to witness Earthrise over the lunar horizon.
Sirius is 8.6 light-years away and still the brightest star in the sky, proximity is its only trick, and it is a good one.