December 30
The Dove at Year's Close
Sun Position
The Sun is in Capricorn, at about 22.9 degrees south declination, retreating slowly northward. Northern Hemisphere days are growing by roughly a minute per day now, gaining momentum into January; southern days are shortening at the same rate.
Sky Highlight
Columba the Dove is a faint southern constellation that passes its highest point for northern observers in late December evenings; from tropical and southern latitudes it is well-placed and worth hunting down for NGC 1851. The broader Canis Major-Columba-Puppis region is the richest section of the winter Milky Way for southern observers.
Deep Sky Object
NGC 1851, the compact globular cluster in Columba, lies about 39,500 light-years away and is one of the more concentrated globulars in the Milky Way's halo, its unusual internal structure, with two chemically distinct stellar populations, has made it a subject of active research into the formation history of globular clusters.
Featured Star
Phact (α Col) is a B7IVe blue-white subgiant about 261 light-years away, returning to close December as the brightest star in Columba the Dove. Its 'e' emission classification places it among the Be stars, rapidly rotating objects whose equatorial spin ejects material into a surrounding decretion disk, a dynamical process ongoing right now above the plane of the solar system.
Around This Date
- December 30, 1924Edwin Hubble presented to the AAS the Cepheid-based distance to the Andromeda Galaxy, showing it to be roughly 900,000 light-years away (a figure later revised upward), definitively settling the 'Great Debate' about whether spiral nebulae were within or beyond the Milky Way.
- December 28, 1882Arthur Eddington was born; his 1919 confirmation of light bending around the Sun made Einstein's general relativity the most tested physical theory of the twentieth century.
Phact spins out a disk we cannot see from here, and the year spins toward its close, both turning on invisible axes.