December 31

December 31

The Footstool of Orion

Sun Position

The Sun is in Capricorn, at about 22.8 degrees south declination, continuing its slow northward return. Northern Hemisphere days are gaining incrementally on darkness; southern days are shortening at the same quiet pace.

Sky Highlight

The last night of the calendar year finds Orion fully cleared of the southeastern horizon by 8 PM for northern observers, with Eridanus winding southward from Cursa near Rigel all the way toward Achernar near the south celestial pole. The full river is visible only from the Southern Hemisphere, but its northern reaches are well placed for everyone tonight.

Deep Sky Object

NGC 1535, a small but bright planetary nebula in Eridanus about 5,000 light-years away, is well placed in December evenings, a blue-gray disk in moderate apertures sometimes called the Cleopatra's Eye Nebula, compact enough to be mistaken for a star at low power but revealing its disk shape clearly at higher magnification.

Featured Star

Cursa (β Eri) is an A3IIIvar white giant about 89 light-years away, sitting at the northern end of Eridanus the River, just northwest of Rigel, traditionally the footstool or footrest of Orion. Its variability is subtle, and at 89 light-years it is one of the nearer stars in December's lineup, a relatively modest giant compared to the supergiants that dominate this month.

Around This Date

  • December 30, 1924Edwin Hubble presented his measurement of the distance to the Andromeda Galaxy to the American Astronomical Society, demonstrating it lay far beyond the Milky Way and establishing that the universe contains billions of galaxies beyond our own.
  • December 28, 1882Arthur Eddington was born in Kendal, England; his theoretical work on stellar interiors and his 1919 solar eclipse observations confirming light deflection by gravity would make him one of the most consequential astrophysicists of the twentieth century.

The river starts here, at Orion's foot, and runs all the way to the southern horizon, a good reminder that every ending is also a place where something else begins.