November 21
The River's End
Sun Position
The Sun is in Sagittarius near -19° declination. Northern Hemisphere days are short and cold at higher latitudes; Southern Hemisphere is moving through its warmest weeks.
Sky Highlight
Achernar, the river's end, transits near midnight from southern mid-latitudes in November, making this one of the best times of year to observe it from Australia, southern Africa, or South America. From the Northern Hemisphere, Achernar is not visible above latitudes of about 33°N, and even near that boundary it barely clears the horizon.
Deep Sky Object
The Small Magellanic Cloud (SMC), irregular dwarf galaxy, about 200,000 light-years away, companion to the Milky Way. In November evenings from southern latitudes, the SMC is well up in the south, an irregular glowing patch in Tucana rich with star clusters, nebulae, and compact stellar nurseries. Entirely invisible from latitudes north of about 15°N.
Featured Star
Achernar (α Eridani) is a main-sequence B-type star 139 light-years away, spectral class B6Vep, but it spins so rapidly that it is visibly oblate: its equatorial diameter is about 56% larger than its polar diameter, the most extreme oblateness measured in any bright star. It marks the end of the river Eridanus, a star so far south that northern star catalogs long struggled to locate it precisely.
Around This Date
- November 21, 1905Albert Einstein submitted his paper on special relativity's energy-mass equivalence (E = mc²) to Annalen der Physik, formalizing the mass-energy relationship central to modern astrophysics.
- November 24, 1639Jeremiah Horrocks observed the transit of Venus from his home in Lancashire, England, making the first confirmed observation of this event.
Achernar spins fast enough to flatten itself into an oblate disk, the most misshapen prominent star in the sky, and you can't tell by looking.