September 17
Dual Allegiance
Sun Position
The Sun is in Virgo near -2° declination. The autumnal equinox is within one week for Northern Hemisphere observers; in the Southern Hemisphere, the spring equinox brings the same milestone.
Sky Highlight
The Andromeda Galaxy is well-placed in the northeast by 9 PM for Northern Hemisphere observers this week, rising earlier each night. A dark sky shows its elongated haze without optical aid; binoculars reveal the oval core and a hint of M32, its satellite galaxy.
Deep Sky Object
M32, elliptical Galaxy, Andromeda. About 2.65 million light-years away, M32 is a compact dwarf elliptical satellite of M31 and appears as a small, bright ellipse just southwest of Andromeda's core. Best viewed from Northern Hemisphere; low in the north from southern latitudes.
Featured Star
Alpheratz (α And) is a chemically peculiar subgiant of spectral type B8IVpMnHg, 97 light-years away, whose surface is exceptionally rich in manganese and mercury, elements concentrated by radiative diffusion in its relatively calm atmosphere. Historically it was shared between Pegasus and Andromeda and designated both α Andromedae and δ Pegasi before the constellation boundaries were formalized in 1930.
Around This Date
- September 17, 1789William Herschel discovered Mimas, a small icy moon of Saturn, during one of his systematic sweeps of the planetary neighborhood.
- September 17, 1930The International Astronomical Union formally adopted standardized constellation boundaries, ending centuries of overlapping and ambiguous stellar assignments, which is why Alpheratz was officially transferred entirely to Andromeda.
Alpheratz once had two names and two owners; the sky's politics were settled by a vote in 1930, but the star itself continues as it always has.