September 12
The Head of Andromeda
Sun Position
The Sun is in Virgo near +1° declination. Day and night lengths differ by only a few minutes; in the Northern Hemisphere, the last warm evenings of summer are giving way to the crisp dusk of autumn.
Sky Highlight
The Andromeda Galaxy (M31) is rising in the northeast by mid-evening this week and will be well-placed for the next several months. September marks the beginning of the prime viewing season for M31 from the Northern Hemisphere.
Deep Sky Object
M31, Andromeda Galaxy. About 2.5 million light-years away, M31 is the nearest large spiral and is approaching the Milky Way at roughly 110 km/s; in about 4.5 billion years the two galaxies will merge. Northern Hemisphere observers find it well up by 10 PM; Southern Hemisphere observers see it low in the north.
Featured Star
Alpheratz (α And) is a chemically peculiar blue subgiant (spectral type B8IVpMnHg) about 97 light-years away, with an unusually high surface abundance of mercury and manganese. It marks the upper-left corner of the Great Square of Pegasus and simultaneously serves as the head of the chained figure of Andromeda, belonging to both constellations in historical catalogs.
Around This Date
- September 12, 1959The Soviet Luna 2 spacecraft became the first human-made object to reach the lunar surface, impacting near the Palus Putredinis region.
- September 12, 1962President Kennedy delivered his "We choose to go to the Moon" speech at Rice University, articulating the commitment to the Apollo program in its most memorable form.
Alpheratz belongs to two constellations at once, a boundary star whose dual allegiance is a small lesson in how the sky's borders are human, not celestial.