September 21
The Bright Beta
Sun Position
The Sun is in Virgo at about -4° declination, one or two days before the equinox. Day and night lengths are very close to equal at all latitudes; twilight is a standard 12-hour division everywhere.
Sky Highlight
The autumnal equinox approaches within one to two days. The days around the equinox are notable for rapid change in day length, roughly three minutes of change per day at mid-latitudes, the fastest rate of the year. Worth noting that equal day and night technically occurs a few days before the equinox due to atmospheric refraction (the equilux).
Deep Sky Object
M77 (NGC 1068), Seyfert Galaxy, Cetus. About 47 million light-years away, M77 is one of the nearest and best-studied active galactic nuclei, it contains a supermassive black hole whose accretion disk makes its core far brighter than a typical spiral. It is rising in the east-southeast and well-placed by late evening from mid-northern and southern latitudes.
Featured Star
Diphda (β Cet) is an orange giant of spectral class K0III about 96 light-years away, despite its Beta designation, it is the brightest star in Cetus by a comfortable margin, outshining Menkar (α Cet). It is a mild variable and a star at a relatively stable post-main-sequence phase, burning helium quietly in a shell.
Around This Date
- September 21, 2003The Galileo spacecraft was deliberately steered into Jupiter's atmosphere after nearly 14 years of operation, ending its mission and preventing any accidental contamination of Europa.
- September 23, 1846Neptune was observed for the first time as a planet by Galle and d'Arrest at the Berlin Observatory, following positional predictions by Le Verrier and Adams.
Diphda has the brightness and the name of a second-place finisher, but it was the Greeks and not the star that got the labeling wrong.