September 13
The Henna Hand
Sun Position
The Sun is in Virgo at roughly 0° declination, within days of crossing the celestial equator. Days and nights are essentially equal worldwide; the astronomical definition of equal day and night (when the Sun's center crosses the equator) arrives in about ten days.
Sky Highlight
Cassiopeia is circumpolar for observers north of about 30°N latitude and is now well-placed in the northeast by mid-evening, its distinctive W ascending. For Southern Hemisphere observers it remains near or below the northern horizon.
Deep Sky Object
NGC 457, Owl Cluster (or ET Cluster), Cassiopeia. About 7,900 light-years away, this open cluster contains roughly 80 stars and two bright ones that form what many observers describe as the eyes of an owl. Circumpolar and well-placed for Northern Hemisphere observers; visible but low for mid-southern latitudes.
Featured Star
Caph (β Cas) is a yellow-white subgiant to giant of spectral type F2III-IV, about 54.7 light-years away, making it one of the closer members of the Cassiopeia W. It pulsates weakly as a Delta Scuti variable, varying by a few hundredths of a magnitude over a period of about 2.5 hours, changes far too subtle for the naked eye but well-documented photometrically.
Around This Date
- September 13, 1922A temperature of 57.8°C (136°F) was recorded at El Azizia, Libya, a reading long held as Earth's highest surface air temperature before being invalidated by the World Meteorological Organization in 2012 due to measurement problems.
- September 14, 2015LIGO's twin detectors simultaneously registered gravitational waves from the black hole merger GW150914, the first direct detection of gravitational waves ever made.
Caph's Arabic name carries the image of a hand marked with henna, a domestic detail that survived two thousand years of constellation renaming because someone wrote it down.