September 19
The Hot Flank
Sun Position
The Sun is in Virgo near -3° declination. Nights are about 20 minutes longer than days at 40°N latitude; at 40°S, days are 20 minutes longer than nights.
Sky Highlight
Perseus is rising in the northeast by late evening, heralding the Perseid meteor shower's parent region though the shower itself peaked in August. The Double Cluster in Perseus is now a fine naked-eye and binocular target all night.
Deep Sky Object
NGC 869 / NGC 884, Double Cluster, Perseus. Two rich open clusters roughly 7,500 light-years away, nearly adjacent in the sky, containing young blue giant stars. One of the finest binocular objects in the northern sky; accessible from southern mid-latitudes but low.
Featured Star
Algenib (γ Peg) is a hot blue subgiant of spectral type B2IV, about 391 light-years away, and one of the bluest and intrinsically brightest stars defining the Great Square. It pulsates as a Beta Cephei variable with a period of 3.64 hours and amplitude of a few hundredths of a magnitude, not visible to the eye, but it makes Algenib a standard reference for stellar pulsation studies.
Around This Date
- September 19, 1848William Bond and his son George Bond, observing from Harvard College Observatory, discovered Hyperion, a moon of Saturn, the first planetary moon discovered by American astronomers and the first known non-spherical moon.
- September 19, 1976Two Imperial Iranian Air Force F-4 Phantom II jets were scrambled over Tehran to intercept an unidentified aerial object; both aircraft experienced instrumentation and weapons-system failures during the approach, and the incident was documented in declassified US Defense Intelligence Agency reports.
Algenib is the Great Square's most energetic member, its steady appearance at dusk hides a star that is quietly vibrating in three-hour cycles.