September 24
The Pulsing Flank
Sun Position
The Sun has crossed into the southern celestial hemisphere, now at about -1° declination and moving south at its fastest daily rate of the year. Nights are growing longer in the Northern Hemisphere; Southern Hemisphere spring days are lengthening correspondingly.
Sky Highlight
Post-equinox nights in late September are excellent for deep-sky observing from both hemispheres, as the nights are now longer than 12 hours everywhere and temperatures remain mild. The Great Square of Pegasus is well-placed high in the southeast after dark from the Northern Hemisphere.
Deep Sky Object
NGC 7009, Saturn Nebula, Aquarius. About 2,000 light-years away, this bright planetary nebula is named for its Saturn-like appearance in larger telescopes, with faint extensions (ansae) on either side of the disk. Well-placed in the south for Northern Hemisphere observers; high and favorable from southern latitudes.
Featured Star
Algenib (γ Peg) is a blue subgiant of spectral type B2IV, about 391 light-years away, the fourth and hottest corner of the Great Square. Its Beta Cephei pulsations (period 3.64 hours) are a consequence of iron-opacity-driven instabilities just below the surface, a mechanism that also drives pulsations in several other hot, compact stars.
Around This Date
- September 24, 1970The Soviet Luna 16 spacecraft landed on the Moon and, the following day, returned to Earth with the first automatically collected lunar soil samples, 101 grams scooped from Mare Fecunditatis.
- September 24, 2014India's Mars Orbiter Mission (Mangalyaan) entered Mars orbit on its first attempt, making India the first Asian nation and the first in the world to succeed on a maiden Mars mission.
The Great Square looks static on any given night, but three of its four stars are pulsing on their own schedules, the sky is never as fixed as it appears.