September 18
The Weak Pulse
Sun Position
The Sun is in Virgo at about -3° declination. Nights are measurably longer than days in the Northern Hemisphere; in the Southern Hemisphere, spring's longer days are just beginning to assert themselves.
Sky Highlight
The autumn constellations (Pegasus, Andromeda, Cassiopeia, Perseus) are now all above the horizon by mid-evening for Northern Hemisphere observers, offering a rich survey sky. The Milky Way's plane is still visible in the northwest.
Deep Sky Object
NGC 869 and NGC 884, Double Cluster, Perseus. Two open clusters about 7,500 light-years away, side by side, containing several hundred stars total and visible to the naked eye as a misty patch. Circumpolar from much of the Northern Hemisphere; accessible from mid-southern latitudes.
Featured Star
Caph (β Cas) is a yellow-white giant of spectral type F2III-IV, 54.7 light-years distant, and one of the sky's nearer Delta Scuti variables, it oscillates in brightness with a period of roughly 2.5 hours, too subtle for the eye but well-documented in photometry. It traces the western tip of Cassiopeia's W and is always above the horizon for observers north of about 30°N.
Around This Date
- September 18, 1984The Space Shuttle Discovery made its maiden flight (STS-41-D), deploying three communications satellites and demonstrating the reusable orbiter system's operational capability.
- September 20, 1945Wernher von Braun and 125 German rocket scientists arrived in the United States under Operation Paperclip, a transfer that directly shaped the trajectory of American rocketry and the eventual Apollo program.
Caph pulses every two-and-a-half hours, not because something is wrong, but because a certain size and temperature make stars vibrate like a bell struck softly.