July 31
The Amber and the Blue
Sun Position
The Sun is in Leo at about +17.4° declination. Northern Hemisphere summer is past its peak but still warm; Southern Hemisphere winter is beginning to ease toward August. Cygnus is near the zenith at midnight from mid-northern latitudes.
Sky Highlight
No major annual event peaks on July 31, but the Southern Delta Aquariids are still active, and the Perseids are beginning their slow build toward their August 12–13 peak. From a dark northern site, a careful watcher can now find a few Perseid meteors per hour in the predawn northeast, radiating from near the Double Cluster in Perseus.
Deep Sky Object
M39 (NGC 7092), a large, loose open cluster in Cygnus roughly 800 light-years away. It contains about 30 stars spread across a wide field, making it better suited to binoculars or a wide-field telescope than high magnification. Its proximity makes individual stars easy to resolve. Best from Northern Hemisphere latitudes; low or inaccessible from southern mid-latitudes.
Featured Star
Albireo (β Cyg) closes out July from 430 light-years away in Cygnus, its orange giant (K3II) and blue companion (B8Ve) among the most-observed doubles in amateur astronomy. The color contrast is immediate and unmistakable: the surface temperature difference between the two components is about 6,600 K, enough to shift the dominant wavelength from the orange-red part of the spectrum to the blue-white.
Around This Date
- July 31, 1964NASA's Ranger 7 spacecraft transmitted 4,316 photographs of the Moon in the final minutes before impact, providing the first close-up images of the lunar surface and showing it was geologically complex enough to support a crewed landing.
- July 31, 1971Apollo 15 astronauts completed their third moonwalk and recovered the Genesis Rock (a piece of the ancient lunar crust dated to about 4.1 billion years old) from the rim of Hadley Rille.
July ends with a double star that makes the physics of temperature visible to the naked eye, two stars, two colors, one honest sky.