October 11

October 11

The Telescopic Double

Sun Position

The Sun is in Libra near -8.5° declination. Across the Northern Hemisphere, the angle of midday sunlight has flattened noticeably since the equinox, casting longer shadows even at noon.

Sky Highlight

Mid-October is an excellent period for observing the Perseus Double Cluster (NGC 869 and NGC 884), which rides high on October evenings for Northern Hemisphere observers. The Double Cluster is one of the finest binocular objects in the sky and lies between Cassiopeia and Perseus, two constellations both well-placed this month.

Deep Sky Object

NGC 869 and NGC 884 (Perseus Double Cluster), an open cluster pair about 7,500 light-years away. The Double Cluster is arguably the finest open cluster pair in the northern sky, two young, rich clusters separated by only a few hundred light-years in space, containing numerous luminous blue supergiants that give them a jewel-box quality in a small telescope. Best for Northern Hemisphere observers in autumn; low or below horizon from far southern latitudes.

Featured Star

Mesarthim (γ Ari) is a blue-white main-sequence pair (B9V + A1V) about 164 light-years away, and it holds a notable place in the history of telescopic astronomy: Robert Hooke split it as a double star in 1664, making it one of the earliest doubled stars resolved through a telescope. The two components are separated by about 7.5 arcseconds, easy in modern instruments. Mesarthim, one of the first double stars ever seen through a telescope, in 1664.

Around This Date

  • October 11, 1968Apollo 7, the first crewed Apollo flight, launched on a Saturn IB rocket, demonstrating that the redesigned command module was flight-worthy after the fatal Apollo 1 fire.
  • October 14, 1947Chuck Yeager flew the Bell X-1 through the sound barrier for the first time, achieving Mach 1.06 at 43,000 feet in a flight that defined the modern era of high-speed aviation research.

Mesarthim is a reminder that the earliest telescopes, turned on familiar stars, immediately revealed the sky to be more layered than anyone had assumed.