August 21
The Summer Triangle Holds
Sun Position
The Sun is in Virgo, having crossed the Leo-Virgo boundary around August 23, but on August 21 it is still in late Leo, at about +10° declination. Days are shortening steadily in the Northern Hemisphere.
Sky Highlight
The Summer Triangle (Vega, Altair, and Deneb) is at its best placement of the year, riding high overhead from northern mid-latitudes by mid-evening and well placed for the Southern Hemisphere in the northwestern sky. This asterism contains some of the most studied stars in the sky and frames the richest portions of the northern Milky Way.
Deep Sky Object
M56, a globular cluster in Lyra, lies roughly 32,900 light-years away and is accessible in binoculars, though modest compared to showpieces like M13. It was discovered by Messier in 1779 during the same observing run in which he tracked a comet across Lyra, making it a coincidental catch. Well placed from both hemispheres on August evenings.
Featured Star
Deneb, the A2Ia supergiant 2,600 light-years away, anchors the top of the Northern Cross and the tail of the Swan simultaneously, the same star serves two geometric roles, which says more about pattern-making than about the star. It is one of the most intrinsically luminous stars known within a few kiloparsecs of Earth.
Around This Date
- August 20, 1977Voyager 2 launched from Cape Canaveral, making it the first of the two Voyager spacecraft to leave Earth despite Voyager 1 departing two weeks later on a faster trajectory.
- August 22, 2003NASA's Spitzer Space Telescope launched, beginning a mission that would map the full infrared sky, peer into stellar nurseries hidden by dust, and detect the thermal emission of extrasolar planets.
Deneb, Vega, and Altair form a triangle that mid-August always delivers directly overhead, three different stellar histories converging in one corner of the sky.