August 18
The Star That Proved the Earth Moves
Sun Position
The Sun is in late Leo at approximately +11° declination. Northern Hemisphere daylight hours continue to shrink; the autumnal equinox is five weeks away.
Sky Highlight
Mid-August brings some of the most favorable Milky Way observing of the year for both hemispheres. Without the interference of major showers, a dark site on August 18 is an opportunity for naked-eye exploration of the galactic band from Cassiopeia through Cygnus, Aquila, and Sagittarius. Southern Hemisphere observers can trace the Milky Way from Centaurus through the Scorpius-Sagittarius core.
Deep Sky Object
NGC 6826, the Blinking Planetary in Cygnus, is a small planetary nebula roughly 2,200 light-years away with a central star bright enough to be seen directly, and which appears to 'blink' when you alternate between direct and averted vision. Well placed for Northern Hemisphere observers in August, this is one of the more interactive objects in a binocular or small-telescope session.
Featured Star
Etamin, the K5III orange giant in Draco at 148 light-years, was the key target of James Bradley's 1725 observations that revealed the aberration of starlight, the tiny annual shift in apparent stellar positions caused by Earth's orbital motion. He aimed his zenith sector at Etamin to detect parallax, and instead found the first definitive proof that Earth goes around the Sun.
Around This Date
- August 18, 1877Asaph Hall discovered Phobos orbiting Mars, just six nights after finding Deimos, completing the only paired discovery of planetary moons in a single brief observing run in the 19th century.
- August 20, 1977Voyager 2 lifted off from Cape Canaveral at 14:29 UTC, carrying instruments that would work reliably more than 40 years into the mission.
Bradley was looking for parallax and found something else entirely, one of the better examples in astronomy of the experiment that answers a question you weren't asking.