Searching for Planets
Close as Mercury, far as Neptune
The moon waxes through our evening skies from a thin crescent at week’s end to first-quarter Tuesday September 2. Friday Luna shines just two degrees above the first-magnitude star Spica low in the southwest.
Sunday the moon appears farther east at sunset, forming a tight triangle with Saturn to the west and Mars to the south. The two planets appear equally bright, shining at magnitude 0.6, but Saturn’s golden glow and Mars’ red hue make them easy to tell apart.
Monday evening the moon shines at the head of Scorpius, which is marked by three slightly mis-aligned stars. The moon is just one degree above the northernmost of the three, Graffias, shining at magnitude 2.5. Ten degrees southeast of the moon is the red giant Antares, the heart of the scorpion.
As the sun dips beneath the horizon around 7:40, look in its wake for Mercury low in the west. Binoculars may help you pinpoint this elusive planet. When most people spot Mercury, they are surprised by its brightness, shining around zero magnitude, brighter than most stars. But the innermost planet orbits so close to the sun that it never appears more than a dozen degrees above the horizon during dark hours.
Mercury has been a fixture of our night sky since the dawn of civilization. The first telescopic observations of the planet were made by Galileo in 1610, but unlike his viewing of Saturn, there was no eureka moment. It was not until more than 350 years later with the fly-by of the Mariner 10 space probe that astronomers learned much more about this elusive planet.
You may be able to see the farthest planet from the sun, Neptune, before dawn this week. On Friday the outermost planet reaches opposition, when earth is directly between it and the sun. Even with binoculars or a small telescope it will appear as little more than a small blue dot low in the west-southwest amid the dim stars of Aquarius.
Venus and Jupiter will greet you in the east before dawn amid the glow of the coming sun.