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August 7: A lunar occultation of the Pleiades
Let’s spend tonight with the Little Gem, a planetary nebula also cataloged as NGC 6818. It is highest in the sky around 11 P.M., when it is some 35° above the southern horizon. It’s located in the northeastern corner of Sagittarius, to the upper left of the Teapot asterism at this time of night.
To find it, locate 3rd-magnitude Beta (β) Capricorni in Capricornus (to the upper left of Sagittarius) and simply slide 9° due west. You’ll want a telescope to enjoy this planetary — a small one will do, but as always, the larger the better. NGC 6818 glows at magnitude 9.3 and spans some 48” on the sky. In a small telescope, it appears about half as large, roughly 25” across. That’s compact enough to lend it relatively high surface brightness, making it a great target for smaller scopes, even if you may see less overall detail.
Once you do find NGC 6818, you may see why it has another name: The Green Mars Nebula. The first half of that moniker comes from the planetary’s color, whose glowing gas appears blue-green to the eye. The second half may be less obvious, but relates to the nebula’s perceived size on the sky — a diameter of 25” is roughly the same size as Mars during its closest approaches to the Sun, so amateur astronomers have likened the appearance of this glowing globe to the Red Planet, for all that their colors are opposite.
Sunrise: 6:05 A.M.
Sunset: 8:05 P.M.
Moonrise: 12:59 A.M.
Moonset: 5:09 P.M.
Moon Phase: Waning crescent (18%)
*Times for sunrise, sunset, moonrise, and moonset are given in local time from 40° N 90° W. The Moon’s illumination is given at 10 P.M. local time from the same location.
The Moon passes 4° north of Mars at 2 A.M. EDT; the pair is visible together in the early-morning sky of August 9, some 30° high in the east an hour before sunrise. They straddle the border of Gemini and Taurus, to the upper left of the familiar hourglass-shaped figure of Orion the Hunter, bisected by his famous belt.
Mars is magnitude 1.3, a bright reddish point of light in this region of the sky, just to the right of the delicate waning crescent Moon. Higher in the sky and to the right of the pair is the red giant Aldebaran, brighter than Mars at magnitude 0.9 but shining with a similar hue, thanks to this star’s advanced age, which has cooled its temperature. Compare both Mars and Aldebaran to Betelgeuse, the magnitude 0.5 red giant shoulder of Orion, which also shines orangey-red.
Far below and to the left is Mercury, just 4° above the eastern horizon an hour before sunrise. Despite its low altitude, it’s shining brightly at magnitude –0.6, making it easy to spot as long as your horizon is clear. Through a telescope, Mercury spans 6” and is 65% lit. Compare that with Mars, whose phase is 93% but appears just a bit smaller, 5”. This is because although Mercury is physically smaller than Mars, the latter now lies farther from Earth, which diminishes its apparent size in our sky.
As always, make sure to put away any optics well before sunrise from your location, which may differ from the time given below.
Alison Klesman is senior editor of Astronomy magazine. She holds a Ph.D. in astronomy and has studied a variety of topics, from minor planets to supermassive black holes.
