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A bright green comet makes its appearance in the night sky this month. Comet 21P / Giacobini-Zinner is now visible with binoculars and telescopes. Meanwhile, an even brighter comet is heading towards us and will become visible to the naked eye in December.
Comet 21P / Giacobini-Zinner, or simply "21P", will make its closest approach to the Earth on September 10th while circling the sun. A good pair of binoculars or a small telescope is needed to spot this bright green comet, but the next comet, 46P / Wirtanen, will be bright enough to see at the naked eye in December.
Sometimes called "dirty snowballs" of space, comets are icy bodies that move around the sun in a very elliptical orbit, leaving a trail of dust and gas in their wake. Comet 21P revolves around the sun about once every 6.6 years. Each year in October, the Earth crosses the "crumb trail" left by comet 21P, treating aerial observers to Draconid's annual meteor shower. A relatively modest meteor shower, which sometimes has spectacular explosions, Draconid's meteorite rain is expected to produce only five to eight shooting stars per hour in 2018, NASA's meteorite expert Bill Cooke told NASA's Space.com. [Amazing Photos: The Comets 21P/Giacobini-Zinner and 46P/Wirtanen of 2018]
Before our planet crosses the debris of comet 21P, air observers can watch the comet slowly approaching the sun this week. The bright green comet "should reach a visual magnitude of 6.5 to 7," according to the skywatching website EarthSky.org. "It means that it will not be visible to the eye … but almost."
Depending on the darkness and clarity of the sky, the weakest stars and other celestial objects that we can see without the aid of telescopes or binoculars have an apparent magnitude of about 6 or 6.5. In lightly polluted areas like New York, for example, skywatchers can see stars up to a magnitude of only 4.0. (Lower magnitudes indicate brighter objects.)
To locate comet 21P at its best level, look east to the modern constellation Auriga – home to Capella, the goat star – after midnight and before dawn. The comet will officially reach the perihelion, or its closest point to the sun, around 10:40 am EDT (0640 GMT) on 10 September. You can still see the comet for several nights before and after this close approach, but how long does it last? will remain in sight depends on the power of your sky monitoring equipment. To locate the exact coordinates of the Comet 21P when viewed from a specific location or time, consult the NASA Ephemeris Calculator.
If you do not have the opportunity to review Comet 21P or if you do not have access to the binoculars or telescopes you need, do not worry. Another comet encounter is just around the corner, and this time you will only need your own eyes to see her in the night sky (provided you have good vision or sports glasses).
On December 16th, comet 46P / Wirtanen will make its closest approach to the sun, and it will be visible just after dark, according to In-The-Sky.org. The periodic comet, which revolves around the sun once every 5.4 years, was discovered in 1948. Its last approach to the Earth took place in 2013, at 567 million miles (907 million kilometers) from the Earth. The last time, Wirtanen gets a lot closer, but Joe Rao, SpaceWatching columnist for Space.com and meteorologist for FiOS 1 News, told Space.com that this time, "it was not really impressive." On his return December 16, the Comet 46P will be only 7.7 million miles (11.7 million kilometers).
Comet 46P / Wirtanen is "one of those little comets" and tends to be visible only with a good telescope or pair of binoculars, Rao said. But this year, "it will be much brighter than it usually is," he added. "It will become as bright as the third magnitude, which is about as bright as the weakest star in the Big Dipper. So, if you know where to look in the sky, it will be against the winter constellations of the bull the bull, near the Pleiades and Hyades star clusters.
To track the 46P / Wirtanen comet and determine its coordinates, check out NASA's ephemeris tool for the 46P.
Editor's note: If you are capturing a beautiful photo of comet 21P / Giacobini Zinner or comet 46P / Wirtanen and you would like to share it with Space.com and our partners for a story or an image gallery, please contact the editor. Chef Tariq Malik at spacephotos @ space.com.
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