The new moon will rise on the morning of Dec. 18, and while you won't be able to find it in the sky, there will be several other celestial sights to enjoy. Without the moon's bright, glowing face in the night sky, it will be a great time to look for planets and constellations.
New moons occur about once every month when the moon is on the same side of Earth as the sun. Because no sunlight reflects off the moon's surface and toward Earth, the moon briefly becomes invisible for about a day. December's new moon also rises on a morning, so it will be not only hidden but entirely absent from the sky. The moon will be at its darkest at 1:30 a.m. local time, after which a tiny sliver of the waxing crescent moon will slowly begin to appear. [Best Night Sky Events of December 2017 (Stargazing Maps)]
Skywatchers will have to stay up late or get up early to see their favorite planets in the moonless sky, as in mid-December, they will rise in the wee hours of the morning.
The first planet to rise will be Mars. It will emerge from the eastern horizon at about 3 a.m. local time in North America depending on your location. Less than an hour later, Jupiter will rise, followed by Mercury and Venus shortly before dawn. Mars and Jupiter will be the two most prominent planets in the sky this month, as the other planets will be close to the horizon when the sun rises. You can find out exactly when each planet will be visible from your location at heavens-above.com.
Mercury will be hard to spot, since it will be only 9.8 degrees above the horizon when the sun rises on the East coast and will be washed out by sunlight before then. Shining at magnitude 1.9, it will be bright but not inordinately so. (Lower magnitudes are brighter.) Venus will also be a bit more difficult to see than usual. Even though it is very bright, Venus won't get more than 3 degrees above the horizon before sunrise. Seeing either of these two inner worlds will require a nearly flat horizon such as the ocean, as well as a cloudless sky and a lot of luck.
Mars and Jupiter will be better prospects, since both will be higher in the sky ahead of dawn. In New York City, the Red Planet will be 24.5 degrees above the horizon at 5:35 a.m. local time, when astronomical twilight begins (when the sun is between 18 and 12 degrees below the horizon). (Your clenched fist held at arm's length measures about 10 degrees.) Jupiter will be at an altitude of 17 degrees, just high enough to clear most buildings and trees. Mars will be in the constellation Virgo east of its brightest star, Spica. Jupiter will be in Libra, on an approximate line from Spica to Mars. And Saturn will rise around 7:30 a.m., 10 minutes after the sun does, so it won't be visible during the day.
For skywatchers in the southern U.S., the planets will be higher above the horizon and easier to see. For example, observers in Miami will see Mercury rise at 6 a.m. — an hour before sunrise — but it will still be only about 11 degrees above the horizon before the sunlight starts to obstruct the view. By about 6:30 a.m., when civil twilight starts (this is usually when streetlights start to turn off), Mercury will be only 6 degrees above the eastern horizon. Venus will rise about 20 minutes before the sun does, reaching a meager 4 degrees altitude before sunlight washes it out. At 5:35 a.m., Mars and Jupiter will be 30 and 22 degrees above the horizon, respectively.
Deep-sky objects like the Orion Nebula offer good targets for observers away from bright city lights and have small telescopes or binoculars. It is located in the constellation Orion (the hunter), which rises about an hour after sunset in midnorthern latitudes and will be high in the southern sky. The Orion Nebula, also known as M42, is basically visible all night, and the absence of a moon in our sky means the nebulosity in it is easier to see.
Orion contains a lot of bright stars in an easily recognizable pattern — the famous "belt" of Orion is marked by the stars Alnitak, Alnilam and Mintaka (as seen from left to right). Betelgeuse is the hunter's left shoulder, while Rigel is his right foot (as seen from the ground). Orion's right shoulder is the star Bellatrix, or Gamma Orionis. All of these stars are many times more massive than the sun. The stars of the belt and Bellatrix are blue giants, and Betelgeuse is a red supergiant. [Star Facts: The Basics of Star Names and Stellar Evolution]
Just below Orion in the sky is Canis Major (the big dog), marked by Sirius, the brightest star in the northern skies. It's also the nearest star that is visible to Northern Hemisphere skywatchers, at only 8 light-years away.
For much of the night, the Andromeda galaxy, M31, will be high in the sky and free of moonlight, and easier to see than usual. At sunset — around 4:30 p.m. on both Dec. 17 and Dec. 18 — the constellation Andromeda, which contains the galaxy, will be in the east. The "head" of Andromeda (Alpha Andromedae or Alpheratz) marks one corner of the "Great Square." The other three corners of the square are in Pegasus, the winged horse.
Planet gazers equipped with telescopes will have good views of Uranus and Neptune. Uranus is in Pisces and Neptune in Aquarius, and both are high in the sky at sunset. By about 5:30 p.m. on Dec. 17, when the sky starts to get dark enough to see stars, Uranus will be about 50 degrees above the southern horizon. It will cross the highest point in the sky (known as the meridian) about 2 hours later. Neptune will be about 40 degrees above the horizon and will have already started the second half of its westward journey.
Uranus will shine at magnitude 5.8, which is too faint to see with the naked eye unless you have very good vision and are at a dark-sky site. However, with a good pair of binoculars, you may be able to spot it. Neptune will be even dimmer, at magnitude 7.9, which is beyond the range of what human eyes can discern in the night sky. However, you can still see the planet with a small telescope or binoculars mounted on a tripod for stability.
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