In the reverse of a full moon, when the moon's face is completely illuminated by reflected sunlight, the new moon isn't visible at all in the night sky. A new moon occurs when the moon and sun share the same celestial longitude, which is a projection of Earth's lines of longitude onto the celestial sphere. This alignment is also called a conjunction, and the same term applies to any two celestial bodies sharing a celestial longitude. (Another term for celestial longitude is right ascension, which is measured in hours and minutes rather than degrees.)
During a new moon, sunlight shines only on the side of the moon facing away from the Earth, and the Earth-facing side of the moon is dark. On top of that, a new moon is always close to the sun in the sky that the sun's glare overwhelms anything nearby. The only time a new moon is visible is during a solar eclipse, when the moon blocks all or part of the sun's disk. The next solar eclipse will be on Jan. 5-6, 2019, when the moon will block up to 62 percent of the sun for viewers in northeastern Asia and the Aleutian Islands. [10 Surprising Lunar Facts]
That said, the period around the new moon offers dark skies in which you can spot objects that the moon might otherwise outshine. In the days preceding and following the new moon, our satellite is visible as a crescent just before sunrise and after sunset, which can offer picturesque views as the moon passes by planets or bright stars.
For example, the moon was in conjunction with the planet Venus on Monday (Dec. 3), about four days before becoming new, according to skywatching site In-the-Sky.org. The moment of conjunction was during the day for observers in the eastern U.S., at 1:43 p.m. EST (1843 GMT), when the two objects were about 3 and a half degrees apart, or approximately seven lunar diameters.
A conjunction with another planet follows on Wednesday (Dec. 5), when the moon will be within a degree and a half, or about three lunar diameters, of Mercury. The moment of conjunction with that planet will occur in the afternoon, at 4:07 p.m. for observers based in New York City. The moon and Mercury will still be close, within four lunar diameters, in the pre-dawn sky. This pairing will be harder to see than the conjunction with Venus, because the two do not get more than 10 degrees above the horizon before daybreak.
After the new moon, when the crescent becomes visible after sunset, the moon will pass by Saturn. The two bodies will be in conjunction on Dec. 9 at 7:19 p.m. EST (0019 GMT on Dec. 10), according to In-the-Sky.org. In this case, the moon and the ringed planet will be below the horizon when they are closest. But with a flat horizon and clear weather, one can see them together in the sky just after dusk, though they will both be only a few degrees above the horizon. Both will set about an hour after sunset.
In early December, Mars will remain visible in the evenings; on the night of the new moon, the planet won't set until 11:25 p.m. in New York, according to heavens-above.com calculations. The planet will be in the constellation Aquarius, which tends to be hard to see in brightly lit areas. But Mars itself, with its distinctive reddish hue, will stand out.
The early December new moon offers opportunities to catch some fainter objects in the winter sky, notably the Orion Nebula, which can sometimes be seen even from city locations. The Orion nebula looks like a fuzzy "star" just south of the three stars in Orion's Belt: Alnilam, Alnitak and Mintaka.
Orion is one of the most recognizable winter constellations for Northern Hemisphere skywatchers, as it has a lot of first-magnitude and brighter stars. Notably, that includes Betelgeuse, a red supergiant that traditionally marks the hunter's right shoulder (assuming Orion faces the observer), and Rigel, a bluish giant star that would be where the left foot is. The entire constellation will be above the horizon by 7:30 p.m. in the eastern U.S., and it will stay visible for almost the entire night.
Joining Orion in the winter sky will be Canis Major, which sits just south of Orion and contains Sirius, also called the "dog star." It's the brightest one in the Northern Hemisphere skies, and by 9 p.m., it will be just high enough to clear low buildings and trees on the southeastern horizon.
Other prominent winter constellations are Gemini, which is just north and west of Orion, and Taurus, the bull, which is just east and north (the bull looks to some observers as though it is charging Orion). Taurus contains the Hyades, a loose cluster of stars that form the bull's head. Gemini's brightest stars are Castor and Pollux. The latter has the distinction of being the closest giant star to the sun, at about 33.8 light-years away from our star. Castor is a system of six stars, which can be seen through a small telescope as three pairs.
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