The new moon of January will occur at 12 a.m. EST (0400 GMT) on Wednesday, Jan. 13. Our natural satellite will make a close pass to Mercury a day later, and while the great conjunction of Jupiter and Saturn has passed, the two planets are still close together in the sky and will make a close grouping with Mercury just after sunset before all three sink below the horizon.
New moons happen when the sun and moon share the same celestial longitude, a condition also called conjunction. Last month the new moon coincided with a total solar eclipse (at least if you were in South America), but that won't happen this time, because the moon will "miss" the sun as it passes in the sky. The reason we don't get eclipses every month is that the orbit of the moon is tilted relative to the plane of the Earth's orbit, by about 5 degrees.
So, this time we just get a dark sky — you can't see the moon from Earth because the illuminated side is facing away from you. (If you could "turn off" the sun, you could see the moon blocking the stars behind it, but that would be all).
Related: 2021 moon phases calendar
On the evening of the new moon, Jan. 12, the sun sets at 4:50 p.m. local time in New York. As it sets, Jupiter, Saturn and Mercury will all become visible as the sky darkens, forming a rough line in the southwest from left to right, with Mercury furthest left, then Jupiter, then Saturn. At 5 p.m. Mercury will be the highest of the three, at 8 degrees above the horizon. According to Heavens-Above.com calculations, Jupiter will be about 7 degrees and Saturn about 5 degrees.
The three planets will all span about three degrees in azimuth (the horizontal direction) — that means the distance between Mercury and Saturn will be about six lunar diameters. Catching the trio will require good weather and a flat horizon, but all three are bright enough that they can become visible before Saturn sets, at 5:33 p.m. local time in New York City. Jupiter follows 13 minutes later, and Mercury at 5:51 p.m.
If you live further south, observing the three planets together becomes slightly easier; the angle the planets' orbits make with the horizon is a bit steeper. From Miami, for example, the sun sets at 5:49 p.m., and by 6 p.m. Saturn is nearly 6 degrees above the horizon, Jupiter at 8 degrees and Mercury at 9.6 degrees. Saturn, the first to set, does so at 6:30 p.m., Jupiter at 6:42 p.m., and Mercury at 6:49 p.m.
Southern Hemisphere observers might have it easier, but that isn't the case, largely because the sun sets much later (as it is the austral summer). In Cape Town, South Africa, the sun doesn't set until 8 p.m. on Jan. 13, (the new moon occurs there at 7 a.m. local time) and by 8:15 p.m. Saturn is only 2.6 degrees above the horizon — the planet sets at 8:30 p.m. Jupiter sets nine minutes after that, and Mercury follows at 8:54 p.m.
Mercury is usually a tough planet to observe, as it is close to the sun and while bright, it is usually difficult to pick out in the first minutes after sunset against the sky (which can still be lit). But this time the young moon will make it easier to spot as it will pass 2 degrees to the south of Mercury on Jan. 14, according to skywatching site In-The-Sky.org. This conjunction won't be visible from New York as it occurs at 3:14 a.m. Eastern, but after sunset the thin crescent moon will still be just east of Mercury and make for an easy signpost to the planet. If you want to catch the actual moment of conjunction, one needs to be in eastern Asia — it occurs at 5:14 p.m. in Tokyo, where the sun sets at 4:50 p.m. on Jan. 14.
Other planets on the night of the new moon will be Mars, which by 6 p.m. Eastern will be high in the sky in New York, about 61 degrees in altitude in the south. Located in the constellation Aires it will be easy to see because of its distinct hue (they call it the "Red Planet" for a reason) and the relative faintness of the stars in Aires.
Venus, meanwhile, continues its tenure as a morning star, rising at 6:13 a.m. Eastern. The sun follows at 7:18 a.m. on Jan. 13. Venus is moving closer to the sun each day; by the end of the month it will be rising only about a half hour before the sun does.
Stars and constellations
Winter constellations are in full swing for Northern Hemisphere observers this month. By about 5 p.m. Orion, the hunter is completely above the Eastern horizon, and one can watch its stars appear as the sky darkens. Orion faces Taurus, the bull to his west and north — the bright star Bellatrix (Gamma Orionis) marks one of Orion's shoulders, and looking to the right and up one will see the Hyades, a cluster of bright stars that is the "face" of the bull. Aldebaran (Alpha Tauri) is among these, recognizable by its orange-yellow color.
Looking left, above Orion's head, one sees Gemini, the twins, and the two stars Castor and Pollux. On the way you pass Auriga, the charioteer and the star Elnath, or Beta Tauri, one of the bull's horns. Auriga contains the bright star Capella, which is directly north of Elnath, and from the latitude of New York City and above it never sets — it is one of the circumpolar stars. Capella looks like one star, but it is in fact a system of four. The stars were resolved via spectroscopy; astronomers couldn't see them visually, but they could separate the stars light into its component colors. Every star has a characteristic spectrum, with the colors showing what elements are present. In the 19th century astronomers noticed that the spectrum seemed to show more than one star, and in the early years of the 20th century it was confirmed that there were in fact four of them.
By about 9 p.m. on Jan. 12 the constellation Canis Major, which contains Sirius, the brightest star in the night sky, is well above the eastern horizon. Sirius is also known for being relatively close — 8.6 light-years distant, basically in our stellar backyard. Sirius also has an unseen companion, which was found only because the star seemed to "wobble" In its motion across the sky, over a period of years. The reason was that Sirius has a white dwarf companion — a star that is so dense it is no larger than the Earth but has a mass equal to that of the sun.
For Southern Hemisphere observers, January is when Puppis, Carina and Vela, the three constellations that make up the ship (connected to the Argo, the famous ship of Jason and the Argonauts). By midnight, Vela and Carina are both near the zenith, and the Southern Cross is just to the east of the meridian. Also visible is Centaurus, home to Alpha Centauri, the closest stellar neighbor to the sun.
From the Southern Hemisphere the winter constellations of the Northern Hemisphere all appear "upside down" towards the north. There isn't a "pole star" in the southern skies, but one can find the south celestial pole using the Southern Cross — the base of the cross points to the southern pole, which is about halfway between the Southern Cross and the bright star Achernar, which marks the end of the River, Eridanus (which starts near Rigel, the "foot" of Orion).
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