September new moon 2021: See Mercury and Venus in the moonless sky – Space.com

The new moon arrives on Monday (Sept. 6), and over the next two days makes a close pass to Mercury and then Venus. 

The moon is officially new at 8:52 p.m. EDT (0052 Sept. 7 GMT), according to NASA’s SkyCal. A new moon means the moon is directly between the sun and Earth, sharing the same celestial longitude — this is technically called a conjunction. (The term also applies to other celestial bodies, such as planets). 

The timing of the lunar phases depends on where the moon is relative to the Earth, so it occurs at the same time all over the world — the only differences being due to what time zone you are in. In Melbourne, Australia, for example, the new moon occurs at 10:52 a.m. on Sept. 7, and in London it is at 1:52 a.m. Sept. 7. 

Related: Best night sky events of September 2021 (stargazing maps)   

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 A new moon is invisible from Earth unless there is a solar eclipse, when the moon passes in front of the sun. The moon’s orbit is tilted by about 5 degrees relative to the plane of the Earth’s orbit, which is why eclipses don’t happen every month. Most of the time the moon is offset from the sun (from the point of view of Earthbound observers). The next solar eclipse isn’t until Dec. 4, 2021.  

 Young moon meets Mercury  

Mercury and Venus will appear low in the western sky just after sunset on Sept. 5, 2021 ahead of September’s new moon. (Image credit: NASA/JPL-Caltech)

On Sept. 8 the moon is in conjunction with the planet Mercury, but the pairing will only be readily visible from southern latitudes. The moon will only be a day old and barely visible as a thin crescent after sunset. If you live in New York City, the sun sets at 7:15 p.m. on that day, according to Time and Date. At that time, Mercury will only be about 9 degrees above the horizon, according to the skywatching site Heavens Above. 

The conjunction of the moon and Mercury occurs at 4:18 p.m. local time in New York City, according to In-The-Sky.org — meaning that it is basically not visible due to the sun’s bright glare. However, if one lives at the latitude of Mexico City or San Juan, Puerto Rico, the situation improves. From Mexico City the conjunction happens at 3:18 p.m. local time, so it is still during the day, but as the sun sets at 7:44 p.m. Mercury will be 17 degrees above the western horizon, so as the sky gets darker, by about 8 p.m. Mexico City time, the pair will become visible. 

From the Southern Hemisphere the situation is even better: from Melbourne, Australia the conjunction occurs at 6:18 a.m local time on Sept. 9, which is well before either the moon or Mercury rises, but by sunset (which is at 6:05 p.m.) Mercury will be about 25 degrees in altitude just north of west. So some 15 minutes later the sky will be dark enough to see them both, setting at about 8:15 p.m. AEST. 

Spotting the moon so soon after the new phase is difficult, but doing so is still important for lunar calendars such as those used by observant Jews and Muslims. Be aware that pointing binoculars or a telescope to an object near the sun is very dangerous; the sun’s light is concentrated and can burn one’s retinas, even at sunset. Such damage is permanent. 

 Related: How to safely observe the sun (infographic) 

 The moon passes Venus 

Venus will shine near the bright star Spica on Sept. 5, 2021. Look for it in the western night sky. (Image credit: Starry Night)

The next conjunction will happen on Friday (Sept. 10), as the moon passes Venus. This one is much better placed for people in mid-northern latitudes. The actual conjunction happens at 1:17 a.m. EDT (0517 GMT), when both the moon and Venus are below the horizon, according to In-The-Sky.org, but they will still be quite close together in the sky that evening. Both will be in the constellation Virgo. Sunset that day is at 7:12 p.m. local time in New York City, and Venus will be about 16 degrees high in the Southwest. By about 7:30 p.m. the planet should be easy enough to see (Venus is one of the brightest objects in the sky) with the moon placed to its left and slightly above it. about 4 degrees away. 

As with the Mercury conjunction those farther south will have an easier time observing the event in the evening sky. Miami skywatchers will see the sun set at 7:30 p.m. Eastern and Venus will be 24 degrees above the horizon. In Cape Town, the conjunction occurs in the morning, at 7:17 a.m. local time, but at sunset, which is at 6:34 p.m, Venus is at 40 degrees above the west-northwestern horizon. By 6:49 p.m. when the sky is dark enough to begin seeing stars, Mercury, Spica (the brightest star in Virgo), Venus and the moon will form a rough line from the horizon going upwards and slightly to the right (north). 

The actual moment of the Venus conjunction will be visible from time zones east of Australia (or west of California); for example in Honolulu, Hawaii, the sun sets at 6:39 p.m. local time on Sept. 9, and the conjunction is 7:17 p.m. local time. At that point the moon and Venus will be about 19 degrees above the horizon. Both will set at about 8:47 p.m. local time. 

 Other planets 

On the night of Sept. 8 in mid-northern latitudes, Mars will only be a few degrees above the horizon at sunset; in New York City, for example, it will only be about 4 degrees high in the west when the sun sets at 7:19 p.m. local time, according to Heavens Above. The red planet will be more visible further south; in Buenos Aires it will be about 9 degrees in altitude at sunset (6.38 p.m. local time) — still challenging to spot. 

Jupiter and Saturn, by contrast, will be visible much of the night. For New Yorkers Saturn rises at 5:41 p.m. and Jupiter at 6:32 p.m. Eastern, so as the sky gets fully dark one will see them low in the southeast by about 9 p.m. with Saturn at about 27 degrees and Jupiter at 24 degrees. Jupiter and Saturn will be in the constellation Capricornus, with Saturn to the west (right) of Jupiter. In the Southern Hemisphere the two planets will be much higher in the sky; from Buenos Aires at 9 p.m. on Sept. 6 (local time) Saturn will be at about 67 degrees and Jupiter at 51 degrees above the horizon in the northeast.  

 Constellations 

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September is when the autumn stars are rising as the nights get slightly longer in the Northern Hemisphere. Before midnight (about 9 p.m.), one can see the Summer Triangle (which consists of the stars Deneb, Altair and Vega) high in the east, with Sagittarius due south and just to the right of it, Scorpius — classic summer skies. 

By midnight the Summer Triangle has moved to the western half of the sky and looking due south is Capricornus, Aquarius and Pisces as one moves east (left). All three are fainter constellations, and difficult to see in brightly lit cities. The star Fomalhaut in Piscis Austrinus (the Southern Fish) is brighter; if one has a clear southern horizon it will be visible and you can use Jupiter to find it as it will be almost directly “below” the planet. 

Meanwhile, turning north, midnight means one can see the “Great Square” of Pegasus, the legendary winged horse, north of Pisces. The constellation Pegasus is brighter and usually just visible even from city locations. Only three of the four stars in the square are in Pegasus, though — the fourth is the head of Andromeda, who was saved by Perseus as he rode in on Pegasus. The star, called Alpheratz, will mark the left corner of the square at around midnight. Continuing north (left), one encounters Perseus, and then Auriga, the charioteer, rising in the eastern sky. 

In the Southern Hemisphere, by 9 p.m. on Sept. 6 the Southern Cross and Centaurus are high in the southwest. Continuing south, almost to the zenith at the latitude of Santiago, Chile, one will see Scorpius, “upside down” from the perspective of antipodeans. The Southern Fish is high in the east, and to its south (on the right) is Grus, the crane. The Crane is a “modern” constellation to Europeans, delineated based on 16th- and 17th-century observations of the southern sky by explorers, but the stars in it were familiar to Arab astronomers and the name for the brightest star, Alnair, is from Arabic. Surrounding Grus (clockwise from the bottom) are Sculptor (which is also next to the Southern Fish), Phoenix and Tucana, the toucan. 

Sculptor is faint but notable for containing the South Galactic Pole — looking in that direction means you are seeing out the “bottom” of the Milky Way galaxy. 

Phoenix is another faint constellation. Its brightest star, Ankaa, is magnitude 2.4 — roughly a tenth as bright as the star Vega — was part of a figure Arab astronomers called a dhow (a kind of boat). Tucana contains most of the Small Magellanic Cloud, a satellite galaxy of the Milky Way. 

Editor’s Note: If you snap an amazing night sky picture and would like to share it with Space.com’s readers, send your photos, comments, and your name and location to spacephotos@space.com.

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