The new moon occurs Thursday, Sept. 17, at 7 a.m. EDT (1200 GMT), according to NASA, a day before Mercury reaches its highest point in the evening sky for the rest of the year.
The new moon occurs whenever the moon is between the sun and Earth. About every 29.5 days the two bodies share the same celestial longitude, an alignment also called a conjunction. Celestial longitude is a projection of the Earth’s own longitude lines on the sky — during new moons, if you drew a line from Polaris, the North Star due south through the sun, you would hit the moon.
Since the illuminated side of the moon faces away from Earth, new moons are invisible to ground-based observers, unless the moon passes directly in front of the sun which creates a solar eclipse. That won’t happen this time — the next one is due on Dec. 14.
Visible planets and constellations
A day after the new moon, on Sept. 18, the planet Mercury will get to its highest point above the after-sunset horizon. This apparition will still be a difficult one to observe; from New York City the planet on the evening of the new moon, the sun sets in New York City at 7 p.m. local time and the planet itself sets at 7:48 p.m., according to skywatching site In-The-Sky.org.
Shining as bright as the star Vega at approximately magnitude 0.0, the planet will only be about 8 degrees above the horizon, and it will take about 15-20 minutes for the sky to darken enough for the planet to be visible. (Magnitude is a measure of brightness, with negative numbers denoting the brightest objects.) The observing conditions improve as one travels south; the planet’s altitude is 13 degrees at sunset in Miami, which on Sept. 18 is at 7:22 p.m. local time. The planet sets about an hour later, at 8:24 p.m.
Venus, meanwhile, will be making its showing as a morning star, rising at 3:07 a.m. local time on Sept. 18 in NYC. By 6 a.m. the planet will be about 31 degrees above the eastern horizon — it will easily be the brightest star-like object in the sky, at magnitude -4.0. A fun observing exercise is to see how close to sunrise one can still see the planet; sunrise is at 6:40 a.m. local time in NYC.
Moving further out in the solar system, Mars, in the constellation Pisces, will rise at 8:31 p.m. local time in NYC. By 10 p.m. it will be about 17 degrees above the eastern horizon. Turning westward, Saturn and Jupiter, both in the constellation Sagittarius, will be about 26 and 23 degrees above the southern horizon, forming a close pair — if you stretch your hand at arm’s length and open your hand, the two planets will be about the same separation as your index and pinky fingers.
In late September, Sagittarius and Scorpius are in the western half of the sky by 9 p.m. and the famous “Summer Triangle” — consisting of the bright stars Deneb, Altair and Vega — is high near the zenith (the point directly overhead). If you follow a rough line east from Sagittarius to Altair, and then look east of that line, you can see the constellation Aquarius. To Aquarius’ east (the left) is Pisces, and to the west is Capricornus. Aquarius, Pisces and Capricornus are relatively faint, and harder to see unless you get away from city lights.
Continuing east and north, you will see the constellations of Pegasus and Andromeda, marked by an asterism called the Great Square. One corner of the square is Andromeda’s head, while the other three mark the wing of Pegasus, the legendary flying horse ridden by Perseus to save Andromeda from a sea monster.
From Andromeda’s head you can trace two lines of stars and find the Andromeda Galaxy, which can be spotted from a dark-sky site with the naked eye.
For those in the Southern Hemisphere, some planets that are low in the sky in mid-northern latitudes are much higher. On the night of Sept. 17, for example, the new moon occurs at 10 p.m. Australian Eastern Standard Time. At that point Mars, for example, will be about 15 degrees above the horizon, while Jupiter is a full 58 degrees above the horizon, and Saturn is at an altitude of 63 degrees. Jupiter sets at 3:06 a.m. local time while Saturn sets 29 minutes later.
Stars visible in the Southern Hemisphere on Sept. 17 will include the Southern Cross, which will lie south of Jupiter and Saturn at an altitude of about 20 degrees, and at around 9 p.m. local time will be just west of south. Just to the north and to the right will be the Centaurus constellation, home to Alpha Centauri, the closest star to Earth.
In the eastern half of the sky observers in the Southern Hemisphere will see Pisces rising, and south of that, the constellation of Cetus, the whale (both “upside down” from their Northern Hemisphere orientations). Looking upward from Cetus, one can spot the Phoenix, and south of that, the Large Magellanic Cloud and the Small Magellanic Cloud, both of which are satellite galaxies of the Milky Way.
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