The new moon occurs on Saturday, Sept. 28, at 2:26 p.m. EDT (1026 GMT) — one day after the moon reaches perigee, or the closest point to Earth in its orbit.
The new moon occurs when the moon is directly 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 celestial sphere. During the new moon, if one were to draw a line from Polaris, the North Star due south toward the sun, that line would also hit the moon.
Because the sunlit 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.
Related: 2019 Moon Phases Calendar
A day after the new moon, the moon will be in conjunction with the planet Mercury, but observers in mid-northern latitudes generally won't be able to see it. The moon will only be one day old, so catching its tiny sliver will require a keen eye and a clear western horizon.
The moon and Mercury will rise after the sun, and because of their close proximity to the sun, they will be impossible to observe safely. When the sun sets, the pair will be only 3 degrees above the horizon, according to the skywatching site in-the-sky.org.
For more southerly observers the situation is better, because the ecliptic, the line that the sun, moon and planets appear to follow across the sky, intersects the horizon at a much steeper angle. From San Juan, Puerto Rico, the sun sets on Sept. 29 at 6:14 p.m. local time, and Mercury sets at 7:08 p.m. — just 10 minutes before moonset, according to timeanddate.com.
When the sun sets, Mercury will be 11.8 degrees above the horizon, and the moon will be at an altitude of 13.5 degrees. The two will be about 6 degrees apart, but they will be hard to see because of their proximity to the sun. The moon's thin, one-day-old crescent is a challenge for even experienced observers, and the sky will take some time to darken enough to see Mercury. (Skywatchers should also note that it is dangerous to try observing objects close to the sun — you should not use optical aids such as binoculars while the sun is still above the horizon.)
Venus will be difficult to see in the solar glare, either on the night of the new moon or in the days after. The planet reached superior conjunction, or the point on the opposite side of the sun from Earth, in August. While the planet is no longer hiding behind the sun, it is still close to the sun in the sky.
From New York City, Venus sets at 7:15 p.m. local time on Sept. 28, or about 30 minutes after sunset. Venus will only be 5.5 degrees above the western horizon, or half the width of a fist at arm's length, at sunset. That said, as autumn progresses the planet will become more visible as an "evening star." As is the case for Mercury, skywatchers farther south will have a better view of Venus. In San Juan, Venus will be about 9 degrees above the horizon at sunset on Sept. 28, and from Santiago, Chile, the planet will be a full 27.5 degrees in altitude at sunset.
Moving further out in the solar system, Mars, in the constellation Virgo, will be visible in the wee hours of the morning on Sept. 28, rising at 6:05 a.m. local time in New York City, while the sun follows at 6:49 a.m.
Jupiter, meanwhile, will be in the western half of the sky on Sept. 28, setting at about 10:12 p.m. in New York City. It will be in the constellation Ophiuchus, above the bright red star Antares. The star's name comes from the Greek phrase for "rival to Ares," the Greek god of war — and the ancient Greek name for the planet Mars. Both celestial objects share a reddish hue. Saturn is to the east of Jupiter, in the constellation Sagittarius, and for New Yorkers the planet sets at about midnight on the night of the new moon.
You can find out exactly when the planets are visible from your specific location using timeanddate.com's astronomy calculator.
Stars and constellations
In late September, the constellations Sagittarius and Scorpio are in the western half of the sky by 9 p.m. and the famous "Summer Triangle" — Deneb, Altair, and Vega — are high near the zenith. If one follows a rough line east from Sagittarius to Altair, and then looks east of that line, one can see the constellation Aquarius.
To the east (left) of Aquarius is Pisces, and to the west is Capricorn. Aquarius, Pisces and Capricorn are relatively faint and harder to see in light-polluted areas. Continuing east and north, you can 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 one 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, planets that are low in the sky in mid-northern latitudes are much higher in the southern regions. Jupiter and Saturn will be well above the horizon after sunset, and those planets won't set until early Sunday morning.
Asterisms, or star patterns visible in the Southern Hemisphere on Sept. 28 will include the Southern Cross, which will be in the southwestern sky, accompanied by Centaurus just to the north of it. In mid-southern latitudes, Centaurus sets by midnight, and the Southern Cross will brush the horizon. However, in Buenos Aires the constellation is circumpolar — it never sets.
In the eastern half of the sky observers in the Southern Hemisphere will see Pisces rising, and south of that, Cetus, the whale (both "upside down" from their Northern Hemisphere orientations). Looking upwards from Cetus, one can spot the Phoenix constellation, 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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