On September 21, 22 and 23, 2017 – as the sun crosses the celestial equator, bringing the September 22 equinox – try to catch the slender waxing crescent moon and planet Jupiter in your western twilight sky. Try looking for them as soon as the sky darkens; both the moon and Jupiter are bright and should show up in the twilight. They might be tough from northerly latitudes on September 21, though. So near the September equinox – with the autumn angle of the sun and moon’s path, the ecliptic, low in the twilight sky – the waxing crescent moon will follow the sun beneath the horizon before it gets good and dark. Meanwhile, from Earth’s Southern Hemisphere, the ecliptic is now making a steep angle with the sunset horizon, and the moon and Jupiter will be higher above that horizon after sunset on September 21.
Click here for a recommended almanac to find out the setting times of the sun, moon and Jupiter in your sky.
From North America – indeed, from the whole world – it’ll be easier to spot the moon on September 22 and 23, when its distance from the sunset will be greater. Each following day will present a wider evening crescent higher up in the west at sunset and setting later after nightfall. Given an unobstructed horizon in the direction of sunset, Jupiter sets better than one hour after sundown at mid-northern latitudes. Binoculars can help you to view the moon and Jupiter all the sooner after the sun goes down.
The sky chart at the top of this post is for mid-northern latitudes in North America. At mid-northern latitudes in Europe and Asia, Jupiter will be similarly positioned in the western sky. However, a thinner crescent moon will be closer to the horizon and will set even sooner after sunset.
The Southern Hemisphere has the advantage on all of these dates. In the Southern Hemisphere now, as we approach the September equinox, spring is coming. In the springtime, from either hemisphere, a young crescent moon rides high above the sunset and is easy to spot.
So, for instance, on the evening of September 21 at Philadelphia, Pennsylvania (40o north latitude), the moon sets one hour and 7 minutes after sunset and Jupiter sets one hour and 9 minutes after the sun.
But directly south of Philadelphia, at Valdivia, Chile (40o south latitude), the moon on this same date (September 21) sets one hour and 27 minutes after the sun and Jupiter sets two hours and 12 minutes afterwards.
The moon and Jupiter rank as the second-brightest and fourth-brightest celestial objects to light up the heavens, respectively, with the sun of course being the brightest. Venus, the third-brightest heavenly body, appears in the east before dawn now, so there’s no mistaking Jupiter for Venus in the September 2017 sky.
Bottom line: How many of you will see the moon and Jupiter after sunset on September 21, 2017? If you miss them, try September 22 or 23.
Bruce McClure has served as lead writer for EarthSky's popular Tonight pages since 2004. He's a sundial aficionado, whose love for the heavens has taken him to Lake Titicaca in Bolivia and sailing in the North Atlantic, where he earned his celestial navigation certificate through the School of Ocean Sailing and Navigation. He also writes and hosts public astronomy programs and planetarium programs in and around his home in upstate New York.