Low in southwest after Sunset – Sept. 1st
by Deborah Byrd
The new moon fell on Monday, August 29. New moon is the time each month when the moon passes most nearly between the Earth and sun. It’s when the moon is traveling across the sky with the sun during the day.
When and where will you next see the moon? At each new moon, the moon passes from the morning to the evening sky. In the several days after new moon, a thin waxing crescent moon shows up in the west at dusk.
That could happen tonight. Will you see the young crescent moon and Spica, the brightest star in the constellation Virgo, and the planet Saturn low in the west after sunset tonight? Good chance, if it’s clear and no obstructions block your view of the southwestern sky.
The moon moves continuously in orbit around Earth. As seen on our sky’s dome, the orbital motion of the moon carries our companion world farther from the sun-Earth line each day. And thus tonight’s moon is farther from the sunset than last night’s moon.
Still, autumn is nearly upon us in the northern hemisphere. That means you’ll have a harder time catching the young crescent moon, Spica and Saturn after sunset than they do in the southern hemisphere. Spica, a spring and summer star, fades in the twilight dusk at this time of year. And the ecliptic – the path of the sun, moon and planets – makes a narrow angle with the horizon at sunset in September and October. This fact keeps the waxing crescent moon low in the sky – and tougher than usual to see.
The whereabouts of the ecliptic in our sky is related to Earth’s orbit around the sun. (Are you getting an idea of cycles upon cycles in the celestial sphere? You should, because that’s the reality.) Earth’s tilt on its axis – which carries the sun low in winter and high in summer – also keeps the young crescent moon from being easily visible after sunset in September and October.
But the sky is nothing if not reliable. The moon will keep moving in orbit around the sun, and thus the crescent moon in our evening sky will keep moving farther from the sunset each night. You’ll see it one night soon, if you don’t see the moon with Spica and Saturn tonight.