Go young moon hunting!
Many skywatchers enjoy seeing the young moon return each month to the west after sunset. If you’re one of them, now is the time to look for that fleet little moon that appears in the west at evening twilight, then sets before nightfall.
Generally, any moon that’s less than one day old (or 24 hours past new moon) is hard to spot with the eye alone, or, sometimes, even with binoculars. For the most of the world on April 6, 2019, however, the moon will be over one day old after sunset. In other words, if you have a clear and unobstructed horizon in the direction of sunset, you have a good chance of catching the whisker-thin young moon after sundown on April 6. Then, as the evenings pass – as Earth spins on its axis day by day, and the moon moves in orbit around Earth, thereby inching farther from the sun on our sky’s dome – you’ll find the moon more easily each evening. As our above chart shows, it’ll appear a little higher in the west after sunset in each of the coming evenings.
The young moon will be hard to catch from Australia, New Zealand and far-eastern Asia, where the moon is only about one day old (or younger) at sunset April 6. By the time that the sunset comes to the Americas on April 6, the moon will be somewhere around 1 1/2 to 1 3/4 days old, making the young moon moon easier to spot after sunset. Each hour, the moon in its orbit edges its own angular diameter farther from the sun on the sky’s dome, and the sliver of its daytime side turns more slightly toward Earth. Moreover, the widening lunar crescent stays out a little longer after sundown.
In other words, an older moon is easier to see … farther from the sunset, with more of its illuminated side showing.
Young moons in early spring are usually much easier to catch than young moons in early autumn. That’s because the ecliptic – the approximate monthly pathway of the moon – hits the sunset horizon at a steep angle in spring yet a shallow angle in autumn. And since it’s now spring in the Northern Hemisphere and autumn in the Southern Hemisphere, you’d normally expect this April young moon to be more easily viewed in the Northern Hemisphere.
But this time around, the April 2019 new moon passes 5 degrees (10 moon-diameters) south of the ecliptic, greatly negating the Northern Hemisphere’s advantage and the Southern Hemisphere’s disadvantage. So – no matter where you live on Earth – give the young moon a try!
If you miss the young moon after sunset on April 6, look again on the evenings after that. Day by day, a wider waxing crescent will shine higher up in the sky at nightfall. Watch for the moon to move into the vicinity of the red planet Mars and the red star Aldebaran on the evenings of April 8 and 9. And if you live at temperate latitudes in the Northern Hemisphere, be mindful that the sparse moonlight over these next several evenings may provide you with your last good opportunity to view the post-dusk zodiacal light until next year.
Bottom line: After sunset on April 6-9, 2019, watch for the young waxing crescent moon to widen day by day as it climbs upward toward the red planet Mars and the ruddy star Aldebaran.