It’ll be easier to catch the young moon – a slim waxing crescent in western twilight – after sunset today in Southern Hemisphere than in the Northern Hemisphere. But, no matter where you are, you should see the moon if your sky is very clear. The moon turned new – and rose and set with the sun – on October 15 at around 12 UTC. So the October 17 young moon will be about two days past new for all parts of Earth – two-and-a-half days for us in the U.S. Look shortly after sunset in a clear western sky. An unobstructed horizon in the direction of sunset is always to your advantage when hunting for a young moon. The planet Mars and the star Antares appear to the upper left of the moon on October 17.
A young moon is easier to spot in spring and harder to find in autumn. That’s because the ecliptic – the pathway of the sun, moon and planets – hits the horizon at a shallow angle on autumn evenings. Meanwhile, on spring evenings, the ecliptic is nearly perpendicular to the evening horizon.
That’s also why angle of the young crescent moon with respect to the horizon is different in spring than in autumn. Keep in mind that the “horns” of the moon always point away from the sun. On autumn evenings, a young waxing crescent in the west is to one side of the sun sunset, and appears tilted steeply relative to the horizon. (See photos above and below)
Meanwhile, on spring evenings – when the ecliptic is nearly perpendicular to the horizon – the young crescent is high above the sunset. The crescent itself is more parallel the horizon, exhibiting a “smile.” Of course, why wouldn’t the young lunar crescent be smiling right now at southerly latitudes? It’s spring!
So the moon will be much easier to see from Earth’s Southern Hemisphere tonight. Also, as seen from the Southern Hemisphere, Mars and Antares are more directly above the moon, rather than to the side.
Let’s do some spot checking, contrasting two different spots on Earth. At Newfoundland, Canada, the moon sets about one and one-third hours after sunset today, yet at Santo Andre, Brazil, the moon sets a whopping two and one-half hours after the sun. What’s more, the Southern Hemisphere has a much better chance of catching Mercury, the solar system’s innermost planet, while people at middle and far northern latitudes will have to have a very clear western twilight sky – or some skill at observing – to see Mercury.
Again, the advantage goes to the Southern Hemisphere because the ecliptic – or path of the sun, moon and planets – is highly inclined to the evening horizon in that part of the world.
If you miss the moon on October 17, you’ll have another chance to catch a wider waxing crescent moon higher up at dusk and nightfall tomorrow and the next day. The moon is waxing now, appearing in the sky for more of the night each day, with more of its lighted portion visible.
Bottom line: How many of you at northerly latitudes will be able to spot the young moon after sunset on Wednesday, October 17? The moon is far enough away from the sunset now to be visible from anywhere on Earth, but you’ll need a very clear sky and an unobstructed view, especially from northerly latitudes. At those northern latitudes – where it’s autumn – the moon sits mainly to the side of the setting sun, so the young moon lurks low in the sky and sets fairly soon after sunset. In the Southern Hemisphere – where it’s spring now – the young crescent stands above the setting sun, and stays out longer after sunset.