A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Frank Reed
Date: 2019 Jan 29, 17:26 -0800
Have you ever looked at Mercury through a telescope? You'll never see a slim crescent, as we do so easily with Venus or the Moon. It's there in principle, of course, but it's effectively unobservable. It's an interesting challenge: can anyone find a photo of Mercury as a thin crescent (let's say less than 20% illuminated) anywhere? Of course Mercury has phases. That's really the only joy you get as an amateur observer when you see it through a telescope. You can just barely make out its tiny half-full disk at maximum elongation from the Sun. Other times, it's a bit gibbous or a "fat" crescent, but never anything like the paper-thin crescent in the artwork.
To see the Moon's horns horizontal when it's very low in the sky, as in the image, you need the Sun-Moon arc (the exact same arc that we measure when shooting lunars) to be perpendicular to the horizon. This is nearly the same as having the ecliptic perpendicular to the horizon. There are some interesting exceptions when the Moon is very close to the Sun in angular terms, but basically you need to be in the tropics (+5° in some years, -5 in other years). So you got that essentially right. By the way, if anyone wants to experiment, you can try this in "Stellarium". Tap the comma key to show the ecliptic. Then run through the calendar year and watch how the angle relative to the horizon (where the ecliptic intersects the horizon) changes during the seasons.