NavList:

You're right that you would never be able to see Mercury close to opposition in high latitudes. There's also an interesting problem in tropical latitudes. 

Let's look at a specific case. We go to the tropics in about five weeks. On March 8, Mercury is about 9% illuminated at an elongation of 11° from the Sun. Venus under these circumstances would be manageable. But Mercury would be practically impossible to see. Its apparent magnitude would be 2.5 ignoring atmospheric extinction, and that's tough to see even in late twilight. But so close to the Sun, Mercury will only be above the horizon in the earliest part of twilight. That means its altitude would be just 5° or so, and atmospheric extinction would knock it down to an apparent magnitude of 3.7 or lower. But the real killer is variable refraction at this low altitude. Mercury will be distorted and smeared out by chromatic aberration and ruined by poor "seeing". If you could manage to find it with a telescope at all, the tiny disk would be turned into a dancing blur. Even modern video photographic tricks and image-stacking would have a tough time showing the planet as a crescent. 

The crescent in the art image we've been discussing (for the past fourteen years!) most closely resembles Venus when it's at its thinnest ... but not quite. And the difference calls attention to another factor in Venus's favor compared to Mercury. The atmosphere of Venus enhances the crescent when it becomes very thin. In fact, when Venus is very close to the Sun (and then observable only in daylight), rather than a crescent, Venus can appear as an extended ring. This is similar to the phenomenon that turns the Moon orange or dusky red during a lunar eclipse. From the point of view of an observer on the Moon, the Earth does not become a perfect slim crescent (like in the opening sequence right at the beginning of the movie "2001"). Instead it is a crescent with glowing extensions making almost a ring in the sky. Because of its atmosphere, Venus maintains its brightness as a slim crescent while the brightness of Mercury plummets under similar geometric circumstances. 

By the way, I suspect that part of the appeal of the "Hideaway" digital artwork arises from its resemblance to the title frame of 2001: A Space Odyssey. See below. 

Frank Reed

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