The Sky Guide Challenge
Spotting the craterlets of Crater Plato.
The Moon presents some great challenges. One of them can be found within the rim of the dark, flooded crater Plato, a distinctive feature on the Earth-facing side of the Moon, which sits on the northern shore of the large basin known as Mare Imbrium, the Sea of Showers. With a diameter of 100km, Plato stands out because its dark floor contrasts with the surrounding bright highlands.
On first glance Plato’s floor looks smooth and rather featureless. This is due to the lava that once flowed within it having levelled out before solidifying. The challenge relates to the tiny craterlets that litter Plato’s floor. You can see some with a small aperture starting at, say 3-inches depending on the seeing conditions. To see the majority of them you’ll need a larger scope or a high-resolution imaging setup. Your timing needs to be right too, because lighting greatly affects the visibility of these tiny indentations.
Normally the best time to view lunar relief features is when the terminator is close by. At such times the oblique lighting from the Sun casts shadows that make these features stand out more. Does this apply for craterlets in Plato’s floor? Well, yes and no. Yes, because many of them have small, raised rims which do indeed cast shadows when the Sun is low in their sky. No, because the direct illumination that you get closer to a full Moon can make some of the craterlets stand out as tiny bright spots.
Experimentation is the best strategy and it’s definitely worth noting how many craterlets you can see at different phases. Seeing has a large part to play, too: if it’s poor, your ability to resolve these tiny features is reduced.
For convenience the craterlets can be divided into size-based groups. Group one (yellow on the image) covers the five largest craters which have diameters ranging down in size from 2.6km to 2.0km. Small scopes with apertures from 3 to 5 inches should cope with these, depending on seeing.
Group two (blue) comprises a further five craters ranging down in size from 1.8km to 1.2km. The largest of these should be visible under good seeing through an 8-inch instrument, but you’ll need a larger scope to stand a chance with the rest.
If you’ve managed to see the craters in Groups one and two purely visually, pat yourself on the back. If you’ve struggled with some of them then consider using a high frame-rate planetary camera setup. Unless you have access to a very large scope, an imaging route is definitely recommended for Group three (red). This is the toughie, containing 12 craterlets ranging in size downwards from 1.1km to 0.9km. Even with digital imaging on your side, don’t expect an easy ride with Group three.