December’s top lunar feature to observe
Mersenius
Type: Crater
Size: 85km
Longitude/Latitude: 49.3˚ W, 21.5˚ S
Age: Around 3.9 billion years
Best time to see: Four days after first quarter (5 and 6 December) or three days after last quarter (19 December)
Minimum equipment: 50mm refractor
Mersenius is a fine lunar crater located on the western edge of Mare Humorum, the Sea of Moisture. With a diameter of 85km it is a large feature that blends into its surroundings rather well. Its rim is battered and irregular in shape. This is particularly evident around its north section, clockwise to the east, which is wide with a relatively gentle slope. Moving anti-clockwise to the west from north, it appears better-defined and initially steeper. The crater’s depth is 2.3km, but the centre of the otherwise flat floor bulges upward by 450 metres. There is no indication of a central mountain complex, the flat floor being pockmarked by mostly tiny craterlets and, under high magnification with larger instruments, a number of fine cracks.
Of the inner craterlets, the largest is 5km Mersenius M which sits near the eastern rim. Towards the southwest rim lies Mersenius N which, at 3km across, is a good test for a large telescope. It sits near to 15km Mersenius H, a larger crater that interrupts the rim of Mersenius itself. If you have a larger scope like a reflector over 200mm, try to make out the form of Mersenius U, which ‘hangs’ south of H and looks like the outline of a drip of water.
“Of the inner craterlets, the largest is 5km Mersenius M which sits near the eastern rim.”
Heading southeast from Mersenius is 34km Mersenius D which, thanks to a smaller crater interrupting the northmost section of its rim, also has a teardrop appearance. Mersenius D sits immediately on the edge of 380km-diameter Mare Humorum and is flanked by 36km Liebig to the southwest and 10km Mersenius E to the northeast. Mersenius E has a sharp rim leading down to a tiny, flat floor section. A small 2.4km craterlet sits perfectly on its eastern limb. With high-resolution imaging equipment you could try to image this crater to see whether you can pick up a dark band running from the 2.4km craterlet down to the 2km floor of Mersenius E.
Mare Humorum has many cracks along its border regions. One that begins just to the east of Mersenius, runs for 230km to the north, passing to the west of the impressive 111km Gassendi crater on the northern edge of the mare. The largest rille of Rimae Mersenius is around 2km wide for most of its length, widening to 3km near to its southern end.
The region to the northeast of Mersenius is dominated by 42km Mersenius P which, like its named parent, is also completely flat inside. Nearby are two sharp craters with small round floor sections: 16km Mersenius S touching the northeast rim of P, and 14km Mersenius C which lies to the east of S, near to where Rimae Mersenius passes.
As the morning terminator sweeps across Mare Humorum to reveal Mersenius, it’s worth spending some time on the shores of the mare trying to trace the various cracks on Humorum’s surface. In particular, look for the arrangement northeast of 20km Liebig G, part of the Rimae Doppelmayer complex.