June’s top lunar feature to observe
Hortensius
Type: Crater Size: 15km
Longitude/Latitude: 28˚ W, 6.5˚ N
Age: Less than 1.1 billion years
Best time to see: Two days after first quarter (9 June) or one day after last quarter (22–24 June)
Minimum equipment: 100mm refractor
Hortensius is a small round crater in Mare Insularum, the Sea of Islands. It sits to the south of the mid-point between two dramatic ray craters; 31km Kepler (not shown) and 93km Copernicus. Another way to locate it, which will be more phasefriendly, is to first identify the circular form of 34km T.Mayer, a crater at the western end of the Montes Carpatus range which borders the southern shores of Mare Imbrium. Hortensius lies 280km to the south of T.Mayer, just south of the midpoint between T. Mayer and 40km Lansberg.
Hortensius itself is bowl-shaped with steep sides leading down to a small, 4km diameter, flat floor. Its nearest large neighbour is 48km Reinhold, 183km to the southeast, although it’s the crater Copernicus t op lunar feature to obser ve which tends to dominate the scene.
Hortensius C, located 40km to the east-southeast, is a 6km mini-replica of Hortensius with the same shape and profile, right down to the circular floor. Continue the line from Hortensius through Hortensius C for the same distance again and you’ll arrive at 15km Hortensius E, an impressive near-ghost of a crater. Completely filled with lava, all that remins of Hortensius E is an elevated outline of its outer rim. Continue the line from Hortensius through Hortensius E for the same distance again and you’ll arrive at Reinhold, mentioned earlier.
The region just north of Hortensius appears relatively flat under direct light, but actually contains a very interesting set of lunar domes. Known as the Hortensius Omega dome field, after the largest example, Hortensius Omega, the region contains six domes, bulges on the lunar surface which are the remnants of volcanic activity. These are shield volcanoes, their characteristic shape formed when viscous lava pushed up through the lunar crust.
The domes rise to a height of 400m and range in diameter from 6–8km. Their relatively low height means they are best seen when the Sun is low in their sky, in other words when the lunar terminator is nearby. Each dome has a 1km pit in its summit, except for one that has two and one that appears to have none. The main form of the domes can be seen with a 100mm instrument, but the pits are tiny and will require either a large scope or a high-resolution imaging setup to resolve properly. To the north of the domes is a rugged area of hills and massifs to the west of Copernicus. West of the largest massifs here is the 13km crater Milichius, itself associated with a single dome called Milichius Pi. This has a diameter of 10km and like four of the Hortensius domes contains a single pit, here 1.6km in diameter.
The line running southwest of Hortensius is similar in nature to that running from Hortensius to Reinhold. First along the southwest line, 59km from Hortensius is 7km Hortensius B.
Continue for a further 50km and you’ll arrive at 10km Hortensius A. Like Hortensius C mentioned earlier, A and B are very similar in appearance to Hortensius, being bowl-shaped with a small, flat central floor.