Moonwatch

March’s top lunar feature to observe

Crater Jacobi is located in the southern highland region of the Moon and this in itself creates something of a challenge when trying to locate it. There are lots of craters in this region which look quite similar. If you’re a newcomer to lunar observing or just not that familiar with this region of the Moon, locating craters like Jacobi is a great way to learn.

One way to find it is to use a similar technique to star-hopping. On the Moon’s surface it’s the craters that provide the navigational guides but there is a twist. The Moon’s appearance is constantly changing as seen from Earth. Its phases and libration – the small rocking and rolling action that occurs due to the Moon’s orbital characteristics – means navigational markers may or may not be visible. Some features also appear to change shape slightly due to libration.

So, how do you find crater Jacobi? Probably the best guide is 114km Moretus, which is located 475km to the south. Moretus is straightforward to identify because it’s large and has welldefined terraced walls leading to a flat floor. Its defining characteristic is a central mountain which appears to cast a dramatic pointed shadow across the crater’s floor; a bit like the hand on a giant lunar clock. From Moretus, head north and slightly east to locate 96km Curtius. Centre-to-centre, Moretus to Curtius are 150km apart. Curtius looks like a battered version of Moretus with no central mountain.

Immediately north of Curtius are two similar-sized craters, 41km Curtius B and 36km Zach A. These lead the way to 71km Zach. Now head northeast by approximately two times Zach’s diameter and you’ll arrive at Jacobi. Before you arrive, be sure to check out the distinctive line of west-east craters Jacobi F (42km), E (23km), G (42km) and A (28km), south of Jacobi.

Jacobi is an old crater which looks like a gentle circular depression on the lunar surface. An equatorial triangle of smaller craters overlays Jacobi; 17km Jacobi O cutting into Jacobi’s northern rim, 19km Jacobi J cuts into the southern rim with 15km Jacobi P sitting just outside Jacobi’s southeast rim. Jacobi’s floor is relatively flat but disturbed by smaller craters. Of particular interest is the chain of craterlets starting at 9km Jacobi K in the southwest, extending through 8km Jacobi N in the northeast. These point to a small channel in Jacobi’s northeast rim, just east of Jacobi O. This channel is formed from four 3km, overlapping craterlets. around 3.5km in depth.

Like much of the southern highlands, the flat areas between craters are pockmarked with tiny craterlets. A great example of this surface type can be seen to the north of Jacobi, between it and the 75km crater Cuvier. Such a spread of craterlet sizes is a great test for high-resolution imaging setups. Starting from relatively large examples such as 6km Cuvier M, see whether you can record 4km Lilius X and craterlets below this size. High-end amateur equipment should be able to resolve down to 700m under stable conditions and there are plenty of craterlets in between the 4km to 0.7km range to test your skills on here.

Jacobi is an old crater which looks like a gentle depression on the lunar surface