Popular Astronomy

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Gassendi, an old lunar favourite (SPA News Circular 271)

Looking through the Lunar Section records of the past three decades it’s clear that a select few craters rank extraordinarily highly on the Moon observer’s hit list, namely: Aristarchus, Clavius, Copernicus, Gassendi, Plato, Ptolemaeus and Theophilus. It’s hardly surprising — each feature is prominent and/or very large, each easily visible through binoculars and small telescopes. Of these features, Gassendi is easily the most observed.

Why this particular crater? Well, for a start it’s pretty large at 110km across, and it occupies a prominent position on the northern border of Mare Humorum (Sea of Moisture) in the southwestern quadrant of the Moon. It comes into view at the morning terminator when the Moon is around 11 days old, presenting as the largest near-terminator crater at around this phase, and it is visible when the waxing Moon has made its presence known in the evening skies. Observers are therefore lured towards Gassendi, a feature which repays close attention and is full of surprises and challenges for those using any size of instrument.

The impressive lunar crater Gassendi, observed by Brian McInnerny on 30 September 2009 using a 200mm SCT (above) and by Graham Sparrow one lunation later, on 29 October 2009 using a 200mm Newtonian (below).

Much of the crater overlies the mountainous plateau to the north of Mare Humorum, but its southern wall projects into northern Mare Humorum, where it narrows, the lava flows almost submerging its southern rim. A complex collection of hills, mountains, ridges and linear rilles covers the crater floor. A group of three large central mountains is surrounded by Rimae Gassendi, a system of narrow valleys which cut across much of the crater’s floor, mostly to the east of the central peaks. A 100 mm telescope will resolve the most prominent rille east of the largest central peak. The other rilles require a 150 mm telescope to resolve adequately. Combined, the largest of Gassendi’s rilles would stretch for more than 300km. 33km diameter crater Gassendi A is superimposed upon Gassendi’s northern rim; south of it a ridge extends a short distance across Gassendi’s floor, causing a prominent triangular shadow to be cast at a morning illumination.

This month Gassendi emerges into the lunar morning sunlight on 26 March; by the evening of the 26th, its intricate topographic details should be visible. On this evening Aristarchus also emerges into the lunar morning, making a splendid sight as it lights up the grey plains of Oceanus Procellarum (Ocean of Storms).

Added by:  Graham Sparrow