The week of Oct. 24-30 takes us from Lunar Day 24 to New Moon. Early Monday morning the Moon will rise around 1:30 AM (depending on how far away you are from the center of your time zone). This week we will highlight the crater Bullialdus, visible on Monday only.

Bullialdus: [SW/M7] If you observed Tycho last night and can recall what it looked like, Bullialdus shares much of the same morphology in spite of being only half the size: a compound central mountain, eye-catching terraces, a flat floor, a thick ejecta blanket, and material in the immediate environs that rained back down after impact. There are so many interesting features that it is surprising that Bullialdus gets such short shrift. It is the most conspicuous crater on Mare Nubium, an area that offers several conversation pieces. Examine the inner terraces for tiny impact craters and evidence of landslides. Can you make out an intriguing raised ridge running from the central mountains southeast to the base of the terraces on the wall?

bullialdusThe floor of Bullialdus creates a small illusion; as a result, there is some disagreement about its shape. Some observers think it is concave, others (most notably Patrick Moore, of Caldwell Object fame) think it is convex. How does it strike you?

Look carefully at the shared terrain between Bullialdus and Bullialdus A immediately to its south. Other than the fact that Bullialdus A is smaller, can you see any evidence that tells you which crater is older?1

bullialdus-causewayBullialdus Causeway2 : There is a wide, shallow valley that extends out from the west rim of Bullialdus and travels N.W. toward Lubiniezky. Toward the end of the valley (a little more than one crater diameter) you will run into two parallel lines, about 7 miles apart, running perpendicularly across the valley. These parallel lines are clearly depicted on Sky & Tel’s Field Map as a short arc and give the illusion of a causeway or a bridge that cuts across the valley at right angles. (You’ll need about 150x.) Whatever it is, it remains something of a mystery, and nobody has come up with an adequate explanation.


On Monday, Saturn, Venus, and Antares form a right triangle low in the southwest during evening twilight. Watch Venus’s progress every night, as by Thursday Venus, at a very bright mag. -4, will make a straight line between Saturn and Antares.

1 Bullialdus A has landed within the already formed ramparts of Bullialdus, indicating that it’s younger. Also, the mere fact that it is smaller usually means it is younger.
2 A purely descriptive term as there is no official name for this feature.


It is highly recommended that you get a copy of Sky and Telescope’s Field Map of the Moon, the very finest Moon map available for use at the telescope. It is available for $10.95 at and on Amazon. All features mentioned in this blog will be keyed to the grid on the Field Map and will look like this: Plato: [NW/D9]

Courtesy of Gray Photography of Corpus Christi, Texas
Lunar photos: NASA / USGS / BMDO / LROC / ASU / DLR / LOLA / Moon Globe. Used by permission

Andrew Planck
Bullialdus: The Most Conspicuous Crater on Mare Nubium
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