The week of February 27 – March 5 takes us from Lunar Day 8 to Day 14. This week we will highlight the moon crater Bullialdus, located in the SW quadrant of the Field Map and viewable on Wednesday night.

Bullialdus and Bullialdus A on the moonBullialdus: [SW/M7; L=22°W] (Depending on libration, Bullialdus may not appear until tomorrow night.) If you recall what Tycho looked like last night (and there may be enough features left tonight to remind you), 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 moon 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? Over the next few nights, observe how the appearance of the central mountains changes substantially.

The 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 moon crater is older?1


On Thursday, Venus is 0.5° north of Jupiter.

1 Bullialdus A has landed within the already formed ramparts of Bullialdus, making it younger. Also, the mere fact that it is smaller usually means it is also younger.


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 Moon Crater on Mare Nubium

Leave a Reply

Your email address will not be published. Required fields are marked *