The week of December 11 – 17 takes us from the end of Day 23 to Day 29. This week we will highlight the crater Bullialdus, viewable as soon as the Moon rises early Tuesday morning.

Bullialdus: Most Conspicuous #MoonCrater on Mare NubiumBullialdus: [SW/M7; L=22°W] If you recall what Tycho looked like Sunday night (and there may be enough features left early Monday morning 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 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? Come back in two weeks and observe how the appearance of the central mountains changes substantially over the ensuing nights.Bullialdus the most conspicuous crater on Mare Nubium.

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. Before looking at the footnote, can you see any evidence that tells you which crater is older? 1

Bullialdus CausewayBullialdus Causeway:2 [SW/M7; L=25°W] There is a wide, shallow valley that extends out from the west rim of Bullialdus and travels NW towards 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.


The Geminid meteor shower will be at its peak on Wednesday night and early Thursday morning. Early morning is the best time, but there should be a significant number of meteors starting at 9:00 PM. You should be able to see at least 60 Geminids per hour, and, under ideal conditions, possibly up to 120! The Geminids are known to produce brilliant, slow-moving fireballs. The Geminids parent asteroid, 3200 Phaethon, will also be in the sky in Perseus—only 1° WSW of Kappa Persei on Thurs. morning at 1:00 AM EST; R.A.3h 10m 03.25s, Dec. +43° 57’ 07.8”. It moves rapidly toward the NW. It will be at magnitude 11, so you will need a medium sized telescope.

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.

2 This is 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: Most Conspicuous #MoonCrater on Mare Nubium
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