The Straight Wall at sunrise
Thursday 26 March 2026 · the Moon, a day past first quarter
| Date | 2026-03-26 |
|---|---|
| Time (UTC) | 19:55 – 21:15 |
| Instrument | 200 mm f/6 Dobsonian |
| Eyepieces | 24 mm (50×), 9 mm (133×), 6 mm (200×) |
| Seeing | Antoniadi II, briefly I |
| Transparency | 2/5, milky; irrelevant for this |
| Sky | Moonlit; NELM about 4.5, not measured |
| Moon | About 8 days old, roughly 60% lit, high in Gemini |
Spring is the season for the evening Moon from here. A first-quarter Moon in March rides high, well over 50° by dark, so for once I am looking straight up through the air rather than along it, over the rooftops. It was hazy, but haze often comes with steady air, and tonight it did.
Whole disc at 50× first, no filter. People say the Moon is too bright in an 8-inch; I find my eye settles after a minute and the filter just takes the colour out. The terminator ran down through the middle of Mare Imbrium, the western half still dark, and on through Mare Nubium into the southern highlands. Copernicus was still in the night, a few degrees past the line.
Rupes Recta was the reason I came out. At 133× it is a dark, dead-straight line on the floor of Mare Nubium, sharp enough to look drawn on, with the Sun just rising over it from the east so that the fault throws its shadow westward. The small crisp crater Birt sits beside it with the faint thread of its rille next to it — the rille came and went, but I am counting it. At the southern end of the wall, the curl of hills that people call the stag’s antlers.
North to Plato, close to the terminator: the smooth dark floor about half drowned in the shadow of its eastern rim, the shadow edge cut into teeth by the rim peaks. Just east of it, the Alps, with the Alpine Valley cutting cleanly through them. At 200× in a steady moment I thought I saw a hint of the central rille along the valley floor, but I can’t honestly say I did. Noted as “suspected”.
Down south, Clavius sitting right on the terminator, its rim lit and its floor still mostly black, with the arc of craterlets across the floor just catching the light one by one. Then Ptolemaeus, Alphonsus and Arzachel in their line, well lit by now, Alphonsus showing its central peak.
Stopped at 21:15 when my feet gave out. Lovely night.
Next timeThe evening after, for sunrise on Copernicus and the long shadows of its terraces.