Moonwatch
“An old feature, its borders are irregular from multiple bombardments”
Despite its impressive 88km diameter, Byrgius appears diminished because of its location on the Moon as seen from Earth. It’s very close to the western limb making it appear foreshortened into an ellipse. Being an old feature, its borders are irregular from multiple bombardments. Towards the west, the rim rises to a height of around 2km and there are breaks in the rim to the north and south.
Equally as impressive are the two smaller craters that lie nearby. 19km Byrgius A is an excellent example of a ray crater, with bright ejecta emanating out of the impact zone for distances up to 400km across the lunar surface; it is on the Association of Lunar and Planetary Observers (ALPO) list of bright ray craters. On the opposite side of Byrgius is 27km Byrgius D, which appears to have a very sharp, well-defined rim that contrasts starkly with the eroded form of Byrgius itself.
Follow the line out from the centre of Byrgius, through ray crater Byrgius A, and the most distinctive feature you’ll arrive at will be 41km Henry Freres. Midway between Henry Freres and Byrgius A is the less prominent form of 23km
Byrgius B, which is striped by ejecta from Byrgius A. This is on the ALPO list of banded craters, features that exhibit radial bands across their inner walls. For a long time the origin of these bands was poorly understood, but it’s now thought that many such bands are simply lines of contrasting ejecta from the impact that formed the crater.
This region of the Moon can be difficult to navigate under high illumination because of its lack of distinctive surface tonal variations. The most obvious feature is the 380km
Mare Humorum, which lies 735km to the east (centre to centre). Roughly midway between the centre of Byrgius and the western shore of Mare Humorum is 56km Cavendish, defined by the 24km crater
Cavendish E, which interrupts the southwest part of its rim.
Located 85km southeast of Cavendish is 30km de
Gasparis. This is a fascinating area to study using a 200mm or larger telescope as it’s crisscrossed by a number of fine rilles, collectively known as
Rimae de Gasparis. Under oblique illumination, it is well worth taking your time studying this area, trying to figure out how far you can follow these narrow features. On average the Rimae de Gasparis are around 3km wide.
Being relatively close to the Moon’s western limb, Byrgius is heavily influenced by the Moon’s libration state. Libration is the collective term used to describe the apparent rocking and rolling action of the Moon’s globe as seen from Earth. This is the result of the variation in the Moon’s orbital speed around its elliptical orbit and the tilt of the orbit relative to Earth’s orbit. Although Byrgius doesn’t disappear around the western limb because of libration, it does get quite close and at such times presents an even narrower, highly foreshortened ellipse.