Moon­watch

“An old fea­ture, its bor­ders are ir­reg­u­lar from mul­ti­ple bom­bard­ments”

Sky at Night Magazine - - CONTENTS -

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.

Byrgius D Byrgius is a well­bat­tered crater while Byrgius A is a fine ex­am­ple of a ray crater Byrgius Byrgius B Byrgius A Henry Fr­eres Cavendish Cavendish E Mare Hu­mo­rum Ri­mae de Gas­paris de Gas­paris

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