ring plain

ring plain

[′riŋ ‚plān]
(astronomy)
A lunar crater of exceptionally large diameter and with a relatively smooth interior.
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References in periodicals archive ?
The normal specimens, which we shall pass over quickly, are Phocylides (114 kilometers diameter) and Nasmyth (77 km), two forgettable flat-floored ring plains that look like the sole and heel of a shoe.