Editing Greenwich Mean Time (section)

== Ambiguity in the definition of GMT ==

Historically, GMT has been used with two different conventions for numbering hours. The long-standing astronomical convention, dating from the work of [[Ptolemy]], was to refer to noon as zero hours (see [[Julian day]]). This contrasted with the civil convention of referring to midnight as zero hours dating from the [[Roman Empire]]. The latter convention was adopted on and after 1 January 1925 for astronomical purposes, resulting in a discontinuity of 12 hours, or half a day. The instant that was designated as "December 31.5 GMT" in 1924 almanacs became "January 1.0 GMT" in 1925 almanacs. The term ''Greenwich Mean Astronomical Time'' (''GMAT'') was introduced to unambiguously refer to the previous noon-based astronomical convention for GMT.<ref>{{cite book
 | title = Astronomical Supplement to the Astronomical Almanac
 | publisher = University Science Books
 | year = 1992
 | isbn = 0-935702-68-7
 | page = 76
}}</ref> The more specific terms ''UT'' and ''UTC'' do not share this ambiguity, always referring to midnight as ''zero hours''.