Emily Dickinson Lexicon

MONTH, n. [Sax. monath, from mona, the moon; D. maand; G. monath; Sw. månad; Dan. maaned; L. mensis; Gr. μην, a month, from μηνη, the moon.]

A space or period of time constituting a division of the year. Month originally signified the time of one revolution of the moon, a lunation, or the period from one change or conjunction of the moon with the sun to another, a period of 27 days, 7 hours, 43 minutes and 5 seconds. This is the periodical month, or as we generally call it, the lunar month. In this sense we still use the word month. But we also apply the term to the space of time in which the sun passes through one sign, or a twelfth part of the zodiac. This period contains 30 days, 10 hours, 29 minutes, 5 seconds, and is called a solar month. In the year, there are twelve solar months, and thirteen lunar months. In popular language, four weeks are called a month, being nearly the length of the lunar month. A calendar month differs in some degree from a solar month; consisting of twenty eight, twenty nine, thirty or thirty one days, as the months stand in calendars or almanacs.