MONTH, a period of time derived from the motion of the moon. The °sidereal') month may be regarded as the period in which the moon, as seen from a fixed star, would appear to make a complete revolution round the earth; it is evidently the period in which she passes through the 12 signs of the zodiac; its mean value during the year is 27.32166 days. The month, more commonly called a month" or "lunation,* is the period dur ing which the moon goes through all her phases. It is usually reckoned from new moon to new moon; to complete the lunation the moon must not only pass through the 12 signs of the zodiac, but also come again to occupy her old position relatively to the sun, which has itself advanced in the zodiac, hence the lunar is longer than the sidereal month. The mean value of the lunation is 29.5306 days. The month is the 12th part of one solar year, or 30.4368 days. The •anomalistic* month is the period in which the moon passes from perigee to perigee of her orbit; it differs from the sidereal month be cause the perigee varies its position. The line of nodes of the moon's orbit varies its position, and the month, or the period of her motion from ascending to ascending node, dif fers from the other months mentioned above.
The 12 civil or calendar months of the year have from 28 to 31 days each. The lunar month was used by the Chaldicans and Egyp tians, and is still by the Jews, Turks and some uncivilized nations. The calendar months are not equal divisions of the year, some (April, June, September and November) consisting of 30, and the remainder of 31 days, except Feb ruary, to which a period of only 28 days is as signed (see LEAP YEAR), with the addition every fourth year of one more day. These dis tinctions often give rise to much confusion as to the time intended to be designated by a month. In popular language it is often under stood to be four weeks, as this is very nearly an equal period, expressed in the division by weeks, to the month. This was even laid down by Blackstone as the legal definition of the term, so that a lease for 12 months was only for 48 weeks. The expression of twelve month," however, has been legally held to mean a solar year. See CALENDAR; MOON; ZODIAC.