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in chronology, the conventional period of a lunation, i.e., passage of the moon through all its phases. It is usually computed at approximately 29 or 30 days. For the computation of the month and its harmony with the solar calendar and for the months in others than the Gregorian calendar, see calendarcalendar
[Lat., from Kalends], system of reckoning time for the practical purpose of recording past events and calculating dates for future plans. The calendar is based on noting ordinary and easily observable natural events, the cycle of the sun through the seasons with equinox
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. For the difference between the sidereal month and the synodic month, see moonmoon,
natural satellite of a planet (see satellite, natural) or dwarf planet, in particular, the single natural satellite of the earth. The Earth-Moon System

The moon is the earth's nearest neighbor in space.
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. Certain stones have in ancient and modern times been connected with the months; these lucky stones, or birthstones, are often given as follows: January [from the god JanusJanus
, in Roman religion, god of beginnings. He was one of the principal Roman gods, the custodian of the universe. The first hour of the day, the first day of the month, the first month of the year (which bears his name) were sacred to him.
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]: garnet; February [Lat.,=expiatory, because of ancient rites]: amethyst; March [from the god MarsMars,
in Roman religion and mythology, god of war. In early Roman times he was a god of agriculture, but in later religion (when he was identified with the Greek Ares) he was primarily associated with war.
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]: bloodstone or aquamarine; April: diamond; May: agate or emerald; June [from the gens Junius]: pearl or moonstone; July [from Julius CaesarCaesar, Julius
(Caius Julius Caesar), 100? B.C.–44 B.C., Roman statesman and general. Rise to Power

Although he was born into the Julian gens, one of the oldest patrician families in Rome, Caesar was always a member of the democratic or popular party.
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]: ruby or onyx; August [from AugustusAugustus
, 63 B.C.–A.D. 14, first Roman emperor, a grandson of the sister of Julius Caesar. Named at first Caius Octavius, he became on adoption by the Julian gens (44 B.C.) Caius Julius Caesar Octavianus (Octavian); Augustus was a title of honor granted (27 B.C.
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]: carnelian or peridot; September [Lat.,= seven; formerly the 7th month]: chrysolite or sapphire; October [eight]: beryl, tourmaline or opal; November [nine]: topaz; December [ten]: turquoise or zircon.
Monthclick for a larger image


The period of the Moon's revolution around the Earth with reference to some specified point in the sky (see table). The differences in the monthly periods result from the complicated motion of the moon.



an interval of time nearly equal to the period of revolution of the moon about the earth. Different types of months are distinguished (see Table 1 and Figure 1). These include (1) the synodic month, which is the period of the sequence of the lunar phases (it serves as the basis for lunar calendars);

Table 1. Length of various types of months
Type of monthMean solar daysMean solar time
Synodic .....29.53058829 days12 hr44 min3 sec
Sidereal .....27.32166127 days7 hr43 min12 sec
Tropical .....27.32158227 days7 hr43 min4 sec
Anomalistic .....27.55455027 days13 hr18 min33 sec
Nodical .....27.21222027 days5 hr5 min36 sec

(2) the sidereal month, during which the moon performs a complete revolution about the earth and returns to its original position relative to the stars; (3) the tropical month, which is the period during which the moon returns to a given longitude; (4) the anomalistic month, which is the interval of time between successive passages of the moon through the perigee; and (5) the nodical month, which is the period of time between successive passages of the moon through the same node of its orbit (this concept is important for the theory of eclipses). In the Gregorian calendar, the year is divided into 12 months, each month containing from 28 to 31 days; this calendar does not agree with the lunar phases.

Figure 1. Difference between synodic and sidereal months. (1) and (3) are the relative posiositions of the sun, earth, and moon at which a full moonon occurs (a time interval of oneone synodic month); (2) is the position of the moon after a complete revolution about the earth (a time interval of one siderealreal month).


The period of the revolution of the moon around the earth (sidereal month).
The period of the phases of the moon (synodic month).
The month of the calendar (calendar month).


1. one of the twelve divisions (calendar months) of the calendar year
2. the period of time (tropical month) taken by the moon to return to the same longitude after one complete revolution around the earth; 27.321 58 days (approximately 27 days, 7 hours, 43 minutes, 4.5 seconds)
3. the period of time (sidereal month) taken by the moon to make one complete revolution around the earth, measured between two successive conjunctions with a distant star; 27.321 66 days (approximately 27 days, 7 hours, 43 minutes, 11 seconds)
4. the period of time (lunar or synodic month) taken by the moon to make one complete revolution around the earth, measured between two successive new moons; 29.530 59 days (approximately 29 days, 12 hours, 44 minutes, 3 seconds)
References in classic literature ?
He returned in the month of August to Kouka; from there he successively traversed the Mandara, Barghimi, and Klanem countries, and reached his extreme limit in the east, the town of Masena, situated at seventeen degrees twenty minutes west longitude.
In 1859, in the month of August, the young traveller, Roscher, from Hamburg, set out with a caravan of Arab merchants, reached Lake Nyassa, and was there assassinated while he slept.
780-781) Avoid the thirteenth of the waxing month for beginning to sow: yet it is the best day for setting plants.
790-791) On the eighth of the month geld the boar and loud- bellowing bull, but hard-working mules on the twelfth.
Between the ages of seven months and eight months those children were changed in the cradle"--he made one of this effect- collecting pauses, and added--"and the person who did it is in this house
From seven months onward until now, A has still been a usurper, and in my finger record he bears B's name.
And while I forgot that I had plunged into the books nineteen hours a day for three solid months, Charley Le Grant shifted my outfit into a big Columbia River salmon boat.
No, I have for the end of the month these bills which have been assigned to us by the house of Pascal, and the house of Wild & Turner of Marseilles, amounting to nearly 55,000 francs; in all, 287,500 francs.
What can I recall of her during the past six months, before I close my journal for the night?
The phrase, "at the month's end," meant, as I understood it, at the last hour of the last day in the month.
Look at the one I got now, not as big as the smallest here, an' me payin' six dollars a month for it.
The papers, as they appeared from month to month, were not the product of those unities of time and place which were the happy conditioning of