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In astronomy, the new moon is the first lunar phase, when the Moon and Sun have the same ecliptic longitude. At this phase, the lunar disk is not visible to the naked eye, except when it is silhouetted against the Sun during a solar eclipse.
The original meaning of the term 'new moon', which is still sometimes used in Calendar, non-astronomical contexts, is the first visible crescent of the Moon after conjunction with the Sun. This thin crescent is briefly and faintly visible as the Moon gets lower in the western sky after sunset, with the smallest circular arc angle possible between 5–7°. The precise time and even the date of the appearance of the new moon by this definition will be influenced by the geographical location of the observer. The first crescent marks the beginning of the month in the Islamic calendar and in some lunisolar calendars such as the Hebrew calendar. In the Chinese calendar, the beginning of the month is marked by the last visible crescent of a Moon.
The astronomical new moon occurs by definition at the moment of conjunction in ecliptical longitude with the Sun when the Moon is invisible from the Earth. This moment is unique and does not depend on location, and in certain circumstances, it coincides with a solar eclipse.
A lunation, or synodic month, is the period from one new moon to the next. At the J2000.0 epoch, the average length of a lunation is 29.53059 (or 29 days, 12 hours, 44 minutes, and 3 seconds). However, the length of any one synodic month can vary from 29.26 to 29.80 days (12.96 hours) due to the perturbing effects of the Sun's gravity on the Moon's eccentric orbit.
The most commonly used was the Brown Lunation Number (BLN), which defines "lunation 1" as beginning at the first new moon of 1923, the year when Ernest William Brown's lunar theory was introduced in the American Ephemeris and Nautical Almanac. Lunation 1 occurred at approximately 02:41 UTC, 17 January 1923. With later refinements, the BLN was used in almanacs until 1983.
A more recent lunation number called the Lunation Number (LN) was introduced by Jean Meeus in 1998, and defines lunation 0 as beginning on the first new moon of 2000 (this occurred at approximately 18:14 UTC, 6 January 2000). The formula relating Meeus's Lunation Number to the Brown Lunation Number is BLN = LN + 953.
The Goldstine Lunation Number (GLN) refers to the lunation numbering used by Herman Goldstine, with lunation 0 beginning on 11 January 1001 BCE, and can be calculated using GLN = LN + 37105.
The Hebrew Lunation Number (HLN) is the count of lunations in the Hebrew calendar with lunation 1 beginning on 6 October 3761 BCE. It can be calculated using HLN = LN + 71234.
The Islamic Lunation Number (ILN) is the count of lunations in the Islamic Calendar with lunation 1 as beginning on the first day of the month of Muharram, which occurred in 622 CE (15 July, Julian, in the proleptic reckoning). It can be calculated using ILN = LN + 17038.
The Thai Lunation Number (TLN) is called "มาสเกณฑ์" (Maasa-Kendha), defines lunation 0 as the beginning of Burmese era of the Buddhist calendar on Sunday, 22 March 638 CE. It can be calculated using TLN = LN + 16843.
There are fifteen moon dates for each of the waxing and waning periods. These fifteen dates are divided evenly into five categories: Nanda, Bhadra', Jaya, Rikta, and Purna, which are cycled through in that order. Nanda dates are considered to be favorable for auspicious works; Bhadra dates for works related to community, social, family, and friends; and Jaya dates for dealing with conflict. Rikta dates are considered beneficial only for works related to cruelty. Purna dates are considered to be favorable for all work.
An attempt to unify Muslims on a scientifically calculated worldwide calendar was adopted by both the Fiqh Council of North America and the European Council for Fatwa and Research in 2007. The new calculation requires that conjunction must occur before sunset in Mecca, Saudi Arabia, and that, on the same evening, the moonset must take place after sunset. These can be precisely calculated and therefore a unified calendar is possible should it become adopted worldwide.Fiqh Council of North America Decision: " Astronomical Calculations and Ramadan "Islamic Society of North America Decision:" Revised ISNA Ramadan and Eid Announcement "
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