History: For approximately five millenia various cultures have used the visibility of the new crescent moon as the basis of their calendars. Although many modern calendars which use lunar months are calculated, some still rely on observations of the Moon. Perhaps as many as one billion people use the first sighting of the new crescent moon to signal the start of the new month. For instance, Islam requires that the sighting of the the new crescent moon should be made with the naked eye. For others, the challenge is not so much to sight the Moon but to observe the youngest possible crescent. Whatever your interest in the new crescent moon, it is useful to have good predictions of the visibility of when the new crescent moon. Predicting the first sighting of the new crescent moon has been attempted since the time of the Babylonians and perhaps earlier still. These early methods relied on the age of the Moon and the time difference between moonset and sunset. More modern methods use the angular separation of the Sun and Moon, the altitude difference between the Sun and Moon, their relative azimuths and the width of the crescent. Some methods even try to model atmospheric effects to improve the prediction and to address the effect of relevant atmospheric phenomena. Studies published by Bradley Schaefer and LeRoy Doggett of Moon Watch programs in the 1980's and 1990's have been used in a method of predicting the new crescent moon developed by Dr. Bernard Yallop, a former Superintendent of HM Nautical Almanac Office (HMNAO). His method uses the angular separation of the Sun and Moon, the width of the crescent and the so-called "best time" of observation to generate a visibility parameter q. Yallop's method uses six criteria based on the value of q to indicate the ease with which the new crescent moon can be seen. They are:    Criterion q range Remarks    A q > +0.216 Crescent easily visible    B +0.216 >= q > -0.014 Crescent visible under perfect conditions    C -0.014 >= q > -0.160 May need optical aid to find crescent    D -0.160 >= q > -0.232 Will need optical aid to find crescent    E -0.232 >= q > -0.293 Crescent not visible with telescope    F -0.293 >= q Crescent not visible, below the Danjon limit Several HM Nautical Almanac Office documents in PDF format are available for download. If you want to find out more about the methods involved in calculating the first sighting of new crescent moon, please consult HMNAO Technical Note No. 69. Dates for Earliest Visibility of New Crescent Moon, 2006-2010 for Karachi, Port Louis, Mecca, Tripoli, Cape Town, London, Boston and Los Angeles can be found in HMNAO Astronomical Information Sheet No. 93. Similar information for London, Leeds and Glasgow can be found in HMNAO Astronomical Information Sheet No. 94. Earliest Visibility of the New Crescent Moon between 2006-2010 in terms of date, time and location can be found in HMNAO Astronomical Information Sheet No. 95. Similarly dates for Earliest Visibility of New Crescent Moon, 2011-2015 for Karachi, Port Louis, Mecca, Tripoli, Cape Town, London, Boston and Los Angeles can be found in HMNAO Astronomical Information Sheet No. 101. Similar information for London, Leeds and Glasgow can be found in HMNAO Astronomical Information Sheet No. 102. Earliest Visibility of the New Crescent Moon between 2011-2015 in terms of date, time and location can be found in HMNAO Astronomical Information Sheet No. 103. HMNAO TN No.69 - A Method for Predicting the First Sighting of the New Crescent Moon HMNAO AIS No.93 - Earliest Visibility of the New Crescent Moon, 2006-2010 for cities around the World HMNAO AIS No.94 - Earliest Visibility of the New Crescent Moon, 2006-2010 for London, Leeds and Glasgow HMNAO AIS No.95 - Earliest Visibility of the New Crescent Moon between 2006-2010 HMNAO AIS No.101 - Earliest Visibility of the New Crescent Moon, 2011-2015 for cities around the World HMNAO AIS No.102 - Earliest Visibility of the New Crescent Moon, 2011-2015 for London, Leeds and Glasgow HMNAO AIS No.103 - Earliest Visibility of the New Crescent Moon between 2011-2015   Glossary of terms: age of the Moon The number of days since the instant of new Moon. altitude (Alt) The angle measured from the horizon (0°) toward the zenith (90°). apparent The designation given to coordinates that indicate that the position includes effects due to the relative positions of the object and the Earth, the movement of the Earth around the Sun and the object in space. ARCL the arc of light is the angle subtended at the centre of the Earth by the centre of the Sun and the centre of the Moon. ARCV the arc of vision is the geocentric difference in altitude between the centre of the Sun and the centre of the Moon for a given latitude and longitude, ignoring the effects of refraction. azimuth (Az) the angle around the horizon measured from true north (0°) through east (90°), south (180°), west (270°) and back to north (360°). best time an empirical prediction of the time which gives the observer the best opportunity to see the new crescent Moon (Sunset time + (4/9)*Lag time). Danjon limit The criterion that dictates how wide the crescent must be in order for it to be observable with the naked eye. All objects which exhibit phases have such a limit, which will depend on distance from the Sun and Earth, the size of the object, and the material that is reflecting the sunlight. DAZ The the difference in azimuth between the Sun and the Moon at a given latitude and longitude. The difference is given in the sense azimuth of the Sun minus azimuth of the Moon. elongation The angular separation between two celestial bodies. geocentric The designation given to coordinates centred on a fictitious observer at the centre of the Earth. GMT GMT (Greenwich Mean Time) is a measure of solar time on the Greenwich Meridian. UT (Universal Time) can be regarded as being the same GMT for most applications. horizon At sea level, the horizon is 90° away from the zenith in all directions. lag time The time interval between sunset and moonset. The lag time is usually given in minutes. It can be negative, indicating that the Moon sets before the Sun. latitude and longitude The coordinates of an observer on the surface of the Earth. Latitude is measure from 0° to 90° degrees, north and south of the equator. North latitudes are positive while south latitudes are negative. Longitude is measured, from 0° to 180°, east and west of the Greenwich meridian. East longitudes are positive while west longitudes are negative. Latitude and longitude may also be used in other contexts. lunation The period of time between two successive new moons. magnitude A measure of the brightness of a celestial object. moonset The time when the upper limb of the Moon is co-incident with the horizon. Usually this includes the effects of refraction (34'), the semi-diameter of the Moon and the parallax. The latter two vary with date and time. new Moon The time of new Moon is when the apparent longitude of the Moon and the Sun are identical. parallax This effect is due to the observer being on the surface of the Earth (not the centre) in combination with the relative proximity of the Moon in astronomical terms. The effect of parallax is about 1°. refraction The effect of the atmosphere on a ray of light as it travels from space through the atmosphere of the Earth and into the eye of the observer. The closer the celestial object is to the horizon, the larger the effect. The maximum value, at standard conditions, is about 34'. semi-diameter Half the angular diameter of an object or the angular radius of the object. standard time In most cases, standard time is based upon some meridian of longitude which differs by a multiple of 15° from the Greenwich Meridian i.e. standard time differs from Greenwich Mean time by a multiple of hours. Standard Time for the UK is Greenwich Mean Time and that for France would be Central European Time (CET) being one hour ahead of GMT. However there are some places where fractions of hours are used as well. Standard Time is not the same as local time when daylight savings time (summer time) is in effect. sunset The time when the upper limb (edge) of the Sun is co-incident with the horizon. Usually this includes the effects of refraction (34') and the semi-diameter of the Sun (16'). time zone The time zone is a convenient measure of time which reflects the offset in longitude of a given region relative to the Greenwich Meridian e.g. Athens is 2 hours ahead of GMT. topocentric The designation given to coordinates centred on an observer on the surface of the Earth rather than a fictitious observer at the centre of the Earth. zenith The point in the sky directly overhead. The horizon is 90° away from the zenith in all directions. zone time The zone time is another name for the Standard Time.   Acknowledgements: Website designed by h3oMEDIA )) Interactive. We would like to thank Duncan Russell for his help in providing a temporary web server for this site.   E-mail: HMNAO can be contacted at this address.