The astronomical new moon occurs when the moon, in its monthly orbit round the earth, is between the sun and the earth and is on the line from the centre of the sun through the centre of the earth. The sun, moon and earth are in conjunction. It does not occur when the moon and sun are in alignment over a particular longitudinal axis on earth such as Greenwich or Jerusalem. The instant when it occurs can be quoted in different time zones.
At the astronomical new moon, the illuminated half of the moon faces the Sun and the dark half faces earth, This means that the new moon is invisible to the naked eye from earth. The astronomical new moon is sometimes known as the dark moon to avoid confusion.
The observed new moon occurs at the appearance of the first visible crescent of the moon, after conjunction with the Sun. This takes place over the western horizon in a brief period between sunset and moonset, and the precise time and date of the appearance of the new moon will be influenced by the geographical location of the observer.
The new moon is the beginning of the month in lunar calendars such as the Islamic calendar, and in lunisolar calendars such as the Hebrew calendar, Hindu calendars, Buddhist calendar, Chinese calendar and the Bible's Autumn and Spring lunisolar calendars. The Islamic calendar has retained the observed new moon so that the new month starts when the first crescent moon is actually seen. Consequently it impossible to be sure when a specific month and in particular the exact date on which Ramadan will begin. In Saudi Arabia, if the weather is cloudy when the new moon is expected observers go up in airplanes. In Iran a special committee receives observations of every new moon from one hundred groups to determine the beginning of each month.
A lunation is the time between successive new moons. Between 1601 and 2401 it varies between 29.272 and 29.833 days, -0.259d or 6h12m shorter and +0.302 or 7h15m longer than the average lunation in that period of 29.531 days.
Delta T (?T) and Universal Time
For many centuries the fundamental unit of time has been the time the earth takes to revolve once on its axis in relation to the Sun. Universal Time or UT, also called Greenwich Mean Time or GMT, is based on mean solar time at Greenwich in London.
However, the earth's rotation is irregular is gradually decreasing. As earth rotates tidal friction is imposed on it through gravitational attraction with the moon and the Sun. Change is also caused by variations in the continental ice sheets which are at either end of the earth's rotationsl axis. As they melt, the land under them moves towards the poles which brings the earth's mass closer to the earth's axis. Earth's rotation on its axis is also subject to short term fluctuations for periods of up to several decades, possibly due to fluid motions in the earth's core which disturb the rotation of the mantle. Changes in climate and variations in sea level may also play a part because they affect the earth's moment of inertia.
The effect of all these forces is to make the true times of astronomical new moons different to calculated times, the differences increasing as we project lunar data back into history. The change may not be constant, and records that enable us to adjust for it only go back 100 years or so. Before then, we have rely on eclipse and occultation observations from medieval and ancient manuscripts.
Stephenson and collaborators have produced a number of works in the field of Earth's rotation over the past several millennia. In particular, they have identified hundreds of eclipse and occultation observations in early European, Middle Eastern and Chinese annals, manuscripts, canons and records. In spite of their relatively low precision, these data represent our only record to the value of ?T during the past several millennia. The further into the past we go, the greater the uncertainty in ?T.
Year ?T -500 4h 40m 0 2h 56m 500 1h 35m 1000 26m 1500 3m 2000 1m
Babylonian Eclipse
In chapter IV of his Almagest, the Greek astronomer Claudius Ptolemy described the observation of an eclipse in Babylon on 23 December 383 BC. The recent discovery of a Babylonian astronomical text also contains a report of a lunar eclipse on 23 December 383 BC that agrees with Ptolemy and indicates that present estimates of ?T at that time are correct to within 10 minutes.
