New light
New light refers to the moment when one can see the contour of the waxing moon as a narrow crescent over the western horizon for the first time ( evening first ) after the new moon . In Central Europe this is possible 1 to 2 days after the new moon. In contrast, the waning crescent moon that is visible for the last time ( morning last ) before a new moon is called the old light .
The new moon phase lies between old light / morning last and new light / evening first : From earth, the moon is invisible in the firmament due to its proximity to the sun and can only be noticed at exactly the new moon through a solar eclipse if it covers the sun.
In some calendars , reference to the old light or the new light determines when a new month begins. The exact time of these phenomena does not only depend on the current moon orbit , but also on the location , visibility and other conditions of observation.
Basics
Even with the same observation location and comparable visibility conditions, the time interval between successive new light phenomena is not the same. A full cycle of the moon phases can take different lengths of time. Such a rotation related to the position to the sun is called the “ synodic period ” and is often given from full moon to full moon; as the moon orbit from new moon to new moon it is also called “true lunation ”. Their duration changes from one lunation to the next and varies between 29 days 6 hours 35 minutes (29.274 d) and 29 days 19 hours 55 minutes (29.830 d). The determined average value is called the “mean lunation” or “ synodic month ” and is 29 days 12 hours 44 minutes (29.53059 d). The current value can deviate from this mean value by more than 7 hours, from one lunation to the next the difference can be more than 4 hours.
Since the orbital speed also varies, it can take different lengths of time after a new moon until the moon reaches the position at which an observer on earth can again recognize a light figure. The sunlight reflected on the structures of the shadowy surface of the moon can then be perceived as a fine sickle, the tips of which may still be missing the arcs near the pole. The narrow crescent moon can only be seen with the naked eye, however, when its apparent distance from the sun has an angle of at least 7 °, not below that ( Danjon limit ).
The crescent moon becomes visible at the respective observation location when it is above the horizon and is not outshone by the sun. Approximately in the middle of the period after sunset and before moonset is the best time to observe the fine waxing crescent moon on the western horizon for the first time as new light.
Visibilities
As an old light, the sickle of the waning moon rises on the morning of the last visibility (morning last) in the eastern sky in front of the sun and is then outshone by it. The subsequent conjunction , the new moon, can take place at different times of the day or night.
In the evening after the conjunction, the angular distance of the moon from the sun can already be so great (over 7 °) that after sunset, under favorable conditions, the young crescent moon (new light) of the waxing moon in the western sky is visible for the first time (evening first) shortly before setting .
The visibility factors anomaly and latitude of the moon change only slightly between the old light and the following new light, so that the two situations with regard to the same new moon date are approximately mirror images. The greatest influence on the visibility of the narrow crescent moon has the angle that the connecting line sun-moon makes with the horizon (see parallactic angle ). It depends on the latitude and the season , as well as on the current position of the moon in relation to the ecliptic.
The time until the new light is visible is around 16 to 24 hours for locations on the equator . In the region from 29 to 32 degrees north latitude , the new light comes on between 18 and 42 hours after the new moon, depending on the season. At the 60th parallel north it can take up to 59 hours, i.e. around 2½ days. The further the observation point is from the equator, the longer it can take until the moonlight becomes visible for the first time and the greater the differences between the different seasons.
The correct date for a possible new light or old light at a certain location can not only be calculated fairly precisely in advance, but it can also be calculated back and compared with records of the moon phases from ancient times. In this way, astronomical phenomenology can independently verify certain dates of historians.
Influences on the visibilities
Three factors have an impact on local and temporal visibility:
- Anomaly of the moon: The continuously changing distance of the moon to the earth causes different angular distances between the sun and the moon for the earthly observer at the same time intervals (see second Kepler law ).
- Latitude of the moon: Since the lunar orbit is inclined towards the ecliptic , the moon can move 5.3 ° above or below the ecliptic. In addition, the daily parallax acts by up to 1 ° and the planes of the moon and sun orbit slowly turn backwards to the moon movement. Therefore, the maximum distances of the moon from the ecliptic vary.
- Position of the ecliptic: The angle between the ecliptic and the western horizon changes between the equinoxes , which lie in autumn and spring. For the 30th degree of latitude, this results in a fluctuation range of 36.6 ° to 83.4 ° (90 ° - 30 ° ± 23.4 °).
The flatter the ecliptic is to the horizon , the longer the moon remains invisible. If the ecliptic is steep, in the spring in the evening sky in the northern hemisphere, the apparent lunar orbit is also steeper. A narrow crescent moon stands higher above the horizon at sunrise or sunset and can thus be seen better. As a result of the obliqueness of the ecliptic (23.4 °), the visibility of both new and old light is more dependent on the time of year with increasing geographical latitude.
Calendar forms
In many lunar and lunisolar calendars , the new light marks the beginning of a new month . In the meantime, the calculated point in time is usually taken as a basis for the beginning of the month, at which - regardless of possible weather influences - the new light should theoretically be visible. Few calendar practices make him dependent on the act of new light sighting.
Until the 2nd millennium BC In BC, however, the beginning of a new month was usually only announced after the sickle of the new moon was actually sighted. In the Jewish calendar , this was the case until the destruction of the Jerusalem Temple (70 AD). A few calendars, for example the non-cyclical version of the Islamic calendar , are still based exclusively on observation today. In Egypt , for example, the beginning of Ramadan is determined by the sighting of the new light in Aswan , whereby the sighted new light is reported to Cairo by telephone and then Ramadan is proclaimed (status before 1985).
literature
- Rolf Krauss : Sothis and moon dates. Studies on the astronomical and technical chronology of ancient Egypt . Gerstenberg, Hildesheim 1985, ISBN 3-8067-8086-X .
Web links
- The new light
- Old light observation in ancient Egypt
- Predicting the First Visibility of the Lunar Crescent
Individual evidence
- ^ Jean Meeus : Astronomical Formulas for Calculators. 4th edition, Willmann-Bell, Richmond VA 1988, ISBN 978-0-943396-22-4 . - according to Eric Weisstein: Lunation. In: World of Science . dated April 26, 2006.
- ^ André Danjon : Le croissant lunaire. In: L 'Astronomie (Societe Astronomique de France). Volume 50, Paris 1936, p. 64, PDF .
- ↑ Mishnah , Treatise Rosh Hashanah II.
- ^ Rolf Krauss: Sothis and moon dates. Hildesheim 1985, p. 23.