Lunar standstill From Wikipedia, the free encyclopedia Jump to: navigation, search
For the Spanish post-hardcore-band, see Standstill (band).
At a major lunar standstill, which takes place every 18.6 years, the range of the declination of the Moon reaches a maximum. As a result, at high latitudes, the Moon appears to move in just two weeks from high in the sky to low on the horizon. This time appears to have had special significance for the Bronze Age societies who built the megalithic monuments in Britain, Ireland, and it also has significance for some neo-pagan religions. Evidence also exists that alignments to the moonrise or moonset on the days of lunar standstills can be found in ancient sites of other ancient cultures, such as at Chimnney Rock in Colorado and Hopewell Sites in Ohio. Contents [hide]
As the Earth spins on its axis, the stars above us appear to move in circles. It appears to us as if all the stars are fixed in a great sphere surrounding us. In the same way that we measure positions on the earth using latitude and longitude, we measure positions of stars on this sphere in right ascension (equivalent to longitude) and declination (equivalent to latitude). If you stand at a place on the earth which has latitude 50°, then the stars directly above you have a declination of 50°.
Unlike the stars, the Sun and Moon do not have a fixed declination. As the Earth travels its annual orbit around the Sun, with its rotational axis tilted at about 23.5° from the "vertical" (a line perpendicular to the orbit), the Sun's declination changes from +23.5° at the Summer Solstice to -23.5° at the Winter Solstice. Thus, in the Northern hemisphere, the Sun is higher in the sky and visible for a longer period of time in June than it is in December. This is why we have seasons on Earth.
The Moon also changes in declination, but it does so in only a month, instead of a year for the Sun. So it might go from a declination of +25° to -25° in just two weeks, returning to +25° two weeks later. Thus, in just one month the moon can move from being high in the sky, to low on the horizon, and back again.
But, unlike the Sun, the maximum and minimum declination reached by the Moon also varies. This is because the orbit of the Moon's revolution about the Earth is inclined by about 5° to the orbit of the Earth's revolution around the Sun, and so the maximum declination of the Moon varies from (23.5°-5°)=18.5° to (23.5°+5°)=28.5°. The effect of this is that at one particular time (the minor lunar standstill), the Moon will change its declination during the month from +18.5° to -18.5°, which is a total movement of 37°. This is not a particularly big change, and may not be very noticeable in the sky. However, 9.3 years later, during the major lunar standstill, the Moon will change its declination during the month from +28.5° to -28.5°, which is a total movement of 57°, and which is enough to take it from high in the sky to low on the horizon in just two weeks.
Wir haben in diesem Monat eine Mondwende - passiert nur alle 18.6 Jahre... Wenn Ihr Zeit habt, schaut Euch die Mondaufgänge und Untergänge an... sie sind ausserhalb der üblichen Bahn am Horizont. Jetzt im Moment am nördlichsten, und um 18. Januar herum am südlichsten..
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19.07.2010 09:22
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