Acronyms
The adjective acronychic (also akronych, acronymic , Greek άκρόνυχτος ( akronýktios ); from άκρας νυκτός: outermost edge of the night ) is added to astronomical events that take place at dusk . In particular, it is the setting or rise of a star . The apparent brightness of the star and the duration of the twilight determine on which day a star setting in the west or a star rising in the east can be seen with the naked eye at dusk for the first time. The opposite of the dawn term is heliacal .
Acronymic rise and fall
Relative movement between the sun and stars
Since the earth orbits the sun once a year , the sun moves once a year on the ecliptic through the starry sky when viewed from the earth . The difference between the setting time (/ rising time) of the stars and that of the sun increases by about four minutes every day. An acronymic setting star, which the sun appears to be passing in the sky, sets after its setting.
Its descent below the horizon can only be seen with the naked eye a few days before the passage at dusk. The star then sets later than the sun. The sky is no longer outshone by sunlight. An acronymic rising star, which is almost in opposition to the sun, rises after it has set.
Acronic culmination and acronic rising and setting of certain stars
In astronomy, culmination is the point in time at which a celestial object in its daily movement reaches the greatest height (upper culmination) or the smallest height (lower culmination) above the horizon. In the acronychic upper culmination, stars reach their apex (“zenith” of the orbit) at dusk , and in the acronychic lower culmination they reach their lowest point in the sky.
In ancient Egyptian astronomy , great importance was attached to the acronychic culminations and sets of the Dean stars . In ancient Egypt , the dean stars mythologically assumed the role of messengers of the gods who functioned either as enemies or friends of the sun . In Sumer , Assyria and Babylonia , some stars took on important functions as signal transmitters for the agricultural calendar with their acronyms. For example, the rising of Sirius in the evening was observed in order to determine the correct time for the leap month in the lunar calendar .
Acronyms of Venus in the evening sky
The apparent relative movement between the sun and the planets is only similar to the stars in the case of the upper planets (but retrograde movement / planetary loop when these are in opposition to the sun). The conditions of the lower planets Mercury and Venus are completely different , of which the acronychic and heliacal events of Venus are the more important because of their great brightness.
Unlike the stars and planets above, Venus (and Mercury) are not periodically overtaken by the sun in the sky. It swings back and forth in front of the sun on both sides, with its largest deflections ( elongations ) being visible for a maximum of about four hours alternately in the evening and morning sky, but never in the middle of the night (there is no opposition to the sun) .
Your visibility as an evening star is limited by two acronyms, one visible for the first time and one for the last time. In the weeks in between, their downfall happens in the first hours of the night. In the case of the stars, there is only ever one last visible acronychic set in the west per year.
Acronymic setting of the moon in the evening sky
Just as the sun overtakes the stars once a year, the moon overtakes it once a month . This means that the (waxing) moon sets later than the sun after it has overtaken it. Shortly after overtaking, you can see it sinking in the west at dusk for the first time, known as evening first. On this day the moon has its new light .
In the case of the stars, there is no acronymic setting visible for the first time , but only one for the last time in the west (per year).
See also
literature
- Alexandra von Lieven : Floor plan of the course of the stars - the so-called groove book. The Carsten Niebuhr Institute of Ancient Eastern Studies, Copenhagen 2007, ISBN 978-87-635-0406-5 .