User:Immanuelle/acronychic
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The adjective acronychic (also akronych, akronyktic ; from Ancient Greek: ἀκρόνυχος, romanized: akrónychos) is attached to astronomical events that take place at dusk . In particular, this concerns the setting of a star .
The apparent brightness of the celestial body and the duration of twilight determine on which day a star or planet setting in the west can be seen for the last time (“last evening”) or an inner planet rising in the east or the moon setting in the west can be seen for the first time (“first evening”) with the naked eye at dusk. The opposite term, referring to dawn, is heliacal (also “first of the morning” and “last of the morning”).
Acronymic rise and fall[change | change source]
Relative motion between sun and stars[change | change source]
Since the Earth orbits the Sun once a year, the Sun, as seen from Earth, moves through the sky on the ecliptic once a year. The difference between the setting time (/rising time) of the stars and that of the sun increases by about four minutes every day. An acronychically setting star that the sun appears to pass in the sky sets after its setting.
Its descent below the horizon is only visible to the naked eye at dusk a few days before the passage. The star then sets later than the sun. The sky is no longer illuminated by sunlight. An acronychical rising star that is approximately in opposition to the Sun rises after the Sun has set.
Acronymic culmination and acronychic rising and setting of certain stars[change | change source]
In astronomy, culmination is the point in time at which a celestial object in its daily motion reaches its greatest height (upper culmination) or its smallest height (lower culmination) above the horizon. At the acronychical upper culmination, stars reach their apex (“zenith” of their orbit) at dusk, while at the acronychical lower culmination, they reach their lowest point in the sky.
In ancient Egyptian astronomy, great importance was attached to the acronychal culminations and sunsets of the decan stars . In ancient Egypt, the decan stars mythologically took on the role of messengers of the gods, acting either as enemies or friends of the sun . In Sumer, Assyria and Babylonia, some stars with their acronychal rising took on important functions as signaling devices for the agricultural calendar. For example, the evening rising of Sirius was observed to determine the correct time of the leap month in the lunar calendar .
Acronymic sunsets of Venus[change | change source]
The apparent relative motion between the Sun and the planets is similar to that of the stars only for the superior planets (but retrograde motion / planetary loop when they are in opposition to the Sun). The situation is completely different for the lower planets Mercury and Venus, of which the acronychic and heliacal events of Venus are the more significant because of their great brightness.
Venus (and Mercury) are not periodically overtaken by the Sun in the sky like the stars and upper planets. It swings back and forth in front of the sun on either side, and at its greatest elongations it can be seen for a maximum of about four hours alternately in the evening and morning sky, but never in the middle of the night (opposition to the sun does not occur).
Its visibility as an evening star is limited by two acronychal sets, one visible for the first time and one visible for the last time. In the weeks in between, their demise occurs in the first hours of the night. As for the stars, there is always only one last visible acronychal setting in the west each year.
Acronymic setting of the moon[change | change source]
Just as the sun overtakes the stars once a year, the moon overtakes them once a lunar month . This means that the waxing moon sets later than the sun after overtaking it. Shortly after overtaking, it can be seen setting in the west for the first time at dusk, which is also known as the setting of the moon. On this day the moon shows its new light . The moon has no “evening last” because it never sets acronychically, but rises heliacally (“morning last”).
Acronymic setting of the stars[change | change source]
For the stars there is no first, but only one last visible acronychal setting in the west per year.
See also[change | change source]
literature[change | change source]
- Alexandra von Lieven : Outline of the course of the stars – The so-called Nutbook. The Carsten Niebuhr Institute of Ancient Eastern Studies, Copenhagen 2007, ISBN 978-87-635-0406-5 .
Weblinks[change | change source]
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