rising and setting of stars
Certain types of rising and settings must be inferred instead of
observed, because the star is obscured by sunlight. These are used only by
- the cosmical rising: the star and the sun rise
together. The star, of course, is obscured by the sun.
- the cosmical setting: the star sets as the sun
- the true acronychal rising: the star rises
as the sun sets.
- the true acronychal setting: the star and sun
Perceptible risings and settings
These risings and settings can be observed directly, and it
is these which have played a part in calendars.
- the heliacal rising: the first visible, though
brief, appearance of a star on the eastern horizon before sunrise. On the
previous morning, sunlight made the star invisible. When the rising of a star is
spoken of, it is usually the heliacal rising that is meant. In ancient Egypt,
the helical rising of Sirius coincided with the annual rising of the Nile at
- the heliacal setting: the last visible setting
of a star in the evening twilight. On the following evening, the star will pass
below the horizon while there is still too much sunlight for it to be seen.
- the apparent achronychal rising: the last
visible rising of a star in the evening twilight. On the following evening, the
star will rise while there is still too much daylight for it to be seen.
- the apparent cosmical setting: the first visible
setting of a star in the morning twilight. On the previous morning, the star
didn't quite reach the western horizon before sunlight made it invisible.
Copyright © 2000 Sizes, Inc. All rights reserved.
Last revised: 8 November 2003.