applets you may determine the days of heliacal or
acronychal rising and setting of stars.
green: rising, red: setting
light gray: civil twilight (-6°)
dark gray: nautical twilight (-12°)
black: astronomical twilight (-18°);
green: rising, red: setting.
Hit return key after entering data in any text field.
check the applet box to plot the altitude of the Sun entered in the text field "Sun Alt."
Click to get the date, time, azimuth of the star, and the times of rise and set of the star (altitude as in "Star Alt." :
Drag to get a time interval (days) between mouse down and mouse up:
The "heliacal rising" (morning first):
is the first day of the year when the star (after a period when it was invisible) rises in the morning before the Sun and the Sun is still far enough below the eastern horizon to make it briefly visible in the morning twilight.
The "acronychal rising":
is the last day of the year when the star (after a period when it was visible at night) rises in the evening after sunset and the Sun is already far enough below the eastern horizon to make it visible in the evening twilight.
The "heliacal setting" (evening last):
is the last day when the star (after a period when it was visible) sets after sunset and the Sun is already far enough below the western horizon to make the star briefly visible in the evening twilight.
The "cosmic setting":
is the first day when the star (after a period when it was visible) sets before sunrise and the Sun is still far enough below the eastern horizon to make the star briefly visible in the morning twilight.
The visibility of the star depends on several faktors:
- the light pollution
- the altitude of the star above the horizon (for instance 3°)
- the depression of the Sun (for instance -7°)
- the brightness (magnitude) of the star
- the difference between the azimuth of the star and of the Sun
- the color of the star
- the transparency/extinction of the air
An important variable is the "arcus visionis", the vertical distance between the sun and the star (for instance av=3°+7°=10°).
Heliacal Rising & Setting
B. E. Schaefer: Heliacal Rise Phenomena
R. D. Purrington: Heliacal Rising and Setting: Quantitative Aspects
The star Sirius in ancient Egypt and Babylonia