Introduction

Our sky is filled with thousands of stars visible with the naked eye. Some of them remain visible all year long. These are the stars which define the geometrical outlines of the northern circumpolar constellations like Ursae Minoris (the Small Dipper), Ursae Majoris (the Big Dipper), Draco, ... On the contrary, the southern circumpolar stars remain invisible to the european, the russian or the north-american observer. This is due to his (her) geocentric latitude. An australian or a south-american observer effectively enjoys a reversed situation : he (she) can see the Triangulum Australe and the Octans but not the Big Dipper nor the Draco, indeed.

In the one and the other cases, most of the stars visible with the naked eye however fill an intermediate area, located between the northern and southern circumpolar areas, which the Sun yearly seems to travel. During its apparent yearly course, the Sun goes near then moves away from everyone of these stars. On their conjunction day, most of them are unseen with the naked eye. Several days, weeks, even months later, they make their heliacal rising, that is to say, their reappearance at dawn. Their heliacal rising puts an end to their yearly period of invisibility which began on their day of heliacal setting, that is to say, on the day they became unseen in the twilight sky.

At intermediate latitudes, most of the stars lie within the northern and southern circumpolar areas. Their daily and yearly periods of invisibility seem all the longer to the northern hemisphere's observer since the star lies far away from the northern circumpolar area.

The yearly period of invisibility of a star - that is to say, the time elapsed between its days of heliacal setting and rising - mainly depends upon its apparent magnitude, its distance from the ecliptic line, the latitude and the altitude of the site chosen for observation, meteorological factors and the visual acuity of the observer. Within the form leading to determine the days of heliacal setting and rising of any of the 5041 stars visible with the naked eye mentioned within the Hipparcos catalogue, simply enter the year, the latitude and the altitude of the site chosen for observation, as well as the visual acuity of the observer. The software, which combines the most recent algorithms of Celestial Mechanics, Astrophysics and Optics, will then determine the days of heliacal setting and rising of the star considered, the time elapsed since the setting of the Sun or before the rising of the Sun, the heights of the Sun and the star, the atmospherical extinction and the brightness of the sky where the star is. A second form will enable you to enter the values of temperature and humidity rate of the ambient air at the time of heliacal setting and rising of the star, in order to sharpen the results.

Author

Karine Gadré, Culture Diff' company-head,
Associate Researcher, Laboratoire d'Astrophysique de Toulouse-Tarbes,
PH.D in Astronomy delivered by the University Paul Sabatier of Toulouse, France.

Dates of first publication and successive updates of this software : november 2002 / november 2003 / september 2006 / july 2010

This software was conceived within the context of the doctoral dissertation I prepared at the Laboratoire d'Astrophysique de Toulouse-Tarbes, Toulouse, France, on the basis of the most recent developments into Celestial Mechanics and Optics. It led to determine the visual acuity of the ancient Egyptian astronomers and to identify every one of the old Egyptian decans to stars visible with the naked eye of the Hipparcos catalogue.

Access to the user interface

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Make a free test of this sofware. Use it to determine the days of heliacal setting and rising of the star of Sirius in 2010 AD, next discover its numerous functionalities and possible use.

Further information

(AstroEgypto) Les décans égyptiens : des étoiles mythiques
(AstroEgypto) Le lever héliaque de Sirius, source de datation historique
(AstroEgypto) Le ciel de l'Egypte ancienne : horloges stellaires, clepsydres et plafonds astronomiques
(HistoSciences) Astronomie antique et moderne : d'observations en modélisations toujours plus poussées

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