A detail of the Dendera Zodiac displayed at the Louvre, Paris. Courtesy © RMN, Musée du Louvre/ H Lewandowski
I wonder if someone knowledgeable could comment about the seeming lack of precession by Sirius as viewed at the latitude of Dendera? Is it so? and if so what causes this?
The fact that Sirius seems to maintain its position relative to the position of the sun was a surprise to most scientists (aware of precession), when it was first noticed by the French scientific community following the Egyptian discoveries of Napoleon (and the Dendera Zodiac) in the early 1800’s. Physicist, astronomer, mathematician Jean-Baptiste Biot (21 April 1774 – 3 February 1862) proclaimed that this phenomenon was an oddity of the latitude and horizon around Dendera, meaning it just seemed as if Sirius was immune to the effects of precession.
The Dendera Zodiac was discovered on the ceiling of an ancient temple complex near the town of Dendera in Egypt by Napoleon’s invading army. It shows illustrations of the constellations of the zodiac – the band of stars in front of which the Sun passes during the year – with the illustrations sometimes the same as the western drawings of the constellations and sometimes different. A drawing of it was published in Paris in 1802 and created great interest among the city’s scientists. Most of them assumed that they were looking at a representation of the sky as on a planisphere at a particular date and set out to calculate that date.
Each year the Sun is in particular constellations during the summer and winter solstices and during the spring and autumn equinoxes. These constellations change over periods of thousands of year due to the precession of the equinoxes. This phenomenon in turn is caused by a slow wobble of the Earth’s axis that takes 26,000 years for a complete cycle. As a result every 2000 years or so, the Sun appears to have moved by one zodiac constellation at the time of each equinox and solstice.
Scientists such as Jean-Baptiste Biot took a guess at the location of the equinoxes and solstices depicted on the Dendera Zodiac and then calculated using precession how long ago it was created. A wide results were obtained that were generally many thousands of years in the past. These results were most upsetting to people with religious beliefs as the proposed times for the Zodiac were not in accord with the biblical time scale.
It was in this context that the relation between the solstices and the star Sirius became important. The Ancient Egyptians had two kinds of year: a civil year of 365 days and a year based on the rising of Sirius before sunrise, known as helical rising. The year based on the helical rising of Sirius is approximately 365.25 days. Years based on the helical rising of most stars would vary considerably over thousands of years due to precession, but for Sirius in the period under consideration and observed from the latitudes of Egypt there was little variation. This may explain why the Egyptians chose Sirius to define one of their two years. It is not that Sirius does not move due to precession, but that it happens to move in such a way that the distance between it and the equinoxes remains approximately constant.
Recently, it has been claimed that planets are also shown on the zodiac and their position in relation to constellations has significance as do two eclipses, one of the Sun and one of the Moon that are included. Based on the planet positions and the eclipses the Zodiac has been dated to between 15 June and 15 August in the year 50 before our era. This dating fits in perfectly with a more cultural dating based on empty cartouches, carved boxes usually containing the name of the ruler, that lie on the remaining part of the temple ceiling not taken by the French to put into the Louvre. The empty cartouches suggest that Zodiac was made during the interval between the death of Cleopatra’s father and her ascension to the throne together with her young son with Julius Caesar.
Mystery of an ancient zodiac by Jo Marchant
Egyptian Stars under Paris Skies, Jed Z Buchwald, Engineering and Science (Caltech), Volume 66 No 4 pp20–31 2003