Four Absolute Seasons
[Disclaimer: The contents of this post are merely the reflections of the author’s opinions an beliefs, the subject matter holds no scientific weight. The aim is to explore alternative views on history.]
By the Middle Kingdom of ancient Egypt an advanced decanate system was established for astronomical purposes. The decanate system makes use of the heliacal rising of a star, or group of stars, within 10° of space on the ecliptic, the decans. Scholars seem to be in consensus that advanced astronomy did not came about until the Eight Dynasty in the First Intermediate Period (c. 2181–2160 BCE). Could prior dynasties have made practical use of the ecliptic? In this post I will explore concepts that suggests they could have by letting go of the heliacal rising and focusing on the midheaven.
Each day the earth makes an axial rotation by which the Sun follows the ecliptic path in our sky. Because of the angle between the celestial equator and the ecliptic, days lengthen or shorten as seasons change. The summer solstice, or midsummer, occurs when one of the earth’s poles has its maximum tilt toward the Sun. The winter solstice, or midwinter, occurs when one of the earth’s poles has its maximum tilt away from the Sun.
The influence of the polar tilt differs for each latitude. Seasons are more pronounced at higher latitudes and almost not existent around the equator. During the vernal and autumnal equinoxes, the Sun crosses directly over the equator, resulting in a rise in intensity of sunlight, and consequently an increase in temperature. Throughout the year, regions close to the equator experience a warm climate with minimal seasonal variation. As a result, equatorial cultures, in the tropics, recognize wet and dry seasons only.
Egypt depends heavily on the Nile whose main two tributaries merge at a latitude of 15° North, which is in the tropics, the Nile ends in a large delta in Egypt at a latitude of 31° North, which is in the subtropics. Agriculturally Egypt is known to have three season: the flooding season, the growing season and the harvesting season. The flooding season more or less coincide with the summer solstice period.
Agriculturally the seasons are split in three, however, this is not the case when it comes to the luminaries. In Egypt the summer solstice daylength is about 14 hours, the winter solstice’s daylength is about 10 hours, this difference is noticeable enough, solstice observances are not disputed.
The flooding season more or less coincide with the summer solstice period. Agriculturally the seasons are split in three, this is not the case when it comes to the luminaries… there can only ever be four “timely markers” in a solar year.
The vernal and autumnal equinoxes are halfway points where the daylength is about 12 hours, therefore there can only ever be four “timely markers” in a solar year. An interesting characteristic of these timely markers is that the local meridian runs perpendicular to the ecliptic. For the equinoxes this happens at sunrise and sunset, for solstices this happens at noon and midnight.
Time behaves linearly, angles between stars can be determined and fixed stars return roughly to the same position each year. A star at the midheaven, during an equinoctial rising or setting sun, points towards the midheaven at the midnight hour of its relevant solstice; sunset during the vernal equinox and sunrise during the autumnal equinox have the same upper sky as the winter solstice’s midnight hour. This insight allows the observer to calibrate between solstices and equinoxes.
This is perfectly understandable because the upper sky drifts at a constant rate of 4 minutes per degree, that is 6 hours for every 90°. Because a calendar is roughly 360° we can take a day for a degree. By tracking the stars much can be said about the date of the year or the time at night. Precision is determined by the skill of the observer and the composition of the upper sky, naturally some stars are brighter than others. luckily when a bright star is defined in its proper space then it will have its purpose for quite some time.
During the Old Kingdom the constellation that we know as was at maximum tilt , it takes roughly 2000 years for the constellation to completely drift from this position, this happens gradually. Again, the celestial pole aligns with the orbital pole on the solstices; poles are perpendicular to their axial rotation per definition. In short when Leo was at the midheaven, the ecliptic crossed directly over the equator on the eastern and western horizon.
Figure 5 shows the precession of the constellation Leo in the winter solstice, midnight, sky. Between 2376 and 2202 BCE three precise opportunities for alignment followed each other, kicking off with Leo’s main star Regulus. Between 2202 and 1704 BCE there are no precise markers available, beyond 1704 BCE the focus would be on Ursa Major.
Nomenclature obviously is irrelevant, what today is referred to as Leo could have had a different name or association for the ancient Egyptians. Date predictions are not limited to solstices and equinoxes, the concept can easily be applied to all zodiacal constellations as long as you have a reference point.
If a certain date needs to be calculated then the relative angle between the referenced stars on the ecliptic is the determining factor, the closer a star is to the ecliptic the more accurate date predictions become. Table 1 below shows, but is not limited to, stars that are within a 5.5° from the ecliptic with the exception of Sirius and Procyon:
Consider the angles between Alcyone and Sirius (45° using the ecliptic coordinate system), and between Sirius and Regulus (again 45°), and also between the angles of Tianguan and Regulus (65°), with Procyon almost exactly in the middle.
In plane geometry, constructing the diagonal of a square results in a triangle whose three angles are in the ratio 1 : 1 : √2, adding up to 180° or π radians. Hence, the angles respectively measure 45° (π/4), 45° (π/4), and 90° (π/2). Constructing a right triangle by equally dividing 65° in two, results in a triangle whose three angles are 32.5°, 57.5°, and 90°. The ratio of the angles are roughly 7 : 11 : 13, with 11/7 ≈ π/2.
Understanding the ecliptic could have led to time tracking, knowledge of the cardinal directions and spark interest in trigonometry. Although general observations are always crude, greater precision can be achieved when bright stars align to the midnight meridian. This we have seen during the period of the construction of the Great Sphinx of Giza and the Great Pyramid of Giza.[1][2]
If there is any truth to this theory, then why did knowledge not simply increase following the Old Kingdom? It just so happens that a dark period followed the Old Kingdom, perhaps due to the mysterious 4.2 kiloyear event; one of the most severe climatic events of the Holocene epoch. It is said to have lasted for about 150 years (c. 2250–2100 BCE).[3] It is possible that later generation did not fully grasp the importance of the ecliptic in the same way as their ancestors did. Perhaps the constellation Leo did not provide adequate markers, even if its importance is unclear to us, and consequently they might have chosen to shift their focus from the midheaven towards the heliacal rising during the First Intermediate Period.
By Orestes_3113
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References
[1] The Great Sphinx of Giza — https://medium.com/@Orestes_3113/the-mystery-of-the-sphinx-a1d6328fdb30
[2] The Great Pyramid of Giza — https://medium.com/@Orestes_3113/the-great-pyramid-of-giza-607eec532892
[3] 4.2 Kiloyear event — https://en.wikipedia.org/wiki/4.2_kiloyear_event
