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The fading importance of heliacal risings and settings

Ancient civilizations used the appearance of certain stars to guide their subsistence, but it’s mostly just astronomers who understand them now

GLENN ROBERTS glennkroberts@gmail.com @chronicleherald Glenn K. Roberts lives in Stratford, P.E.I., and has been an avid amateur astronomer since he was a small child. He welcomes comments from readers at glennkroberts@ gmail.com.

Long before the formal adoption of calendars as a means of marking the passage of time, ancient peoples relied on the observable objects in the heavens to mark specific annual events or occasions.

Most notably, they watched for signs and events that had to do with the timing of planting and harvesting of crops and with the turning of the seasons.

While many of these observations were based on the movement of and location of the sun and the moon in the sky throughout the year, they also were attuned to the heliacal rising and setting of other celestial objects, usually stars, star clusters and planets. Some of the oldest astronomical treatises from the ancient Greeks, Romans, Egyptians and Babylonians concern the risings and settings of celestial objects.

The term heliacal refers to the position of a celestial object in the sky relative to the sun, particularly with respect to the first rising or the last setting of the object after invisibility due to its conjunction with the sun. A morning heliacal rising of a celestial object is the first day when it is visible in the eastern sky in the light of the dawn just before sunrise, when the sun is still below the horizon (Venus, as the morning star, is a prime example). An evening heliacal rising is the first day when the object is visible in the eastern sky opposite from the twilight just after sunset.

Likewise, a morning heliacal setting is the last day when the object is visible in the western sky opposite the light of dawn just before sunrise and an evening heliacal setting is the last day when the object is visible in the western sky in the twilight just after sunset.

SIRIUS AND THE PLEIADES

The most famous heliacal risings for which there are a great many documented records are those of the heliacal rising of the bright star Sirius (from the Greek Seirios, meaning glowing or scorching) in Canis Major — the Great Dog and the heliacal rising of the Pleiades star cluster in Taurus — the Bull.

The ancient Egyptians placed great significance to the heliacal rising of Sirius; it was even personified by the goddess Sopdet, who wore a star (Sirius) on her head. The annual inundation or flooding of the Nile River deposited large amounts of silt that fertilized the floodplains along the Nile used by the ancient Egyptians — even today — for farming and the grazing of their cattle. Having knowledge of when the inundation was to take place allowed them time to harvest the crops and to move the cattle and buildings off the floodplain to higher ground.

At the time of the establishment of the Egyptian calendar during the Old Kingdom (c. 2700-2200 BC), the heliacal rising of Sirius occurred in early July, which served as the zero-point of the start of the Egyptian solar civil calendar of 365 days, as seen from the then-capital city of Memphis. Nowadays, due to the precession of the equinoxes, it occurs in early August.

The 365-day Egyptian calendar was instrumental in the later reformation of the calendar used by the Roman Republic (509 - 27 BC), which, in turn, gave way to the Julian calendar (under Julius Caesar in 46 BC) and, eventually, our modern-day Gregorian calendar, introduced in 1582 by Pope XIII.

To the ancient Romans and Greeks, the heliacal rising of the Pleiades (from the Greek plein, meaning to sail) or The Seven Sisters star cluster in the spring sky signified the beginning of the seafaring and farming seasons and its setting in the autumn meant the end of these seasons.

Many other cultures around the world incorporated the Pleiades star cluster into their cultures and mythologies, including the ancient Celts, Indigenous Australians, Hawaiians, Arabs, Mayans, Aztecs, Hindus and the Sioux, Kiowa and Cherokee nations of North America.

Perhaps because we are no longer attuned with the rising and setting of celestial objects as guideposts for activities or events in our lives, heliacal risings and settings somehow seem antiquated observations suitable only for astronomers. As well, the significant increase in light pollution in our skies since the Industrial Revolution, not to mention the ever-increasing pollution of our night skies with tens of thousands of artificial satellites, has dramatically interfered with our ability and pleasure in watching for the first or last appearance of a celestial object.

While the heliacal risings and settings of some objects can still, upon occasion, be seen, we are, nonetheless, poorer for the loss of the opportunity to observe the night sky as ancient peoples did, in its true, pristine condition.

IN THE SKY THIS WEEK

Venus (mag. -3.9, in Aquarius — the Water Bearer), now the evening star (see heliacal evening setting above), becomes visible 16 degrees above the southwest horizon by about 5:45 p.m. before heading towards the horizon and setting at around 7:30 p.m.

Jupiter (mag. -2.2, in Pisces — the Fish) appears in the southwest sky at about the same time as Venus, but much higher, at 36 degrees above the horizon before it too drops towards the horizon and disappears from view around 9:40 p.m.

Mars (mag. -0.3, in Taurus — the Bull), now slowly moving away from Earth, is visible 59 degrees above the eastern horizon by about 5:55 p.m., reaching its highest elevation of 68 degrees in the southern evening sky around 7:40 p.m. and remaining observable until almost 2:30 a.m., when it drops below nine degrees above the northwest horizon.

Mercury is not readily observable, reaching only four degrees above the southeast horizon by dawn. Likewise, Saturn, heading towards superior solar conjunction (passing behind the sun), is only nine degrees of angular separation from the sun (as seen from Earth) and is not observable.

COMET E3 ZTF

Hopefully, many of you caught a glimpse of Comet E3 ZTF as it passed between Ursa Major and Ursa Minor last week. I viewed the comet from the National Park on P.E.I.’S northern shore on the evening of Jan. 27. With 15x75mm binoculars, I found the comet quite easily where it sat near the bright star Kochab in Ursa Minor.

The comet's coma was extremely diffuse, but the nucleus quite bright and concentrated (using averted vision I could just make it out with my naked eye). Though now fading rapidly as it pulls away from Earth on its voyage back into deep space, the comet will still be visible in binoculars and telescopes for a while yet.

On Feb. 5, the comet, at a predicted mag. +8.3, will sit within the triangle of stars known as The Kids (baby goats) in the constellation of Auriga — the Charioteer, which sits almost directly overhead by 8 p.m. these evenings.

After that date, the comet, still in Auriga, begins to move southward in the night sky, drawing closer to the planet Mars.

On Feb. 10, shining at a predicted mag. +8.8, it will be just two degrees to the upper left of the Red Planet.

On Feb. 12, it leaves Auriga and enters the constellation of Taurus — the Bull, where, at a predicted mag. +9.1, it will appear to the upper left of the bright star Aldebaran. Don't forget to use the The Sky Online Planetarium link to help you locate the comet in the night sky at your particular location.

Until next week, clear skies.

SCIENCE

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2023-02-07T08:00:00.0000000Z

2023-02-07T08:00:00.0000000Z

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