The heliacal rising of a star is the first occasion on which it is seen briefly in the dawn sky, before the rising of the Sun (Greek hêlios).
From then on, as the Sun progresses away eastward, the star will be each day slightly farther from the Sun and easier to see. It will progress higher into the darker sky.
A star rising at the same time as the Sun, and then at sunrise on later mornings
So the heliacal rising marks the beginning of the star’s annual journey across the night sky. This ends when the star is last seen over the western horizon in the evening after the Sun sets – the star’s heliacal setting.
Heliacal risings are of importance in history, because ancient peoples used them to recognize seasonal moments in the year. Most notably, the heliacal rising of Sirius told Egyptians that the Nile was about to flood; and the heliacal rising of the Pleiades told the Greeks that the sailing season could begin.
The glimpse that brought the flood
Ancient Egyptians watched for the heliacal rising of the brightest star. That star was called by them Sopdet, later rendered in Greek as Sothis; the Greeks called it Seirios, the “scorching,” later Latinized as Sirius.
In the morning twilight of the days of the first half of summer, priests stared at the eastern horizon: the line of highlands along the right bank of the Nile. They had come to know that with or soon after the first glimpse of the star came the first stirring of the water. Sirius’s revealing of itself heralded the Nile’s annual flood.
That flood was vital to Egypt. Indeed it had created Egypt. It laid down the black silt that made the narrow valley floor fertile. Also it filled the channels dug among the fields so that those could continue to be irrigated after the flood had subsided. In years when the flood failed or was weak, the fields were poor and parched. In years when the flood was too strong it tore soil away.
In the cover picture story for Astronomical Calendar 1978 I described the vast hydrological system that governs this flooding of the Nile: the weather that comes from the Atlantic over Africa to the mountains of Ethiopia and sends water at different times and volumes down the rivers that join, south of Egypt, to make the Nile. Also the discovery that the interval between heliacal risings of Sothis was 365 days – the year. And that it became more than that by a quarter of a day over a Sothic cycle of 1460 years, giving rise to the myth of the Phoenix reborn after that time. And the Canicular or Dog Days of heat that the scorching star, in its dog-shaped constellation, also brought. And the slightly lesser Dog Star that precedes the rising of Sirius: Procyon, “fore-dog.” And even the cat that was first domesticated in Egypt’s granaries and worshipped as Bast.
What I did not then try to fix was:
When does a heliacal rising happen?
It’s fairly simple at first approximation: for a given location on the Earth (latitude and longitude), find the first day in the year when, at sunrise, Sirius has also risen – has altitude of zero or more.
Let’s use the year 2000 as the “present” epoch, so as to compare with round numbers of centuries in the past. There would be little difference for 2022. Then, for a representative American location of 40° north and 90° west, Sirius is first above the sunrise horizon on August 2.
As for Egypt: the land is long (stretching from latitude 32° to 22°) and the ancient history was long, from around 3150 BC to 340 BC (the end of the 30th and last native dynasty). Let’s choose 2000 BC, near the beginning of the Middle Kingdom and the 11th dynasty, which moved the capital from Memphis, in the north near the pyramids and modern Cairo, to Thebes, in Upper Egypt, at latitude 26°.
(There were three ancient cities that the Greeks called Thebes, in three continents; Egypt’s “Hundred-Gated Thebes” was the greatest. It was also the capital during the height of Egyptian civilization in the 18th dynasty around 1500 BC.)
In our own time, at Thebes, Sirius heliacally rises on July 22.
Because Thebes is 14° farther south, the celestial equator stands up more steeply. Put another way, the horizon is tilted more down at the right in relation to the Sun, and the occasion when Sun and Sirius reach it simultaneously comes 11 days sooner.
Precession from ancient time
Adjusting for the remote past (or future) is another matter, mainly because of the precession of the equinoxes. This is an almighty phenomenon, “Vaster than empires and more slow,” to quote Andrew Marvell, though he was talking about love, not precession.
Because our rotating Earth wobbles like a spinning top, with a period of 25,800 years, the map of the sky continually changes. What stays (essentially) fixed is the ecliptic, the plane of Earth’s orbit. The celestial equator tilts around in relation to it. The equinox points – where the equator crosses the ecliptic – keep sliding westward along the ecliptic. So do the solstice points, where ecliptic and equator are widest apart. Thus the map position of any star, measured from the vernal or March equinox point, moves by a certain amount each year.
So here is the sky for Thebes, precessed backward four thousand years.
We are in the time of pharaoh Mentuhotep III. Falcon-headed Mentu or Montu was the patron god of Thebes. We might call this the Mentuhotep picture.
There are many things to be pointed out in this picture of an ancient sky.
First, and least important: the year chosen, 2000 BC, is arithmetically -1999, because the year before 1 AD was not 0 but 1 BC. So the span between our 2000 BC and AD pictures is not 4,000 but 3,999. If we choose to talk about “round” years, we’re not talking about an exactly “round” span of years. Never mind.
In these four millennia of precession, the equinox and solstice points have slid almost 56° westward. That is how the celestial equator has re-tilted, like a hula hoop, transforming the relation of everything to the Sun’s position along the ecliptic at a given time in the year.
Sirius, four thousand years ago, came level with the rising Sun – rose heliacally – 19 days earlier in the year, on July 3.
The detail that most clearly moves, between the 2000 BC and AD pictures, is the solstice point, where the Sun is northernmost. In our own time it is between Gemini and Taurus. In 2000 BC it was in Leo – on the Sun’s other side in the picture, below the horizon! The Sun apparently had not yet reached it by the date of the heliacal rising. And yes, I find that back then the “June” solstice was on July 10! (And the “December” solstice, Jan. 5; the “March” and “September” equinoxes, April 7 and October 9.) I’d be happier if we had magister Jean Meeus to confirm this. His 3,000-year list of equinox and solstice dates in his Astronomical Tables starts with the year 1. We carefully speak of the June solstice, since “summer solstice” is not true for the southern hemisphere; if it was anciently in July, we might have to rename it the northern solstice.
As noticeable, or more so because actually visible, is Procyon. In ancient time it was indeed the Fore-Dog, the precursor to Sirius; now, for Thebes, it is below the horizon when Sirius rises.
Proper motion
Precession is the largest but not the only factor in the change of stars’ positions. They also have “proper motion”: real motion in space, relative to our Sun.
You may have noticed in the 2000 BC picture that Sirius is shown in two places. One, the smaller white dot, is used as a point in the drawing of the constellation formlines, and is not shifted for proper motion; the other is. Sirius is one of the nearest stars (8.6 light years away), so its map position changes relatively rapidly. It appears to shift per year 1.2 seconds south and about 0.04 second of right ascension west.
I’ve let this difference between positions with and without proper motion show, for Sirius and some other stars that are bright and relatively near, because it’s interesting to see. (But also, I should confess, because I realized that applying proper motion to the formline stars would require reconstructing a whole catalog!) It does allow you to see that, compared with precession, proper motion makes little difference for the timing of a star’s climb into view.
Hazy horizon
So July 3 was (for Middle Kingdom Egypt) the first morning when Sirius was at or above the eastern horizon at or before the rising of the Sun. But was it seen?
If an Egyptian was looking across the Nile from a mortuary temple on the left bank, the horizon of the Red Sea Hills had a certain height; if he was looking from Luxor or Karnak (the two parts of Thebes) on the right bank, the horizon was nearer and higher. And a shrub or rock or hut on the skyline could happen to be in front of the star!
And the low sky is brighter, reducing contrast with the star. And atmospheric extinction means that the length of the light’s travel through the atmosphere reduces the brightness of a star by 3 magnitudes at altitude 1°; by 2 at 4°; by 1 at 10°.
However, the density of the low atmosphere also refracts the star’s light upward by nearly half a degree if its true altitude is 0°; 0.2° if 4°; 0.1° if 10°.
A dominating factor is elongation: angular distance from the glaring Sun. At sunrise at Thebes on 2000 BC July 3, Sirius was 40° rightward along the horizon from the Sun. That was its difference in azimuth (angular distance around the horizon). Next day, Sirius was not only a bit farther from the Sun but also a bit higher at sunrise; both the azimuthal difference and the altitude difference had increased. The line from Sun to Sirius was not horizontal but sloped up slightly. So this is quite a complex factor: the difficulty of seeing the star changes not only with the length of the line between them but also the angle of that line.
Some of these factors can be different for different times and places, and for stars of different brightness. So it would be tricky to calculate how long after the geometrical heliacal rising comes the observable heliacal rising. We could guess; or find records of sightings; or experiment, by going out and looking. (See Part Three.) Sirius is so piercing that it is sometimes seen down almost to the horizon – its white light split into the many colors that sometimes led to its being called red.
We’ll refer to heliacal risings in the geometrical sense, and guess that the star was actually seen within the next four days.
As those days approached, Egyptian priests perhaps began to watch the eastern sky’s almost-as-bright precursor stars – Aldebaran, Betelgeuse, finally Procyon – knowing roughly how far below Procyon lurked Sirius. But the Sun intervened. Until the morning when the Sun intervened late enough to give time for Sirius to be glimpsed, and the water began to lap higher against the temple steps.
Sudden change?
Since the change in the date of the helical rising is caused mainly by precession, we expect it to be similar to the general rate of precession. And, from July 3 in 2000 BC, the Thebes Sirius date did advance gradually, to July 8 in 1500 AD. Then it seems to have taken a wide jump to the July 22 of our own time.
But that is because of the change of the calendar we use for dates before and after 1582, when October 5 Julian was immediately followed by October 15 Gregorian. The Thebes Sirius heliacal rising was on 1582 July 8 Julian; then 1583 July 19 Gregorian.
If we were to use the Gregorian calendar (closer to the sidereal year) for all dates, instead of only those after 1582, then 2000 BC July 3 would be June 16. (I think.) This explains why the Sun in the picture for 2000 BC, at July 3, has not yet reached its “June” solstice position.
Heliacal setting
Each day after Sirius’s heliacal rising, the Sun has moved on and Sirius rises about 4 minutes sooner, therefore in a darker sky and easier to see. As the months go on, Sirius is rising in full darkness and is seen in the middle of the night sky. Then it is rising after midnight; then in the twilight hours before dusk. There comes the last evening on which Sirius can be seen before sunset.
This occasion, when a star is last above the horizon when the Sun has gone below, is the heliacal setting of the star. The last time the star is actually seen may be some days before it geometrically sets along with the Sun.
The heliacal setting of Sirius is, for modern time and the American location, on May 27.
From then until the August 2 heliacal rising (68 days), Sirius is below the horizon all night.
For Thebes in 2000 BC, the heliacal setting was on May 19 or 20.
You can see that in this instance proper motion could have made the heliacal setting a day later.
The span of daytime-only Sirius was 45 days – shorter because of the angle of the ecliptic: the Sun was more rapidly moving northward.
We can compound our giddiness by positing yet further kinds of event:
The heliacal setting of a star in the west, at or just before the rising of the Sun in the east. And the heliacal rising of a star in the east, just at or after the setting of the Sun in the west!
Elsewhere in sky and vocabulary
We distinguish the geometrical and observed heliacal rising (or setting). Other terms have been used: “true” for the geometrical events; or “cosmical” for the geometrical rising in the morning and setting in the evening, “acronycal” for the geometrical rising in the evening and setting in the morning, leaving “heliacal” for the others… They seem inconsistent, confusing, and unneeded.
Greek hêlios, “Sun”; kosmos, “ornament” or “arrangement” or “that which is perfectly arranged, the universe.” And akro-, “top” or “end”; nyx, “night” – the two limits of the night.
A star has to be within a certain wide band around the sky for it to experience heliacal rising or setting. Circumpolar stars cannot, because they either never set or never rise. If you are at latitude 40° north, stars above declination 50° north are north-circumpolar and never set; stars south of 50° south are south-circumpolar and never rise. But these sets of stars change gradually with precession, because the declination of all the stars changes.
The Pleiades
Whereas Egypt is one long narrow river valley incised across the world’s largest desert, Greece is a salad of peninsulas and islands, and the season to be watched for was the sailing season. The heliacal rising of the Pleiades was the signal that it was safe, or safer, to put out into the Aegean Sea. Plein, “to sail.”
Let’s use Athens, the chief maritime power. For the date, we might use the “golden age” of Athenian democracy, drama, literature, philosophy, and architecture, between 480 and 400 BC.
I thought of choosing 700 BC, around the time Hesiod composed his epic Works and Days. It is a farmers’ manual, preserving lore on, for instance, the seasons at which to plant crops. For that earlier year, the heliacal date would be 3 days earlier. Hesiod lived in Boeotia, an inland region – whose chief city was the other great Thebes. The distance of little more than 30 miles from Athens would make little difference.
What makes more difference is that the Sun passes much closer to the Pleiades (only about 3° north of the ecliptic) than to Sirius (about 40° south of the ecliptic). And the magnitude of the cluster is about 1.6, as against Sirius’s -1.46, meaning that the combined light sent to us by the seven stars is about 16 times weaker; and it is spread out (with low “surface brightness,” like a nebula). The Pleiades, about 400 light-years from us, are the most readily noticed deep-sky object, but much less readily than the one piercing star.
So the delay from the geometrical rising of the Pleiades, at April 18, to the first actual sighting would surely have been longer. And indeed the sailing season is said to have started in May.
And to have lasted till the heliacal setting of the Pleiades in November. No.
The heliacal setting was only a few days later, April 22. In our sky scene, the arrow through the Sun is its motion over 5 days, against the starry background. You can see that it doesn’t take the Sun long to get past the Pleiades, to where they will be level on the sunset horizon.
In May and on through summer, the Pleiades were higher at each dawn; in October they were on the meridian at midnight.
What happened in November was the heliacal rising at sunset, on the horizon opposite from the setting Sun!
The geometrical heliacal rising at sunset was on October 16. Then at each following sunset the Seven Sisters became higher, calling attention to themselves – “Don’t look west at the sunset, here are we over in the east!” – reminding fishers that November weather was coming. The Etesian north-easterlies of summer, that could waft them home, were about to die down.
It needs to be mentioned that translating ancient Greek dates into our calendar is a headache for scholars. Each district or city-state had its own calendar.
More
The subject could ramify on. Apparently the Maori began their year with the heliacal rising of the Pleiades – in June. Good old Allen’s Star Names quotes these bits from Hesiod:
Forget not, when Orion first appears,
To make your servants thresh the sacred ears
And:
When in the rosy morn Arcturus shines,
Then pluck the clusters from the parent vines
In 700 BC, the heliacal rising of Betelgeuse would have been in mid June, or the end of June for the whole figure of Orion; that of Arcturus, in September.
And there are probably many others such references to be found. But enough for now.
Call for observations!
To get a realistic idea of the date when the Egyptians or Greeks first – or last – caught sight of Sirius or the Pleiades, we need to look.
Much the easier observation is of heliacal setting, both because it’s comfortably in the evening, and because we can see it coming. (Rather as watching a star move toward the Moon and be occulted is easier than waiting for it to pop out on the other side.)
Handily, in our time, and in our northern hemisphere, Sirius and the Pleiades go down together into the evening sky of spring (because they are parallel to the northern-spring horizon). And you could start a program of watching them on May 1.
At sunset they are both around 20° above the horizon. Does that put Sirius or, much less likely, the Pleiades far enough out of the glare? Do they become discernible as the minutes pass? – how many minutes, how many degrees do you think their altitude has shrunk to, what are your sky conditions, and what is your location?
Each successive evening they are closer to the Sun. The limiting dates, when they are on the horizon at sunset, are May 21 for the Pleiades and May 28 for Sirius.
To test the heliacal rising dates, you have to be an early riser yourself, as apparently were the Egyptian priests and the Greek sea captains.
The Pleiades, for our time and place, are on the horizon along with the Sun on May 16, Sirius not till August 9.
How many dawns later will each of them actually become visible?
No need to worry about it this Christmas Eve. I aim to get back to the subject before May, with a reminder and more detail about how it may differ for different locations, perhaps other stars. But one more thought:
The star that the wise men saw in the east. Do you think that could have been a heliacal rising, and of what star at what season? Fred Schaaf could probably tell us.