Heliacal Rising of Sirius
Robert Tulip
The day each year that Sirius is first visible in the pre-dawn sky, known as its heliacal rising, was used in ancient Egypt to mark the annual cycle of the heavens. In historical times, the heliacal rising is near the summer solstice. Sirius is invisible below the Egyptian horizon for about 70 days each year since its setting with the sun after the spring equinox. But these dates change, and the period of invisibility used to be much longer. This paper examines the change in the location and date of the heliacal rising of Sirius caused by precession of the equinox. The diagrams were produced using astronomy software SkyGazer 4.5. They show the approximate moment of dawn, with the sun at the left on the horizon, on the approximate day on which Sirius is first visible again on the south-eastern horizon. There is some variance between the diagrams which could be made more precise by using the exact moment when both the sun and Sirius are on the horizon, but this small error does not affect the overall result, which is to show how the heliacal rising point of Sirius, as viewed from Cairo, oscillates between the south and south-east horizon.

The Egyptians used the heliacal rising of Sirius as a marker for the annual rising of the Nile. Due to their use of a 365 day calendar, comprising twelve months, each with three x ten day weeks, plus five epagomenal days, the heliacal rise of Sirius drifted around their year over a 1461 year period known as the Sothic Cycle. Egyptian knowledge of this Sothic Cycle shows that they understood the period of the tropical year as 365.25 days.

Sirius 12000BC Heliacal Rising Cairo.gif 51.11 KiB

In the year 12,000 BC, a date well before historical records, this diagram shows the southeast horizon from Cairo, and all locations at the same latitude of 30° north, at just before dawn on 25 June. Here we see that Sirius, the brightest star in the diagram, crossed the meridian (the vertical green line), the most northerly point for all stars in their daily cycle, just below the horizon (the curved green line), and so was then never visible at that latitude over the course of the year. This diagram also shows the spring equinox point of the sun, marked by the meeting point of the celestial equator (white) and the ecliptic, the path of the sun along the zodiac (yellow). The equinoctial point was then in the constellation of Virgo, opposite its current position in Pisces. This diagram also shows the bright star Canopus, half way between the South Celestial Pole (the axis of the earth) and the South Ecliptic Pole (the axis of the sun). Over the slow 25,765 year period of precession of the equinox, Canopus and Sirius inscribe a circle around the South Ecliptic Pole. Apart from proper motion, the relative movement of stars in the galaxy, these stars have inscribed the same path for millions, even billions, of years. Each day all the stars appear to rotate around the celestial poles. In circling the pole, Sirius never rose above the horizon at Cairo in 12,000 BC. Over the slow sweep of the succeeding millennia, first Sirius, and then its southern bright companion Canopus, gradually moved into view from Egypt.

Sirius 11000BC Heliacal Rising Cairo.gif 24.07 KiB

In 11,000 BC, the date of the highest northerly position of Sirius at dawn had moved back several weeks to 17 July. We see that Sirius was slightly closer to the horizon, but was still invisible at Cairo. The spring point, where the ecliptic crosses the equator, has precessed through the constellation of Virgo.

Sirius 10000BC Heliacal Rising Cairo.gif [ 25.02 KiB
In 10,000 BC, Sirius reached the horizon from Cairo just east of due south on 25 July, so barely came into view. The path of each star across the sky forms a circle around the South Celestial Pole, so in this picture, Sirius rose just above the horizon for a few days each year. Interestingly, this epoch of the first visibility of Sirius from Cairo is sometimes coincidentally referred to as ‘the first time’ or Zep Tepi, in Egyptian myth, and is generally marked by the fact that the spring point was in the constellation of Leo. I am not aware of previous study of the movement of Sirius in this way though that would associate it with the Zep Tepi myth.

By 9000 BC, the heliacal rising point of Sirius had moved to 165° east of north, still at about 25 July, and the spring point was in Leo. The millennial movement of the heliacal rising point now starts to accelerate eastward.

Sirius 9000BC Heliacal Rising Cairo.gif 25.35 KiB

By 8000 BC, Sirius rose at about 155° east on 12 July, while the equinox point had moved to between Leo and Cancer. Here, and above, we can also see Sirius’s neighbour, Orion, in its northward climb towards its current position on the celestial equator. The movement east of the Sirius heliacal rising point is also a move towards the celestial equator, which always crosses the horizon at 90° from north.

Sirius 8000BC Heliacal Rising Cairo.gif 26.85 KiB

In 7000 BC, Sirius rose on 10 July at 145° east, while the equinox point was in Cancer.

Sirius 7000BC Heliacal Rising Cairo.gif 26.45 KiB

Continuing, we see that by 7000BC the heliacal rising point of Sirius had moved nearly halfway along the horizon from due south, where it first only appeared on one day of the year, to its present position close to southeast, where it is visible from Cairo for all the year except for 70 days.

In 6000 BC, the rising point had moved to southeast on 12 July, making Sirius readily visible for several months low in the southern sky. The equinox had reached Gemini.

Sirius 6000BC Heliacal Rising Cairo.gif 27.23 KiB

In 5000 BC, Sirius rises closer to east than south on about 8 July, and its companion Orion is about half way between the horizon and the equator. The equinox point is in Gemini.

Sirius 5000BC Heliacal Rising Cairo.gif 24.77 KiB

In 4000 BC, approaching historical times, Sirius is now rising at 125° east on 6 July, as the equinox occurs in Taurus.

Sirius 4000BC Heliacal Rising Cairo.gif 28.52 KiB

3000 BC marks the approximate beginning of the dynastic age in Egypt. The position of the heliacal rising of Sirius has shifted only 5° in the last thousand years, compared to about a 15° degree shift over a millennium around 9000 BC. and the date has stayed about the same at 6 July. Sirius is moving towards its point of greatest distance from the south celestial pole, in its slow orbit around the south ecliptic pole over the course of the Great Year. The rest of the stars continue to precess, with the equinox now in the middle of Taurus, and Orion even closer to the equator.

Sirius 3000BC Heliacal Rising Cairo.gif 29.32 KiB

In 2000 BC, at the height of Egyptian civilization, Sirius has barely shifted its rising point and date over the last millennium, moving only by about 3° and one day. Norman Lockyer, founder of the journal Nature, and co-discoverer of helium, made intensive study of how the Egyptians aligned their temples to the heliacal rising of Sirius. It appears that after many centuries when alignments no longer worked, new temples were built on old foundations, shifting the axis slightly to put them in line with the rising point of Sirius. The equinox now occurs in Aries.
Sirius 2000BC Heliacal Rising Cairo.gif 28.03 KiB

By 1000 BC, still during the glory days of Egypt, the rising point of Sirius had shifted a further 5° north, and the equinox was in the middle of Aries. Orion is now astride the equator, with Betelgeuse having moved for the first time into the northern hemisphere.

Sirius 1000BC Heliacal Rising Cairo.gif 28.18 KiB

At the time of Christ, the moment still marking the turning point of our calendar, the great civilization of Egypt lay in ruins beneath the swords of the marauding hordes of Assyria, Babylon, Greece and Rome. The rising point of Sirius has barely moved over this time of upheaval, sitting at 110° east. The equinox point is now crossing the first fish of the constellation Pisces.

Sirius 1BC Heliacal Rising Cairo.gif 27.75 KiB

By 1000 AD, in the middle of the Dark Ages, observation of the stars had largely been forgotten except by the Arabs. The equinox point is now in the middle of the constellation Pisces. Sirius rises slightly later, on 10 July, at almost exactly the same place on the horizon where it rose at the time of Christ.

Sirius 1000AD Heliacal Rising Cairo.gif 30.61 KiB

The current situation, shown in this diagram from 2000 AD, shows the equinox nearly at the end of Pisces and just about to enter Aquarius. Sirius still rises close to the same point it has risen for the last 2000 years. Orion’s Belt has reached the celestial equator, near to its most northerly position in about 2300 AD.

Sirius 2000AD Heliacal Rising Cairo.gif 31.2 KiB

Over the next twelve or so thousand years, as shown in the remaining diagrams below for 3000 AD, 10,000 AD and 15.000 AD, we see the rising position of Sirius reverse its northerly motion, returning to invisibility from Cairo again at the end of one cycle of precession, the 25765 year-long Great Year of precession of the equinox caused by the spin wobble of the earth.

3000 AD: After its heliacal stasis over the Christian Aion, Sirius has begun its movement back to the south. The equinox is in the middle of Aquarius.

Sirius 3000AD Heliacal Rising Cairo.gif 31.47 KiB

10,000 AD: The heliacal movement over the Great Year cycle accelerates at its mid point. The September equinox is now shown, in Taurus, opposite the position of the March equinox in Scorpio.

Sirius 10000AD Heliacal Rising Cairo.gif 26.85 KiB

The year 15,000 Anno Domini: Sirius is back at the point where it becomes invisible from Cairo, with the March equinox shown once again in Virgo, and about to enter Leo, its position in the old Egyptian Zep Tepi. Sirius has returned to the heliacal point where it first appeared visible from the latitude of Cairo over the long sweep of the Holocene and the Anthropocene. However, the proper motion of Sirius, it actual shift relative to other stars, is among the fastest of all stars because it is so close to earth, only eight light years away. It appears from this diagram that over the next ten thousand years Sirius will shift to a more northerly actual location, so will not again become invisible from the north for as long as it used to. I hope humans will still be around to see it.

Sirius 15000AD Heliacal Rising Cairo.gif 23.69 KiB