Heliacal Rising Calculator — Arc of Vision, Sirius and Suhail

What a heliacal rising is

A heliacal rising is the first dawn a body is seen again in the eastern glow before sunrise. For weeks the Sun has outshone it. Then the geometry turns, and the body rises early enough to stand in a darkening sky while the Sun is still below.

The twin event is the heliacal setting: the last dusk a body is seen in the west after sunset, before it slips into the Sun's beams for good. Between the two, the body is said to be hidden.

Two heights set the date, not one. The body must be high enough to survive the glare, and the Sun must be low enough to let it through. So the real measure is the gap between them. That gap is the arc of vision.

The date depends on your horizon. Sirius returns on 5 August 2026 over Cairo, on 14 August over Philadelphia, on 27 August over London and on 15 September over Reykjavik. From Longyearbyen it never comes back at all, because it does not rise there.

The arc of vision, and the value we use

Rita Gautschy sets the term plainly in her study of the rising of Sirius. The arc is the altitude of the body minus the altitude of the Sun, both read at the same instant. She calls a realistic first sighting one made at an apparent height of 2 to 3 degrees, and she varies her arc between 8 and 11 degrees.

Her worked example is exact: with the body 2 degrees up and an arc of 8, every Sun altitude between −6.0 and −7.0 degrees meets the rule. Our model gives −6.05 degrees for that case over Philadelphia, and −8.64 degrees at an arc of 10. Same band, same arithmetic.

So this tool asks the body to stand 2 degrees high by default, and lets you slide the arc from 8 to 12. Positions come from the ephemeris, and the reported altitude already carries refraction, which lifts a body near the horizon by about a third of a degree.

One more rule keeps the answer honest. A dawn event must sit in the pre-sunrise half of its civil day, and a dusk event in the post-sunset half. A star that merely arrives in bright daylight is not appearing before the Sun, so it is not published as an event.

Four appearances, not two

Between one absence and the next, a body crosses the Sun's light twice, and each crossing has a name:

• Heliacal rising: the first pre-dawn sight, as the body comes out of its absence. A dawn event.
• Evening first appearance: the first dusk sight, as the body leaves the Sun's glow on the evening side.
• Heliacal setting: the last dusk sight, after which the body is lost in the beams.
• Morning last appearance: the final pre-dawn glimpse before that loss.
• Cosmical rising: the same morning, but with no atmosphere at all — the body's centre on the horizon while the Sun's centre is still down. It lands earlier than the heliacal date, and Gautschy's sources count it as the phase the ancients often meant.

Sirius over Cairo in 2026

Take the most famous case in the tradition. Over Cairo our model hides Sirius from 27 May, when it sets 8.4 degrees under the Sun's glow, until the first August dawn it survives.

At an arc of 10 the rising falls on 5 August 2026 at 02:35 UT, which is 05:35 local. The Sun stands at −8.45 degrees, Sirius at 2, and the gap reads 10.6. The absence runs 69 days. Loosen the arc to 8 and it returns on 3 August after 65 days; tighten it to 12 and it waits until 7 August, 73 days.

Those July and August dates match the published range for the Egyptian sites, and 69 days sits on the classical seventy-day phase. Gautschy also notes the calendar problem: the Egyptian civil year ran 365 days against a solar year near 365¼, so a fixed date slid one day in four, and around the sky in roughly 1,460 years.

One caution about the story. Sirius lies some 39 degrees off the ecliptic, so the Sun never passes in front of it. At its Cairo rising the two centres sit 47 degrees apart. What buries the star is twilight, not overlap.

Suhail over Arabia: a live 2026 test

Canopus is Suhail in Arabian tradition, and its return is still a seasonal event. The Saudi Gazette reported on 16 August 2026 that its astronomical rising over the Northern Borders begins on 26 August, at about 5:51 a.m., some one minute before sunrise at 5:52. The writer M. E. Al-Huzaimi added that the star cannot be seen then, and reaches the naked eye during the first third of September, climbing higher above the southern horizon before dawn.

Run the same sky here. Over Rafha the star reaches its atmosphere-free rising on 28 August at 05:47 local, and it clears a 2-degree height in the pre-dawn glow on 3 to 7 September, depending on the arc you set. At an arc of 10 that is 5 September, 05:16 local, low in the south-east at an azimuth of 158.7 degrees.

Move 140 kilometres north to Arar, the dateline city of that same report, and the model puts the cosmical rising on 1 September at 05:50 local — one minute ahead of sunrise there, just as the article describes, with the naked-eye return on 9 September. Four days of date for 1.4 degrees of latitude. That is the whole lesson of this tool: the sky answers where you stand.

Canopus is a hard case, and that makes it a good one. Its best moment over Rafha is only 7.7 degrees above the southern horizon, so it never climbs far out of the glow.

What the report returns

Pick a target, a place and a year, and the panel prints:

• Every appearance and disappearance of that body in that year, to the second in Universal Time, with the local clock of your place beside it.
• The Sun's altitude, the body's altitude, the arc of vision and the azimuth of the event, so you can check each date by hand.
• The length of the hidden phase, and the cosmical date that ignores the atmosphere.
• A sensitivity table across five arcs, which shows how much of the date is convention.
• The best altitude the body ever reaches over your horizon, which is why some targets report nothing at all.
• A live line for tonight: the arc your sky is offering right now, against the arc you asked for.

How to use the result

Start with your own horizon, not a book's. Search your birth town or your current city and read the date for that latitude. Then compare it with the printed station presets, which are the places the literature argues from.

Read the event's zodiac position next. Each line lists where the body stands at its return, so you can see whether it falls on a natal point or in a natal house. That is the honest way to bring a sky-wide fact into a personal chart.

Use the pair, not one date. A rising and the setting before it bracket a season, and the bracket is the useful part. Arabia's Suhail season runs 52 days in four stations of thirteen, and the Egyptian phase of Sirius runs some 69.

Treat the symbolic layer as story. Return dates mark a season that everyone under that latitude shares. They do not cause the events they are said to herald.

What this tool does not claim

The date is a range, not a point. Across the published arcs of 8 to 12 degrees, the Cairo return of Sirius moves over five days and the Suhail return over four. Anyone quoting one date is quoting one convention.

Latitude can cancel the question. Canopus never clears the horizon at Philadelphia, where its best altitude is −2.7 degrees, nor at London at −14.2. North of roughly 37.5 degrees it does not rise. That limit is arithmetic: ninety degrees minus the gap between your latitude and the star's declination.

Latitude can also make the event meaningless. Over Sydney Sirius stands 72.9 degrees up at its best and is never lost in the Sun's beams, so it has no true heliacal cycle there. The panel says so instead of inventing a date.

Some years report one direction only. Mars has a heliacal rising on 1 May 2026 over Philadelphia and no setting inside the scanned span, because that disappearance fell in 2025. A bright winter star can also show a rising in October and a setting in November, which puts its absence in the other half of the year.

The sky is not the only variable. Real seeing needs a flat, clear horizon, no haze and no Moon. We model an arc, not weather, and a thin crescent of Venus low in twilight glare may stay hidden for days past its computed date.

Ancient dates need calendar care. Before 1582 the Julian calendar applied, and precession moves the stars against the seasons, so a date quoted for Memphis or Babylon is not the same date for the same star today.

Our method in one line

Positions come from sub-arcsecond astronomy (VSOP87/ELP via astronomy-engine), reduced to your horizon with refraction and the observer's place taken into account. Each appearance is located by refinement to the second, not read off a daily table, and one scan of 503 days feeds every arc in the sensitivity table.

Every figure on this page is reproduced by the Batch E-3 engine probes, including the arc arithmetic, the Cairo band, the Rafha and Arar Suhail dates and the latitude limits. Reviewed by Dr. Amara Osei, Astronomy Reviewer.