Wednesday, August 10, 2016

A Real Scorcher! — Sirius At Heliacal Rising - Sky & Telescope

A Real Scorcher! — Sirius At Heliacal Rising

Make a connection to a time when stars were used to track seasons and predict natural events by watching the heliacal rising of Sirius. 

Sopdet, the ancient Egyptian personification of the star Sirius. She wore a star on her head.
Jeff Dahl / Wikimedia Commons / CC BY-SA 4.0

Heat, humidity, the searing Sun. Must be the Dog Days of summer. This hottest time of year runs from mid-July to mid-August in the northern hemisphere. Despite the image of dogs sleeping off the heat during endless afternoons, the canine association has naught to do with pups and all to do with the heliacal rising of the star Sirius.

Sirius marks the nose of Canis Major, the Greater Dog. That much I'll concede. In ancient Egypt circa 3000 BC, the star's return at dawn after its ~70 day hiatus in the daytime sky coincided with the flooding of the Nile River, the lifeline of Egypt then as today. Floods deposited precious silt that fertilized the farmlands along the river.

The first sighting of Sirius and its association with the rebirth of the Nile was so important that its heliacal rising marked the start of the Egyptian calendar year. Heliacal relates to the star's proximity to the Sun (Helios in Greek). At the time, Sirius made its first appearance in early July as seen from the ancient capital of Memphis, but due to the precession of the equinoxes, the star now rises into view in early August.

Our association of the star with the Dog Days comes from the ancient Greeks who, just like us, were impressed with the star's great brightness. At magnitude –1.46, it's almost twice as bright as the next brightest, Canopus. They called it Seirios, Greek for "sparking" (referring to its near-constant twinkling), "fiery," or "scorching," and associated its heliacal rising with hot weather common in the Mediterranean region during late summer.

Ancient Greeks blamed Sirius in part for the intensity of the summer heat because they believed its fiery, scorching nature added to that of the Sun. After all, the two stars rose at nearly the same time and occupied the same quadrant of the sky in summer.

This map shows the sky (as seen when facing east) on July 15, 3000 BC from the ancient city of Memphis (near Cairo) in Egypt. Sirius stands 3° high 32 minutes before sunrise around the time of its heliacal rising.

"Dog Days" translates directly from what your average Roman would have said while mopping sweat from his brow:  "Dies caniculariae iterum!" or "The canicular (dog) days again!" Romans believed the heat made dogs rabid. Mine just slinks from the Sun and sleeps in a cool cavity in the earth she excavated behind the kennel.

The heliacal rising of Sirius was a key calendar marker in ancient times. Sirius is visible near the bottom of the frame.
Bob King

Witnessing the heliacal rising of Sirius depends upon several variables: latitude (tropical latitudes favored due to Sirius's southern declination), atmospheric transparency, and something called the arc of visibility or "arcus visionis." The arc of visibility is the difference in altitude between the star and the Sun at the moment when the star is observed at the horizon without consideration for refraction. For Sirius, the arc comes to about 11°. Assuming transparent skies and a minimum altitude of about 3° for sighting Sirius, the heliacal rising of the star would occur with the Sun about 8° below the horizon.

For current day Cairo at latitude +30° N, this occurs on the morning of August 3rd. Under exceptional skies, you might catch sight of the star at 2° altitude, shifting the date of first sighting to August 2nd. Since stars rise four minutes earlier and climb approximately 1° higher with each passing day, Sirius quickly becomes more accessible after its heliacal rising.

The further north you go, the farther Sirius slides to the south and the longer observers have to wait to see their own personal heliacal rising of the star. Like to give it a try? Much of Europe, the U.S., Canada, and Asia still await the Dog Star's appearance at dawn. I plan an outing on the 17th when the star rises for my latitude.

To find out when Sirius eases over the horizon at your house, look up your latitude and then use the table below to get the heliacal rise date. A perfect horizon isn't required, but you'll obviously need to dig way down in the southeastern sky. Plan to be out about 45 minutes before your local sunrise.

Sirius stands about 3° above the southeastern horizon during its heliacal rising in Boston, Massachusetts. (latitude 42° N) on August 13th. The star will be about 50° to the right (south) of the bright spot near the horizon in the sunrise direction.
Map: Bob King; Source: Stellarium

While the first sighting of Sirius may not signify anything as momentous as the annual flooding of the Nile, seeing it tenderly twinkling at dawn can take us back in time to when it was commonplace for people to use stars to mark important events in their lives. How far we've strayed.

Latitude north Approximate date of heliacal rising
32° August 3
33° August 4
34° August 5
35° August 6
36° August 7
37° August 8
38° August 9
39° August 10
40° August 11
41° August 12
42° August 13
43° August 14
44° August 15
45° August 16
46° August 17
47° August 18
48° August 19
49° August 20
50° August 21

* I want to thank Sky & Telescope's David Dickinson and Ed Kotapish for the data used in the table above.

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About Bob King

Amateur astronomer since childhood and long-time member of the American Association of Variable Star Observers (AAVSO), Bob King also teaches community education astronomy and writes the blog Astro Bob. The universe invites us on an adventure every single night. All we need do is look up. Check out my forthcoming book "Night Sky with the Naked Eye" (on Amazon and BN) about all the great things you can see at night without any special equipment.