The hot breath of summer greets us each morning as we enter the Dog Days, traditionally the steamiest time of the season. In the northern hemisphere the Dog Days run from mid-July to mid-August.
Our pup Sammy used to dig herself a comfy hollow in the ground behind her kennel to avoid the worst of the heat. I think we all have an image in our brains of dogs sleeping away endless hot afternoons and otherwise avoiding the hot sun. But the term is about another kind of canine entirely — Sirius the Dog Star. The nickname comes from its location in the constellation Canis Major the Greater Dog.
Sirius is the brightest star in the night sky both because it's larger and hotter than the sun but also because it's only 8.7 light-years away. As stars go, that's just down the block. In ancient Egypt circa 3000 BC, the star's return at dawn after its an approximately 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 heliacal rising of a star is when it first becomes visible at dawn near the eastern horizon just before sunrise. Heliacal comes from helios, the Greek word for the sun. In ancient times as now, the heliacal rising of Sirius coincided with the hottest part of the summer.
Because Sirius and the sun appear near one another in the sky at the star's heliacal rising it was thought that Sirius's bright rays contributed to the sultry summer heat, making July and August that much hotter. Nowadays, we know that Sirius can't affect the weather on Earth because it's much too far away. But in ancient Egyptian and Greek times no one knew the distances to the stars, so it seemed plausible that Sirius could bump up the temperature. The name Sirius comes from the ancient Greek Seirios, for sparkling or scorching.
In ancient times, the helical rising of Sirius occurred in early to mid-July, but due to a periodic wobble of Earth's axis called precession, the date has shifted into August. It also varies according to latitude. Sirius rises earlier from Phoenix, where heliacal rise occurs on August 3rd compared to August 16th for Minneapolis, where the star follows a much lower arc along the horizon.
Latitude also plays into Sirius's arc of visibility — the difference in altitude between the sun and the star when you can first spot it with the naked eye. For Sirius, the arc is about 9-10 degrees. Assuming transparent skies, the heliacal rising of the star occurs when the sun lies about 7 degrees below the horizon at the same time Sirius appears 2-3 degrees above the horizon (7 + 2 = 9 degrees).
Sirius may be the most brilliant star, but the atmosphere can take it down a peg. Air, along with humidity and dust, absorbs starlight. The closer a star gets to the horizon the fainter it appears. Standing high, Sirius shines at magnitude -1.5, but at helical rising, when it's just 2 or 3 degrees above the horizon, it's closer to -0.5, a full magnitude fainter. The more transparent the atmosphere, the closer to the horizon you'll see it shine. The ubiquitous wildfire smoke of recent weeks is a concern for many observers. Let's hope it clears soon.
In my experience, to see Sirius at its heliacal rising you need an exceptionally clear sky and an unobstructed view almost down to the southeastern horizon. Go to the Sunrise and Sunset Calculator for your local sunrise time, then get your latitude at LatLong.net. The list below will show the helical rise date for your location.
The hour will be early, but watching the return of Sirius at dawn makes a great little naked-eye observing project. It also connects us to a time in the distant past when our ancestors routinely used stars to predict important cultural and religious events.
Plan to be out about one hour before sunrise. If you don't see Sirius on the first try, go out the next clear morning. Since stars rise four minutes earlier and climb approximately 1 degree higher with each passing day, Sirius will quickly become more accessible.
Sirius heliacal rising dates by latitude
32° North — 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
The ancients might balk, but I don't see anything wrong with using a pair of binoculars. With them you can sweep the horizon for the star. If you live in the southern U.S., start looking about 30 minutes before sunrise on your heliacal rise date, and about 35-40 minutes before sunup if you're in the central and northern part of the country. Search about five fists (~50 degrees) to the right of the bright spot on the horizon where the sun will eventually rise. Once you've found Sirius, lower the binoculars and try to see it without optical aid.
Good luck! I hope you'll share your observations on my Facebook page, Astro Bob's Astronomy for Everyone. I'd love to hear your results.
** Note: Data for the table is from Sky & Telescope's David Dickinson and Ed Kotapish.
"Astro" Bob King is a freelance writer for the Duluth News Tribune. Read more of his work at duluthnewstribune.com/astrobob.