July 24, 2014

Astro Event of the Week: The Heliacal Rising of Sirius.

Sirius Rising as seen from Astroguyz HQ on August 1st, at Lat. 28 North...Radio Telescopes pending! (Created in Starry night).

Sirius Rising as seen from Astroguyz HQ on August 1st, at Lat. 28 North...Radio Telescopes pending! (Created in Starry night).

 

   August is traditionally a real roaster for the northern hemisphere. This month usually sees the onset of what’s known as the Dog Days of summer… but did you know that this term has an actual astronomical tie-in? We’re talking about the dawn appearance of the Dog Star, Sirius.  The ancient Egyptians knew this star as the dog-headed god Anubis, and the first sightings of Sirius preceded that most important of calendar dates in their world; the flooding of the life-giving Nile. This generally occurred around the July time frame, as upper to lower Egypt spans between latitude 25°-30° degrees north. Our friend, the Precession of the Equinoxes has also played a role, moving this astronomical event firmly up into August in modern times. Recently, reader and calculator of all things astronomical Ed Kotapish sent us a ephemeris of dates for this simultaneous heliacal rising (see below) of the Sirius by longitude;

Latitude

HR Date

50

8/21/2010

49

8/20/2010

48

8/19/2010

47

8/18/2010

46

8/17/2010

45

8/16/2010

44

8/15/2010

43

8/14/2010

42

8/13/2010

41

8/12/2010

40

8/11/2010

39

8/10/2010

38

8/9/2010

37

8/8/2010

36

8/7/2010

35

8/6/2010

34

8/5/2010

33

8/4/2010

32

8/3/2010

31

8/2/2010

30

8/2/2010

29

8/1/2010

 

This covers most heavily populated blog reading northerly latitudes… if you are reading this from the Honduran jungle, drop us a line and we’ll be happy to handcraft a table, just for you. An interesting effect to note is that Sirius may appear a tiny bit before the date above because of atmospheric refraction; it may appear later, however, because of ground clutter. Elevation or an eastward facing horizon out to sea or over a large lake may help in this astronomical endeavor. This is a fun, and above all, naked eye challenge, although a good pair of binoculars might help sweeping the horizon…or do you want to keep the challenge pre-optical, just like the Egyptians?  Be sure to give this ancient astronomical sighting a try; now’s a good time to set those Sothic calendars!  

This weeks’ astro-term is Heliacal Rising.This is simply the date that an astronomical object rises simultaneously with the Sun from a given vantage point on the Earth’s surface. This varies dramatically with latitude, and the brighter the object, the more likely you are to spot it during heliacal rising. Fixed stars gain about one degree per day on the Sun relative to its solar longitude along the ecliptic, which traces out the plane of the Earth’s orbit about the Sun. Heliacal risings were a good way for ancient societies to pin down the start date of a solar calendar, and Sirius was the brightest star with which to do it.

Comments

  1. brobof says:

    “The ancient Egyptians knew this star as the dog-headed god Anubis”
    Plain wrong.
    Sirius was associated wit the Goddess Sopdet Sothis (Gr) and also latterly as Isis. Sopdet & Sah equated to Isis and Osiris. The linkage, such as it is, between Sopdet and Anubis is tenuous and Greek!
    http://en.wikipedia.org/wiki/Sopdet
    http://www.thekeep.org/~kunoichi/kunoichi/themestream/sopdet.html

  2. Jeff Nisbet says:

    Is there an online source that would tell me the date of the heliacal rising of Sirius at various locations around the world on any date in the last few hundred years?

    Many Thanks,

    Jeff

  3. David Dickinson says:

    I’ve never seen a dedicated online resource for calculating helical risings of Sirius versus latitude, but the original resource for this is J. Meeus’ book Mathematical Morsels… perhaps this signifies an online resource awaiting construction.

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  1. [...] This post was mentioned on Twitter by tavianne, David Dickinson. David Dickinson said: http://bit.ly/9GiKTO Astro Event of the Week:The Helical Rising of Sirius. A chance to duplicate an annual historical sighting. [...]

  2. [...] Astro Event of the Week: The Helical Rising of Sirius. : Astro Guyz [...]

  3. [...] of dog days, at least in the Northern Hemisphere, are those 30 to 40 days surrounding the heliacal rising of Sirius, the Dog [...]

  4. [...] week of August to try that feat that the Ancient Egyptians depended on, with a sighting of the dawn heliacal rising of the bright star, Sirius. The first sighting of Sirius in the dawn sky marked an all-important [...]

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