Wednesday, July 4, 2012

Dog Days of Summer start on Aug. 3, 2012 with heliacal rise of Sirius

If you've ever wondered where the expression "Dog Days of Summer" comes from, check out the Wikipedia page:
The Dog Days originally were the days when Sirius rose just before or at the same time as sunrise (heliacal rising), which is no longer true, owing to precession of the equinoxes. The Romans sacrificed a brown dog at the beginning of the Dog Days to appease the rage of Sirius, believing that the star was the cause of the hot, sultry weather.
As the page notes, the heliacal rise of Sirius no longer corresponds to the beginning of the Dog Days, which were observed between July 24 - August 24 in Ancient Rome. However, that doesn't mean Sirius has ceased to rise at (or just before) dawn in the Eastern sky.

According to my Star Walk app, Sirius will rise with the Sun on August 3, 2012. Over the next week it will rise just before sunrise, making it visible in the light pre-dawn sky.

dog days of summer heliacal rise of sirius 2012
Sirius will rise with the Sun on August 3, 2012

dog days of summer heliacal rise of sirius 2012
Over the next week, Sirius will be visible rising just before the Sun

Stanford.edu provides an animation of the heliacal rise of Sirius alongside the sun. This screenshot shows a very basic idea of what can be expected.

dog days of summer heliacal rise of sirius 2012

Obviously, once the sun is high enough in the sky, even the very bright Sirius will be lost in the light from the Sun.

1 comment:

  1. If Sirius is 13 degrees in Cancer, it will be conjunct the sun on July 5 of any year. Whether you can see it or not it would rise along with the sun on that day.

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