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Aug 11

The Dog Days of Summer & Perseids

Twinkle
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Perseids Meteor - Milky Way
Photo by AJZ Photoblog
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"Twinkle, twinkle, little star, How I wonder what you are."

The end of the Dog Days mark the beginning of the Perseids meteor showers, prolific displays of shooting stars associated with the comet Swift–Tuttle. The Perseids are so called because the point from which they appear lies in the constellation Perseus. Meteorites contain different elements which emit different-colored light when they burn. Iron, one of the most common elements found in meteors, glows yellow. Silicates, which contain a form of the element silicon, glow red. A green glow, clearly visible in the trail of this shooting star, indicates the presence of burning copper.

Twinkle twinkle little star ...

 

The Dog Days of summer, generally reckoned to begin July 3rd and ending August 11th, coincides with the rising of the Dog Star, Sirius, in the constellation Canis Major.

Coinciding with the end of the Dog Days is also the beginning of the Perseids Meteor showers.  The Perseids are so called because the point from which they appear to hail (called the radiant) lies in the constellation Perseus.  In the years when these meteor showers coincide with a thin crescent moon, the views can be spectacular - up to 70 to 200 meteors per hour. 

 

Sirius, a binary and possibly ternary star system, is the brightest star in the sky, and is also the star that "twinkles" or scintillates the most in the night sky.

The phrase “Dog Days” conjures up the hottest, most sultry days of summer though this is coincidental to its original meaning.  For the ancient Egyptians, Sirius appeared just before the season of the Nile’s flooding, so the Dog Star was used as a “watchdog” for that event.  Since its rising also coincided with a time of extreme heat, the connection with hot, sultry weather was made for all time:

 

“Dog Days bright and clear

indicate a happy year.

But when accompanied by rain,

for better times our hopes are vain.”

 

Sirius  is roughly 8.5 light years away from Earth, making it one of the closest stars to us.  As seen with the naked eye, Sirius appears to twinkle or shimmer more than other stars.   Because it is very bright, atmospheric effects are amplified, especially when it is nearer the horizon and seen through the denser part of the atmosphere which can be turbulent and contain many different particles and dust.

Additionally, when a star is near the horizon, refraction is strong enough to create images of the star in every color of the rainbow and cast them about in different directions. To our eye, the star looks like a continuous sparkle of varying colored light as split-second variations in moving air pockets make it dance about. 

Similarly, different chemicals in the meteors produce different colors as they burn up while entering the Earth's atmosphere. For example, meteors made from primarily calcium will give off a purple or violet color, while those made out of magnesium will appear to have a green or teal color.

This tartan, by Carol A.L. Martin, represents a twinkling star in a dark night sky.  

Fascinatingly, in ancient times, Sirius was often referred to as a red star, though it is known to be a blue-white star (the color of which would not have changed over the time period of interest).  For an analysis of the Sirius "color change mystery" click the photo of Sirius.