The July setting sun casts its brilliance
over Lake of the Woods, Ontario

The principle of Rayleigh Scattering explains the red or orange sunsets.  As the sun moves lower in the sky, its light has to travel through "more" of the atmosphere since the sun is at a much lower angle.  In fact at the horizons on a level plane, the atmosphere is about 40 times more thick than it is directly overhead.  Particles and molecules in the atmosphere will scatter out the shorter wavelengths (blue in the color spectrum) more easily than longer ones (red).  Thus, we are left with the reddish portion of the light spectrum to see.  

NEWS:

This time of year from late July to mid August are often referred to as "the dog days of summer."  Although the mid and late summer heat can make us all feel lazy, the dog days get their name from Sirius, the "dog star" and not from your sleepy pooch.  Sirius is the brightest visible star in Earth's sky and this time of year, it is quite near the Sun.  The ancient Greeks, Romans and Egyptians thought the heat from the Sun and Sirius combined to produce the year's hottest weather.  Of course, we know now that Sirius is much too far away to have any bearing on Earth's temperature.  Sirius played a critical role in the daily life of ancient Egyptians, including a prediction of sorts of future harvests in the Nile Valley.  As the Nile floods every year depositing fertile soil, a clear and bright Sirius when it first appeared in the early morning would announce an abundant harvest.  Conversely, a red or "dusty" Sirius signaled a poor harvest to come.  Sirius' first appearance in the morning dawn also marked the beginning of the Egyptian year.  Actually, thousands of years ago, Sirius first appeared in the morning sky in early July.  But today, due to the wobble of Earth's axis, the first appearance of Sirius in the pre-dawn hours is closer to late August.

The astronomical page was developed in response to assist the public with inquiries concerning astronomical data.   If you have any questions or comments, contact Mike Fuhs of the Sioux Falls NWS staff. 

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