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Period When The Sun Is Just Below The Horizon

Even on a foggy morning, there is often enough indirect natural light to allow for the majority of outdoor activities to be conducted without artificial illumination. This is the start of civil twilight, the interval between dawn and sunrise. It comes to an end when the Sun's top edge meets the horizon.

Explanation and occurrence dates [adjust] The still widely accepted inaccurate theory asserts that the Earth's post-sunset and pre-sunrise atmosphere absorbs and disperses only the sun's shorter blue wavelengths and scatters the longer, reddish wavelengths, which accounts for the hour's blue tint. [4] Indeed, the blue hour occurs when the Sun is low enough below the horizon that the blue wavelengths of sunlight dominate owing to ozone's Chappuis absorption. [2]

Scaliger discovered that all three cycles coincided in the year 4713 bce, according to the Julian calendar. According to the knowledge he had, he thought this to be a date far in advance of any historical occurrences. As a result, he fixed the start of the first Julian era at January 1, 4713 bce. Although the Julian years are no longer used, the day number is retained in astronomy and in the preparation of calendar tables, since it is the sole record in which days are not combined into weeks or months.

Mars has begun a new cycle of visible activity in our sky. It will brighten and move into our evening sky during 2022, as Earth approaches Mars in our smaller, faster orbit around the sun. On December 1, the Earth and Mars will be at their closest approach. Our two planets will be closest to a line in space on December 7-8, bringing Mars to its once-every-two-years opposition. Saturn

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