A year of San Francisco sky

Artist Ken Murphy created this interesting video by installing a camera on the roof of the Exploratorium Museum in San Francisco. It took a picture every ten seconds for the full year, starting in July 2009.  The upper left corner shows the sky on July 28, 2009, and then each successive day is in the next rectangle over all the way to July 2010.

Notice that in the summer day frames (top and bottom rows), the days are longer because the Sun rises earlier and sets later.  The winter days are in the middle rows, and the Sun rises later and sets earlier.



The difference in the Sun's rising and setting times throughout the year is due to the fact that the Earth's axis of rotation is tilted approximately -23.4 degrees from the 90 degrees from the imaginary plane of its orbit around the Sun (its ecliptic).  This causes the northern hemisphere to be tilted slightly towards the Sun at the end of March-September and away from the Sun at the end of September-March. 

Because of this tilt, each spot in the northern hemisphere either receives slightly more time on the "day-side" of the Earth, or less time.  As the Earth continues around its in its yearly orbit, this pattern flips so that they southern hemisphere no receives slightly more time on the "day-side".  The increase of Sun-time and the more direct angle the Sun's light reaches the Earth's surface in the northern hemisphere are the reason we have seasons here on Earth (not really the Earth's proximity to the Sun).

Video from NASA's METEOSAT-9 meteorological satellite showing the shifting of Earth's day-night line as the Earth orbits the Sun.



Around 6 a.m. local time each day, the Sun, Earth, and any geosynchronous satellite form a right angle to the day-night line (the edge between the shadows of nightfall and the sunlight of dusk and dawn).

[NASA Source]