Star trails (17 min exposure time)
I hiked to the top of mount Tamalpais with a friend after midnight to see the Perseid meteor shower. There was a nice shooting-star show around 1am, but unfortunately the meteorites were too dim and too far apart to show on this photo. I just captured nice star trails.
Star trails are caused by the apparent motion of the stars due to the rotation of the earth around the north-south axis. The complete circle (one full rotation of the earth) is would take about 23h 56min, but during half this time, the stars are invisible because of day-light. Note that in 24h, earth rotation is more than 360 degres (in fact it's about 361 degres), because the earth also moves on an orbit around the sun, so it advances on its orbit during one day and needs to rotate more than a full circle to get the same meridian aligned again with the sun. So in the 24h period, stars apparent motion is a bit more than a full circle, which is why different constellations are visible at different seasons.
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