From Astronomy Picture of the Day
Fix your camera to a tripod, lock the shutter open, and you can make an image of star trails – graceful concentric arcs traced by the stars as planet Earth rotates on its axis.
Of course, the length of the star trails will depend on the exposure time.
While exposures lasting just five minutes produce a significant arc, in about 12 hours a given star would trace out half a circle.
But in any long exposure, the background glow from light-polluted skies can build up to wash out the trails.
Still, astronomer Josch Hambsch produced this
stunning composite of star trails around the South Celestial Pole
with an effective “all night” exposure time of almost 11 hours.
To do it, he combined 128 consecutive five minute long digital exposures recorded in very dark night skies above Namibia.
In his final image, the background glow on the right is due in part to the faint, arcing Milky Way.