There is also a natural inclination to relate the horizon to the foreground objects on Earth, which you can see are quite distant. The effect is caused in part by our lifetime visual experience with clouds. Notice that the overhead sky seems squashed, much closer to you than the areas near the horizon. Does it? You don't need to wait until dark to check it out any cloudless day will do. The universe stretches for billions of light-years in every direction, so the sky should look like a perfect hemisphere, as distant overhead as it is at the horizon.
It looks like a hole only because human vision tends to see dark patches as gaps. This giant nebula contains roughly 1 million times the mass of the sun. The stars of the Milky Way are packed as tightly there as anywhere else, but vast dark clouds of dusty hydrogen block their light. Noted since antiquity, this gap is where the mythological "river of milk" divides into two parallel streams. Nearly overhead you will see the Great Rift, a section where the light of our galaxy is strangely missing. Take a look at the ghostly band of the Milky Way, optimally situated in the sky this month. If you, like most casual stargazers, go out to look when the sky turns dark, the constellations seem to slide westward at half their average rate. New stars still rise four minutes earlier, but sunset occurs earlier too. During late summer and the first several weeks of autumn, the heavens seem nearly frozen in place. Nightfall therefore brings a slightly different set of stars into view from one day to the next-but not now. This movement, caused by Earth's orbit around the sun, means that stars rise four minutes earlier each day, or two hours earlier each month: A constellation that rises at 9 p.m. Over the course of a year, constellations appear to shift steadily to the west.
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