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Most professional observatories are located on mountain peaks high above the thicker part of the atmosphere. According to Merle Walker of the Lick Observatory, the best sites are "peaks near seacoasts having cold ocean currents offshore that reduce the height of the [temperature] inversion layer". The inversion layer can trap clouds and air pollution below it, and prevent them from interfering with the skygazing taking place above.
Most of us are not willing to move family and possessions to the nearest cold ocean current in order to get a better look at the sky. But it will pay off in the long run to devote some thought to where to do your skygazing. While there is no one perfect spot, the criterion for finding a good location are simple and straightforward.
Far and away the most important fact in choosing a site is the absence of light pollution. Light pollution is any form of light, whether it comes from automobile headlights, buildings and most dreaded of all, street lights. The objects in the sky, except for the Moon and Sun, are very dim. The eye can adjust to the small amount of light coming from the sky unless the eye receives a greater amount of light from a bright object at the same moment. It takes at least several minutes for the eye to readjust again to the dim light coming from the sky.
If you live out in the country, just stay away from roads and houses. City dwellers have to search a bit before they can find a suitable location. I live in an apartment building with a small park behind it. A section of the park is unlit. The area is surrounded by two and three-story buildings, so I do not have a clear view of the horizon, but it is a perfect place for a short session of skygazing. Another good city location to check out is the roof. If there is a particularly high building in your neighborhood, ask for permission to use it. The important thing is to stay away from bright street lighting. Unfortunately, there is no way in a large city to rid yourself completely of light pollution. Light emanating from points miles away from you will still affect your view of the sky.
The other major factor besides light pollution is air pollution. Unlike light pollution, there is no way to easily shield yourself against air pollution other than to get far away from it, although even moving yourself a few miles can make a large improvement in your seeing.
Another factor, albeit a non-technical one, is simply convenience. Anybody willing to travel far enough and long enough will eventually find an almost ideal spot for skygazing. A skygazer who enjoys camping has only to find a fairly open area and to give up his campfire that night to have a near-perfect view of the sky. If it seems overdoing it a bit to travel long distances and undergo some hardships to look at the sky, you probably have not yet been fortunate enough to see a really good sky. Next time you are driving through an isolated area, stop the car and walk about a hundred feet away from the road. On a moonless night you will see the absolutely black sky crowded with thousands of stars. Instead of having to search for the Milky Way, it will appear almost creamy across the black background and occasionally the white streak of a shooting star will pass overhead.
Another important thing to remember is safety. Deserted unlit areas can be lovely for skygazing, but in many cities may be quite dangerous. Use your common sense when selecting a site.
Even if you do not find the one perfect spot from which to do your skygazing, after a bit of searching you will probably have a list of good locations. I use the park behind my building for a quick look at the sky almost every night. I have to walk a few minutes if there is something I wish to see close to the southern horizon. When I want to give myself a treat, I take about a twenty-minute drive out of the city to my favorite hilltop.