In Search of Steady Air

The seeing quality depends on the weather, but not by simple rules that apply everywhere. Poor seeing does seem more likely shortly before or after a change in the weather, in partial cloudiness, in wind, and in unseasonable cold. Any weather pattern that brings shearing air masses into your sky is bad news. Good seeing is most likely when a high-pressure system settles in to bring clear skies for several days running. Keep a seeing-versus-weather log for your locality, and you may discover correlations that will become your key to sharp viewing.

Seasonal patterns are more predictable. The seeing is often mediocre in the cold months over the northern United States and southern Canada, when the high-altitude jet stream flows above these latitudes. The jet stream being overhead always spells trouble. The very best seeing often comes on still, muggy summer nights when the air is heavy with humidity and the sky looks unpromisingly milky with haze. Some astronomers claim that a blanket of industrial smog steadies the air as effectively as summer humidity — or rather that it results from the same tranquil air masses.

Time of night also plays a role, but again there are few universal rules. Right after sunset the seeing is apt to be excellent, so start your planetary observing as soon as you can find a planet in twilight. The seeing is apt to deteriorate before dusk fades out. Some observers find that their seeing improves after midnight; others say it goes to pieces. This depends largely on local topography; observers in valleys might get worse seeing as the night goes on and cold air flows down to pool in the valley. Just before sunrise may be another excellent time.

For observing the Sun (use an astronomer's solar filter!), the best time is early morning before the Sun heats the landscape. The very worst seeing of the 24-hour daily cycle comes in the afternoon.

Geography is critical. Smooth, laminar airflow is the ideal sought by observatory-siting committees worldwide. The best sites on Earth are mountaintops facing into prevailing winds that have crossed thousands of miles of flat, cool ocean. You don't want to be downwind of a mountain; the airstream breaks up into turbulent swirls after crossing the peak. Nor do you want to be downwind of varied terrain that absorbs solar heat differently from one spot to the next. Flat, uniform plains or gently rolling hills extending far upwind can be almost as good as an ocean for providing laminar airflow. You may learn to predict which wind direction brings the best seeing to your observing site.

One easy countermeasure when observing bright objects such as the Moon and planets is to use a color filter. Different colors seem to shimmer out of phase with each other in the seeing (that's why bright stars twinkle in colors), and in a telescope this contributes to the general fuzzing up. A planet's blue image may line up with its yellow image one instant and separate from it the next. If you isolate just the yellow light, for instance, the planet will often appear to quiet down noticeably — at least when seen through a small-aperture scope.

A color filter is especially useful when you're aiming at altitudes lower than 45° above the horizon. The seeing is always worse at low altitudes in the sky because you're looking through more air. In addition, you face more atmospheric dispersion. This is the smearing out of a celestial image into a short spectrum, with blue on top and red on the bottom. Even as high as 60° up, the far-blue component of an image appears 0.9" (0.9 arcsecond) above the far-red component. The difference is 1.5" at 45°, 2.5" at 30°, and 5" at 15°. Your eye is fairly insensitive to light at the extreme red and blue ends of the spectrum, so dispersion really doesn't look quite as bad as this. Still, filtering out all but one color in a swarm of chromatic aberration will sharpen your view. In the summer of 1994 I found a yellow or orange filter invaluable for following the dark spots on Jupiter caused by the impacts of pieces of Comet Shoemaker-Levy 9; because Jupiter was quite low near the horizon.

Mostly, though, beating the seeing is just a matter of patience. Just keep watching, and intermittent good moments may surprise you. One reason why experienced observers see more detail on the planets than beginners do is that they simply watch longer, ignoring all but the steadiest moments. Moreover, the seeing can change as radically from minute to minute as it does from second to second. When that perfect minute comes along, the dedicated observer is the one most likely to be there at the eyepiece to catch it.