Once in awhile, observers will note that "It’s a good night for nebulas". Obviously, we wouldn’t be out observing deep sky objects unless the sky was clear and dark, but some nights just seem to particularly favor nebulae more than other types of deep sky objects. Without a careful comparison of atmospheric conditions, I couldn’t predict whether it’s going to be a "Nebula Night".

First there’s the matter of seeing color in nebulae. One autumn night about fifteen years ago I was sweeping through Cygnus with an 8-inch reflector at Apple River Canyon State Park. At one point I noted a slight pinkish tint to the background. Was this an artifact of the telescope or my tired eyes? Upon checking the star atlas, I discovered that I was peering into the heart of the North America Nebula. That was the only time I had ever noted any coloring in this object when observing with the 8-inch. Several years later, using my 10-inch scope from the high and dry skies of New Mexico, I just barely detected color in the North America. Another observer using only a 4½-inch reflector reported seeing a pink tinge in M42. He had never seen that before and has never seen it since. He wrote to Sky & Telescope, and they replied categorically that it would be impossible to detect color in nebulae with a telescope that small. It is generally true that any color we’re able to detect in nebulae will be in the green or blue range, but the visual detection of the reddish emission of ionized hydrogen requires a larger scope. But how large?

Then here’s another effect. During the fall several years ago, I was observing the diffuse nebula NGC 281 in Cassiopeia from our site in Hebron. I found it easily with the help of a broadband nebula filter. Oddly, it seemed to be pulsating very slowly – one minute it was obvious, then it would fade, then brighten again. But NGC 281 is not known as the "Great Pulsating Nebula"! I surmised that perhaps this was the result of passing cells in the atmosphere. There were no clouds moving through the field. As a general rule, that night was excellent for nebulae. Could it be that conditions were just on the dividing line between "good" and "great", and that is what caused the pulsating nebula? On that night, Jeff Inman remarked about seeing a definite purplish hue in M42 through his 13½-inch scope. I had also glimpsed the elusive Cocoon Nebula, together with the streamer of dark nebulosity (B168) that terminates at the Cocoon.

As soon as we started observing that night, someone had looked at a nebula and remarked that it promised to be a "good night for nebulas". In general, conditions were typical of a good deep sky night; the planets were unsteady. It was cool (in the 30’s) with only moderate humidity. The winds were light but fairly steady from the southeast. Early in the evening, the skies were perfectly clear, though later a line of broken clouds overran us from the west. Judging by the low level of light pollution, there wasn’t much atmospheric scattering that night. Galaxies were fairly easy to spot, but details in them did not stand out particularly well. However, I've encountered similarly clear nights on which the views of nebulae were downright poor, as related below.

A hint as to what affects the appearance of nebulae may have been offered on a breezy December night in 1998. That particular night was virtually cloudless; however, both nebulae and galaxies showed somewhat poorly. The Flame Nebula adjacent to the star Zeta Orionis was just barely visible. As I observed the Crab Nebula (M1) it seemed to be periodically changing in brightness. The daytime temperature had been about 15 degrees warmer than normal and at the time of observation, it was still around 30^o; on the following morning, the weather report gave the relative humidity as 80%. Aha! Atmospheric cells and high moisture content may have been absorbing the faint light of deep sky objects.

Have you encountered circumstances such as described here? If so, what were the atmospheric conditions? Are there identifiable conditions that make nebulae really stand out well? Is there some phenomenon that lets us see color in nebulae when by all rights we shouldn’t expect to? …Or is this just "averted imagination"? And is there such a thing as a "Nebula Night"?

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Some amateurs specialize in observing nebulae. Those of you with Internet access will find a great web site that's devoted to planetary nebulae. It includes details on the best planetaries and how to observe them. The URL is http://www.blackskies.com.