The moon is at new phase on Sept. 21 and during this upcoming week will appear first as a gradually diminishing crescent of light in the predawn hours, and during next week as a slender sliver of light in the early evening skies, and won’t be much of a hindrance at all to stargazing. This combined with the fact that at this particular time of the year the hazy skies of summer are giving way to clearer skies and cooler overnight temperatures.
This means that this is an optimum week to check out the beautiful summer Milky Way. As soon as darkness falls, it becomes evident as a wide glowing arch of variety and beauty, stretching across the sky from the northeast to southwest.
Sweep with binoculars from the Scorpion’s tail in the Scorpius constellation, through the Summer Triangle, and then down to Cassiopeia and Perseus. You’ll find concentrations of stars, clusters, large apparent gaps (such as the “Great Rift” in Cygnus), and more stars than you probably thought existed.
City dwellers always miss out
Unfortunately, because of the tremendous increase in light pollution over the past 50-years, the majority of our current generation has never really seen the night sky in all its grandeur. Indeed, the Milky Way has been one of the chief victims of atmospheric pollution — by light and other factors. In most major metropolitan areas, there is very little hope of ever seeing this broad path of light at all (save for a major power blackout).
When I was growing up in the Throggs Neck section of The Bronx, I would regularly observe the night sky from my backyard. On some nights, I could see a fair number of faint stars, though sighting the Milky Way was always just out of reach. Today, it would be quite impossible to see any stars at all because the public school across the street from where I used to live has installed brilliant security lighting on its roof.
In addition, LED streetlights add their own dazzling illumination to such a degree that the general surroundings are bathed in a sort of artificial twilight; it’s now so bright that you can readily read the fine print in a newspaper in the middle of the night, if you were so inclined.
So bright it cast shadows!
Our eyes have some detail-discerning properties that not even the best long-exposure photograph can match. This advantage is considerable in the case of the Milky Way, which has such great extent that it does not require a telescope. I can recall with great fondness one particular night 50-years ago, spent under the dark skies of upstate New York’s Adirondack Mountains, near the community of Thurman, about 8 miles northwest of Warrensburg.
It was there, during the overnight hours of August 14-15, 1975, that I and one of my closest friends, Glenn Schneider, experienced an incredibly dark and starry night. In my nearly sixty years as an amateur astronomer, I rarely have seen such a spectacularly beautiful sky as what Glenn and I came away seeing after a waxing gibbous moon set soon after midnight.
The two of us — then in our late teens — had our telescopes on hand. Mine was a 4.25-inch Edmund Space Conqueror, while Glenn had his RV-6 Dynascope — a classic amateur telescope of that era. I must now also confess here, however, that we hardly used them at all that night, except to pick out deep-sky objects (star clusters or nebulae) of interest. Looking at Via Lactea (the Milky Way’s Latin name) with a telescope — except to view certain highlights — is akin to closely scrutinizing a painting with a magnifying glass, which only reveals the coarse canvas, and not the actual art itself.
And so, we ended up spending a considerable amount of time using just our eyes savoring the Milky Way in all of its magnificence. So clear was the sky, that rather than appearing as a filmy band of light, the Milky Way appeared granulated in texture, and glowed so bright that it actually cast faint shadows!
A night to remember
Together, we made many estimates of what the “limiting magnitude” of the sky was — that is, what was the faintest star that we could perceive using just our naked eyes alone. I knew this was a special night, when I could readily see M33, otherwise known by some as The Great Triangulum Spiral. According to the legendary deep sky observer, Walter Scott Houston (1912-1993), this galaxy “… is a favorite of photographers and the despair of many visual observers” primarily because it is extremely difficult to well-nigh impossible to perceive with just bare eyes alone.
Yet, on this night, both Glenn and I were able to see it as a conspicuous patch of light. I have never been able to repeat that observation in all these years since.
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Later than morning, we both finally agreed that we had seen stars down to magnitude +7.3. Keep in mind that for most people with average vision, the threshold of naked-eye visibility for faint stars is generally accepted to be +6.5 (the higher the value, the fainter the star).
Because the sky was exceptionally transparent, and our eyes were fully dark-adapted — that process by which the human eye becomes more sensitive to light in low-illumination conditions — we were seeing roughly twice as many faint stars as we might otherwise view under normal circumstances. This occurs as the retinal rods increase their sensitivity by regenerating rhodopsin, a light-sensitive pigment, over a period of time. Full dark adaptation can take 20 to 30 minutes and allows the eye to detect stars and other objects that are invisible to it in daylight or under light-polluted conditions. But we had already been outside for several hours.
“We’ll probably remember this night for the rest of lives,” Glenn noted.
Wistfully, in recent days, I been thinking back to that magical night, and of Glenn who for over 30 years served as an astronomer and principal investigator at the University of Arizona and also served as the project instrument scientist for the Near Infrared Camera and Multi-Object Spectrometer (NICMOS) installed on the Hubble Space Telescope. He was also one of the world’s foremost eclipse chasers.
Sadly, Glenn passed away last winter. He was 69.
The Bortle scale
Another long-time friend of mine and a well-known authority on comets, John Bortle, assiduously observes the night sky from Dutchess County, New York. Back in 2001 he developed a nine-point scale which allows a person to judge just how dark (or bright) their local sky is. For the kind of sky that now exists around my old stomping grounds in The Bronx, Mr. Bortle would likely call it a “Class 9.” Specifically defined: The entire sky is brightly lit. Many stars making up familiar constellation figures are invisible, and dim constellations such as Cancer and Pisces are not seen at all. The only celestial objects that really provide pleasing telescopic views are the moon, the planets, and a few of the brightest star clusters (if you can find them).
People who live up to 50 miles (80 km) from a large metropolitan area probably have a “Class 5” sky on the Bortle scale. That’s a sky where the Milky Way is very weak or invisible near the horizon and looks rather washed out overhead. Light sources are evident in most if not all directions. Over most or all of the sky, clouds are quite noticeably brighter than the sky itself.
People who live in a rural location, out to 75 miles or more from a major city, likely have access to a “Class 4” sky: Some indication of light pollution is still evident along the horizon, but now the Milky Way appears somewhat impressive but still lacks detail. Surroundings are clearly visible even at a distance.
But for the ultimate in skywatching, there is the “Class 1” sky, the kind that Glenn and I enjoyed in the Adirondacks all too many of those years ago: From such a sky the Milky Way is capable of casting obvious diffuse shadows on the ground. The presence of Jupiter or Venus in the sky actually hinders your attempt to adapt to the darkness. Airglow (a very faint, naturally occurring glow most evident near to the horizon) is readily apparent. If you are observing on a grass-covered field bordered by trees, your telescope, companions, and vehicle are almost totally invisible.
Or as Mr. Bortle so eloquently put it: “This is an observer’s Nirvana!”
Joe Rao serves as an instructor and guest lecturer at New York’s Hayden Planetarium. He writes about astronomy for Natural History magazine, Sky and Telescope and other publications.
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