Science: Get Ready for Stargazing in October, the Clearest Month

What month is the clearest of the year for stargazing? The clearest time for the Pacific Northwest is around July; for the Midwest, August; for the Great Plains, it seems to range from July through October. But the most striking pattern is in the large region from New England, south and west into the Gulf States and Texas, where long-term climatological records show that October is by far the clearest month of the year. 

In New York City, for example, October typically has 12 clear days, more than in any other month in that location. Easterners can thank high-pressure systems, which migrate from the west and tend to stall and spread out during October in the vicinity of West Virginia. 

Besides the clarity of the sky, a lack of haze leads to transparent views. This gives us a glimpse of those faint stars near the threshold of naked-eye visibility that can make October nights so stunning. Strong cold fronts periodically dropping south out of Canada often cleanse the atmosphere, with showery rains traveling ahead of them and crisp, dry, clean air following behind them, bringing a few days of superb transparency. Indeed, in contrast to the hazy skies of late summer, we are now treated to days when the sky appears a richer or deeper shade of blue and nights with some of the best observing of the year. [Best Night-Sky Events of October 2018 (Stargazing Maps)]

In addition, the ambient nighttime air temperatures are comfortable. Yes, some nights might be a bit on the chilly side, but just think ahead a month or two and imagine trying to observe in near- or sub-freezing conditions. 

Moreover, October provides you with the best of both summer and winter skies. 

Right after sundown, we still have an excellent view of the summer Milky Way stretching from nearly overhead, down toward the southwest horizon. With binoculars, we can sweep through the glittering star fields in Cygnus, the swan, all the way down to the dazzling star clouds around the center of our galaxy in Sagittarius, the archer.

And if you’re up before the break of dawn, you can enjoy a preview of the midwinter sky, with Orion, the hunter, and his brilliant retinue of Taurus, the bull; Gemini, the twins; and such twinklers as blue-white Sirius, the brightest of all stars, and yellow-white Capella, glowing from a point almost directly overhead. Check out the beautiful Pleiades and Hyades star clusters with binoculars, or look to what many think is the showpiece of the sky, the Great Orion Nebula, a vast gaseous nebula that is often described in stargazing guides as one of the most wonderful telescopic objects in the heavens. 

If this is not enough incentive for you to dust off your telescope and get out to do lots of stargazing, you need only see what happens once we transition from October into November.

The 19th-century English poet Thomas Hood wrote a poem that, I think, best sums up November:

No sun — no moon!
No morn — no noon —
No dawn — no dusk — no proper time of day.
No warmth, no cheerfulness, no healthful ease,
No comfortable feel in any member —
No shade, no shine, no butterflies, no bees,
No fruits, no flowers, no leaves, no birds! —
November!

And as far as stargazing is concerned… well, as you can imagine from Hood’s words, overall conditions rapidly deteriorate. Statistics clearly show that clear-sky coverage across much of North America rapidly takes a turn for the worse. In fact, once we move into November, we enter a protracted spell of widespread cloud cover. 

According to the National Oceanic and Atmospheric Administration’s local climatological data for the United States, the cloudiest month falls between November and May for 98 percent of weather stations sampled.

Put another way, from November through April, roughly 25 to 35 percent of the United States sees more than 70 percent cloud coverage in various months. In contrast, in October, only small portions of Maine and the Pacific Northwest endure such conditions. 

As a final note, however, I should point out that in some years, even October can be unkind to stargazers.

Unfortunately, those very same cells of “fair-weather” high pressure that predominate during early fall in the East can sometimes work against skywatchers. These fronts also tend to produce light winds, which, combined with the waning level of solar radiation coming to Earth, sometimes create what is known as a “temperature inversion.” At such times, a layer of stagnant warm air aloft puts an effective lid over the atmosphere’s lower layers, preventing human-made pollutants from venting upward in the usual manner. 

This results in a layer of smog that thickens in the late-night and early morning hours. Just such an October weather pattern occurred 53 years ago, unfortunately precluding predawn views for most observers east of the Mississippi when the spectacular Comet Ikeya-Seki appeared. Ironically, this was the most brilliant comet of the 20th century. 

Joe Rao serves as an instructor and guest lecturer at New York’s Hayden Planetarium. He writes about astronomy for Natural History magazine, the Farmers’ Almanac and other publications, and he is also an on-camera meteorologist for Verizon FiOS1 News in New York’s Lower Hudson Valley. Follow us on Twitter and on Facebook. Original article on Space.com.

Leave a Reply

Your email address will not be published. Required fields are marked *