August Skies 2018

August 2018 Skies as seen from Denver Colorado.
Viewing southward in Denver at 10:00 PM on August 15th. Telrad circles are centered on the Lagoon Nebula, M8; note position of the Trifid Nebula, M20, just inside the 4° circle, and thus in most finderscopes’ field of view (remember that M20 will be shown at the bottom of an inverted finderscope view). Vesta passes between b Ophiuchi and C Ophiuchi this month, as illustrated by the thin curving arrow at center-right. Look at last month’s chart to get a feel for this area of Ophiuchus, located right above Scorpius (just off this month’s chart at bottom-right). –Object positions, constellation and meridian lines charted in SkySafari, and then enhanced. (Tap on image above for larger version.)

© Zachary Singer

The Solar System

If you follow the planets, you’re likely aware that Mars is just past opposition at the beginning of August, and thus more or less at its biggest and brightest for the year. Dust storms, though, have blanketed the planet, cloaking surface details that should’ve been visible even in moderately sized telescopes. Recent NASA reports suggest that the planet’s weather is quieting down, and that atmospheric clarity should begin to improve.

That time frame could still span several weeks, long enough to miss the closest look at the red planet. Putting all the “closest approach” hype aside, though, Mars’ apparent size will shrink only slightly by mid-month, and it will still be a very large 21” at the end of August. Even at the end of September, Mars’ disk will still span a good 16”, more than enough for you to make out surface detail, under good seeing conditions. Observing will be more convenient, too—by mid-August, Mars transits around 11:30 PM, and a full hour earlier at the end of the month.

Mercury is lost in the dawn for the first half of August, but it gains enough angular separation from the Sun that dedicated observers might find it after mid-month. At that point, Mercury will be relatively dim at magnitude +2.2, and just 5° up in the east, 30 minutes before sunrise. If you can make out an image telescopically, your reward will be a thin crescent more than 9” across. As the days progress, the disk will shrink, but it will widen and brighten—all the way to magnitude -0.7, with a slightly gibbous disk, at month’s end. (Greatest elongation, the widest angular separation from the Sun, is August 26th.)

Venus boasts a brilliant, magnitude -4.2 “lemon wedge” disk at the beginning of August, about 16° above the western horizon a half-hour after sunset. By the end of the month, the disk appears almost half-again larger, or almost 30” across, and its phase narrows to a subtle crescent. The changes come as Venus swings around in its orbit, coming closer to the Earth and narrowing the angle we see between Venus and the Sun. In turn, though, that narrower angle means that at month’s end, Venus stands less than 10° above the horizon a half hour after sunset.

As for Jupiter, catch it while you can. As August begins, the planet is already past the meridian after sunset, appearing in the southwest as darkness falls, and getting low (just over 20° up) by 10 PM. By mid-month, the planet is low by 9 PM—and it sets just before 10:30 at month’s end.

The shadows of two of Jupiter’s four largest moons, Io and Europa, will cross the planet’s face on the 26th, starting around 8:35 PM Mountain Daylight Time, joined by the moon Io itself. Unfortunately for us, Jupiter will be just 22° above the horizon and sinking, so Denver’s usual challenging seeing conditions will be made that much worse.

Still, it’s worth trying—even if the view is too blurry to see the shadows, you should at least be able to witness the end of the Io transit, when the moon finishes crossing Jupiter’s disk and begins to become visible as a separate object to the west. (It will join Europa there, making for a striking change.) According to SkySafari software, the end of the transit occurs at 8:59 our time; be ready to observe several minutes before and after, to see the moon’s dot come into view—you’ll see a subtle “bump” before there is a clear separation.

Saturn passed its opposition early last month, so it’s technically no longer at its prime—but on a practical basis, it’s in a great position for observing! The ringed planet is at its highest in the south (and thus offering the sharpest image) around 10:30 PM in early August, a much easier hour for observing than we had last month. By month’s end, Saturn will be in about the same spot by 9:30, so August is a great time to show this planet to the kids (as many of you know, it’s among the most popular targets for “kids” of all ages). Better still, Saturn will continue to be a good target well into the fall.

At mid-month, magnitude +6.6 Vesta cruises directly between two stars we used for finding last month’s targets, b Oph and c Oph—Vesta’s path is marked on the chart above. (If you missed last month’s issue, catch up in “July Skies,” on pp. 6-8; the file itself is at:

Putting a finderscope’s crosshairs between b and c Oph will show Vesta within its field for most of August—if you got familiar with b, c, and Theta (θ) Oph last month (or do so now), identifying Vesta should be simple after about the 7th or 8th, when the minor planet makes a noticeable triangle with b and c Oph. That triangle shrinks noticeably in size, until Vesta swings close to c Oph between about the 19th and 22ndthey’ll be just 7’ apart on the night of the 21st.

I usually avoid covering meteor showers here, but this year’s Perseid shower is expected to be a decent one, with the peak on the nights of August 11th-12th (Saturday night) and, perhaps better, the 12th-13th (Sunday). The best time to watch a shower is when its radiant (its apparent point of origin in the sky) is well above the horizon—with the Perseids, that’s at about 2 AM. Some years, moonlight washes out the meteors, but the Moon won’t be a factor this time—for best viewing though, find a dark sky, away from obscuring city lights.

Stars and Deep Sky

Unlike last month, when we toured some less commonly visited objects, this month’s are better traveled—the first is a bright nebula in Sagittarius, and the other is a beautiful open cluster.

Our first target, the Trifid Nebula, or M20, at 18h 04m, -23° 02’, is noted—and named—for its “three-lobed” appearance. This well-known object has several components; the first is an emission nebula, similar to the nearby Lagoon Nebula, M8, and the Orion Nebula, M42. These nebulae are glowing clouds of ionized hydrogen gas, often also referred to as H II regions (the “H II” designation means that these clouds are singly ionized; H I is neutral).

H II regions are typically pink or reddish in photographs, but the nebulae are usually too dim for our eyes to perceive the color, so we see them as gray. (I have seen some observing reports, though, that suggest that color may be visible in the Trifid under very good conditions.) The clouds glow because they’re energized, or “excited,” by ultraviolet light from stars within them; the idea is similar to that of a fluorescent tube.

In front of M20’s great glowing cloud, from our point of view on Earth, lies a complex pattern of molecular gas and dust, forming dark lanes that obscure parts of the cloud behind it, and causing the “split” appearance of the bright cloud behind it. That is, the main glow is all one cloud, with the silhouette of the dark area superimposed on it—the Horsehead Nebula in Orion is another famous example. The gas here is very much denser than in the glowing area, and in spite of its darkness, it actually conceals numerous new stars and proto-stars (these were long suspected, but it took recent infrared images to finally show them).

To the north of the emission nebula lies a reflection nebula. Such nebulae commonly appear blue in photographs for the same reason that Earth’s sky does during the day—the blue component of starlight or sunlight is more easily reflected than longer wavelengths, like red. As with the emission nebulae, though, we usually can’t see the color ourselves in a telescope, so this part of M20 also looks gray—if you can see it at all.

Ordinarily, the Trifid’s central (lobed) emission cloud should be easily visible under dark skies in an 8- or 10-inch ’scope, and not too tough in a 6-inch (though detail may suffer in the latter). A UHC or O III filter should boost contrast noticeably.

In early July, though, my experienced observing partner and I were only barely able to make out the dust lanes, using a UHC on my 6-inch Newtonian at 100x (that was the best view); just a few minutes earlier, the nearby Lagoon Nebula was an easy target, with and without the filter. Rather like many galaxies, the Trifid really needs good transparency, along with dark skies—with our recent fires here and in the West, transparency along the Front Range has been awful on most nights. In short, if you don’t have any luck with M20 over the next few weeks, don’t give up on it—the problem may have as much to do with our “temporary” conditions as with anything else.

The easiest way to get to the Trifid Nebula, M20, is to find the Lagoon Nebula, M8—if you’re familiar with the Lagoon, then you know that its bright glow is visible to the naked eye, just above the “spout” of the “teapot” in Sagittarius (if you’re not familiar with this patch of sky, bear with me). M20 is just “above” or northward of M8, lying so close that the two objects share the same finderscope field. In the usual straight-through type of finderscope, with an inverted view, M20 appears as a small, hazy patch of light towards the bottom of your finderscope—the nebula may not be visible, but a few of the stars peeking out from within its clouds will be.

If you’re not familiar with the Lagoon Nebula or Sagittarius, have a look at the August 2015 issue of the Observer ( “Getting Your Bearings,” on page 3, will help you become familiar with this fascinating region (it looks toward the central part of our galaxy, the Milky Way), and to recognize Sagittarius and the Summer Triangle—there’s lots to look at here! M8 itself is covered in the “August Skies” column, on page 7 of that issue. (Read up on the star Polis, too, because we’ll need that for our next target, below.)

Our second target, M23, at 17h 58m, -18° 59’, is an open cluster in the upper, northernmost reaches of Sagittarius. Visually, it’s quite bright, at magnitude +5.5; my notes from recent observations on the 6-inch record a loose, “salt-and-pepper” cluster, with “lots of stars sprinkled all over the place.” M23 might remind experienced observers of M35, a well-known showpiece cluster in Gemini—M23 is only slightly dimmer.

M23 doesn’t get as much attention as it should, because it’s in a part of the sky dominated by even richer objects, like the Ptolemy Cluster, M7, in nearby Scorpius. That’s a shame, because M23 is a lovely cluster to gaze at, and with a beautiful Milky Way background. In the 6-inch, it was already quite good at about 60x, in a 1° field, but better at about 100x—though the eyepiece field had narrowed to just less than 2/3°, the cluster still fit well enough, and the extra magnification helped. Arguably, the view was better still at 150x, though the cluster no longer fit completely, because all the stars shone brightly and the entire view was filled with stars—it was very impressive! The cluster is bright enough to take reasonable magnification well, so try different eyepieces to see what works best for you—in the end, visual observing is a subjective experience.

The hottest stars found in M23 are of class B9, still quite hot and blue—but the absence of even hotter B-class stars and all of the really hot O-class tells astronomers that these “missing” stars have already lived through their life spans. That suggests an age of about 300 million years, or roughly half-again the age of the brighter M7 cluster.

To get to M23, first find Polis, aka Mu (μ) Sagittarii, a magnitude +3.8 star a little less than 6° above, or northward, of the top of the teapot—you’ll know it’s Polis when you see that it’s about as far from M8 (the Lagoon Nebula) as M8 is from the top of the Sagittarius teapot’s spout. From Polis, look about 10° northwest, or “up and right” when this area is near the meridian, to find Xi (ξ) Serpentis, a slightly brighter star than Polis. Xi Ser, as it’s known for short, is 6½° due east of Sabik, aka Eta (η) Ophiuchi, which we used to find our targets last month. Once you have these two stars, Polis and Xi Ser, place your Telrad halfway between them, and M23 should appear near the center of your finderscope field. 

Saturn currently sits very near Polis, making the star less noticeable than usual—my observing partner a few weeks ago didn’t see Polis until it was pointed out: “Look right above Saturn.” Ordinarily, the star would stand out well on a similar night, but Saturn is quite a distraction—and the planet will remain within 2° of the star well through October. It’s not a terrible problem, but you should know about it—and for now, you can take advantage of the situation by using Saturn, the brightest object in the area, to get you looking near Polis. Next year, Saturn will have moved eastward, towards Capricornus, and finding Polis will return to normal.

—See you next month.

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