In August, Venus, the evening “star,” gets interesting for telescopic viewers. Up to four bright planets and 11 bright stars are visible before dawn, when the annual Perseid meteor shower is also at its best. The Milky Way excels—and there’s a near-total lunar eclipse on the last Thursday evening!
Mornings this month are prime for sky-watching. The month begins with the moon and three planets in a long grouping across the sky within the belt of zodiac constellations. On Aug. 1, their arrangement from west to east, an hour before sunup, is: the constellation Capricornus, about to set in the west-southwest; the moon, in the southwest in Aquarius; Saturn, high in the south in Pisces; and the compact Pleiades star cluster, well up in the east, with ruddy Aldebaran, eye of Taurus, nearly 14° below, closely accompanied by stars of the Hyades cluster, completing a letter “V” marking the head of the Bull. Both star clusters are very rewarding sights through binoculars! The red planet Mars, also in Taurus, is 14° to the lower left of similarly colored Aldebaran on Aug. 1, and shifts about two-thirds of a degree daily, passing between the Bull’s horns on Aug. 2-3. Look daily and notice the change, which will bring Mars into Gemini by Aug. 12. The easternmost visible planet in the morning sky in early August is Mercury, found very low in the east-northeast, 27° to the lower left of Mars on the first few mornings.
The moon shifts its position eastward by an average of 13.2° per day, completing one trip around the zodiac in 27.3 days. Because of the sun’s apparent motion of nearly 1° per day against the zodiacal backdrop—the result of the Earth’s orbital revolution around the sun—it takes an extra 2.2 days for the moon to catch up and pass the sun twice in succession, so a cycle of lunar phases (from full moon to full moon, for example) takes up 29.5 days.
Go out each morning in early August, to follow the waning moon, passing Saturn on Aug. 3 and 4, and passing just below the second- and third-magnitude pair of stars of Aries, the Ram, on Aug. 5. By Aug. 6, the moon is a fat crescent, to the upper right of the Pleiades. If you get out very early the next morning, Aug. 7, not long after moonrise, you can catch the Pleiades cluster very close to the moon’s earthlit upper limb, and watch the moon pull away from the cluster until morning twilight intervenes.
An hour before sunup on Aug. 8, the crescent moon cozies up to the tip of the Bull’s northern horn, with Mars below. On Aug. 9, a thinner crescent moon will appear closer to the red planet, about 6° to Mars’ lower left. That same morning, look to the lower left of the moon for three objects arranged in a diagonal straight line. From lower right to upper left, they are: Mercury, the brightest; Pollux, 6° to Mercury’s upper left; and Castor, the faintest, 4.5° to Pollux’s upper left. On the morning of Aug. 10, one hour before sunrise, the 7 percent crescent moon appears 6°-7° to the right of the “Twin” stars Pollux and Castor, 19° to the lower left of Mars, and 13° to the upper right of Mercury. On Aug. 11, the moon’s last morning, look for the 2 percent thin crescent very low in the east-northeast, with Mercury just 1.7° to the lower right of the center of the moon’s disk. Binoculars will give the best view. If the sky is very clear, and you have an unobstructed view in that direction, keep watch with binoculars for the next 15 to 30 minutes, until you can see Jupiter 6° below and slightly to the left of the moon-Mercury pair. Each successive morning, Mercury—heading toward superior conjunction on the sun’s far side on Aug. 27—drops a little lower, while Jupiter, emerging from solar conjunction because of Earth’s faster orbital motion, climbs higher. By Aug. 15, the two planets will appear virtually the same height above the horizon, with fainter Mercury passing within 0.6° to the north (left) of Jupiter. You can fit them in the same telescope field! Within a few days, Mercury will be gone, and Jupiter will be the dominant morning “star” until Venus dethrones it at the end of October.
The moon returns to the morning on Aug. 27, when it’s full and low in the west-southwest an hour before sunup. Then the lineup of bright zodiacal objects from west to east will be the moon, Saturn, Aldebaran, Mars, Pollux (a bit to the north of the solar system’s plane) and Jupiter. The waning gibbous moon will glide past Saturn on Aug. 30 and 31. On Aug. 31, Mars. in central Gemini, forms an isosceles triangle with Pollux and Castor, 12° from each.
Other bright objects in the morning sky include, north of the zodiac plane, the Summer Triangle of Vega, Altair and Deneb in the west to northwest, beginning to set in twilight in the latter half of month; and Capella, the “Mother Goat” star, ascending in the northeast. South of our solar system’s plane, look for Fomalhaut, Mouth of the Southern Fish, far to the lower right of, or below, Saturn. Orion, with his 3-star belt flanked by red Betelgeuse and blue Rigel, has already emerged into the eastern sky by the start of August. Before midmonth, watch for the rising of Procyon, the “before the Dog” star, rising just north of east, to the lower left of Betelgeuse; and Sirius, the Dog Star itself and the brightest nighttime star, rising in the east-southeast, nearly in line with Orion’s belt extended downward. Sirius, Procyon and Betelgeuse form the nearly equilateral Winter Triangle. Capella, the northernmost of the 21 stars of first magnitude or brighter, can be located midway between Orion’s belt and Polaris, the North Star.
The best occasions for viewing the Milky Way in the morning before twilight begins are the moonless pre-dawns of Aug. 11-24, when the Milky Way will be seen in the Summer Triangle, in the west to northwest, up through Cepheus, Cassiopeia high in the north, and down through Perseus, Auriga, the horns of Taurus, and the feet of Gemini in the northeast to east-northeast. When we look a few degrees east of Elnath, tip of Taurus’ northern horn, we’re facing outward in the plane of our Milky Way, directly away from the center of our galaxy.
Included within this span of dark mornings is the peak of the Perseid meteor shower, which is expected to offer its richest show before morning twilight on Aug. 13. The sky will be dark and moonless throughout the night of Aug. 12-13, coinciding with the new moon. Adjacent dates should also be good for planning a meteor watch.
Evening highlights: By far, the brightest “star” in evening twilight is Venus, of magnitude -4.3 to -4.6, low in the west to west-southwest at dusk. Venus is of increasing interest in the closing months of its evening apparitions, because it draws closer to Earth and appears larger in size, and begins to move between Earth and sun and become backlighted, as a crescent. The transition from gibbous phase, though half-full to a crescent, takes place around Aug. 14, the date of Venus’ greatest elongation, when it reaches its maximum angular distance from the sun, 46°. This month, watch Venus change from 55 to 39 percent full, while growing from 21” (arcseconds) to 30” in angular diameter.
On Aug. 1 at dusk, find Venus just south of west at dusk, and locate the star Spica in the southwest, 29° to the upper left of Venus. Watch Venus approach this star all month, until it passes just 1.5° south (to the lower left) of the star on Sept. 1. Find another bright zodiacal star, the red supergiant Antares, heart of the Scorpion, 46° east of Spica, or in the south at dusk on Aug. 1, drifting into the south-southwest by Sept. 1.
The moon in August can be seen on 15 consecutive evenings at dusk, culminating with a deep partial lunar eclipse. On the first evening, Aug. 13, just half an hour after sunset, it may be a challenge to catch the 3 percent crescent very low, just north of west, and 28° to the lower right of Venus. On Aug. 14, you can look for the easier 7 percent crescent a bit later in twilight, 15° to the lower right of Venus. Aug. 15 will present a great photo-op: a 14-percent crescent moon centered about 3.5° to the lower right of Venus! Continuing eastward, the 22 percent crescent moon at dusk on Aug. 16 will appear in the west-southwest, 10° to the left of Venus and 5° to the lower right of Spica. On Aug. 17, the 31 percent moon will appear 9° to the left of Spica. At dusk on Aug. 19, the first quarter moon will sit in the south-southwest, 13° to the lower right of Antares. On the next evening, Aug. 20, the 60 percent gibbous moon will stand within 2° to the lower right of Antares at dusk, and not long after 11 p.m., as seen from western Nevada, shortly before moonset, the upper edge of the moon will slide just more than one degree below the star. (From the South Pacific Ocean and southern South America, the moon will actually occult, or cover the star.) At dusk on Aug. 21, the 69 percent moon appears 11° east of Antares and a few degrees west of the central region of our Milky Way, and on Aug. 22, the 77 percent moon is east of the galactic center and near the spout of the Teapot, where the Great Sagittarius Star Cloud is located—definitely not good nights for observing the Milky Way! On Aug. 23, the 85 percent moon appears near the handle of the Teapot.
For the next several evenings, the moon makes its way through a fainter region of the zodiac. On Aug. 24, the 91 percent moon appears in the far eastern part of Sagittarius, nearly 15 degrees east of the second-magnitude star in the Teapot’s handle. In August’s final week, the moon passes through constellations containing no star brighter than third-magnitude: two nights each in Capricornus and Aquarius, followed by August’s last three nights, rising soon after sunset, in Pisces.
On Thursday evening, Aug. 27, we’ll find the full moon rising very shortly before sunset, already deeply embedded in the dusky but not very dark outer portion of the Earth’s outer shadow, called the penumbra. Not until 7:33 p.m. does the moon begin to enter the dark, central core of Earth’s shadow, called the umbra, and the partial eclipse gets under way. From Reno, the moon has just risen, and the sun is about to set—and, at least from a high place with ideal, unobstructed horizons, an observer would experience the rare sight of the disks of both the sun and penumbrally eclipsed moon sitting upon opposite horizons, 13° north of west and 13° south of east, respectively.
For the next nearly 100 minutes, more and more of the moon’s disk is immersed in the dark umbral shadow, and quite soon, the circular outline of Earth’s shadow edge becomes apparent. When enough of the moon’s disk is immersed in deep shadow, reddish colors typically become noticeable, from sunlight which has been reddened by its passage through the Earth’s atmosphere, and refracted into the shadow, on its way from the sun to the moon.
This lunar eclipse won’t be total, but it is a significant partial, with 93 percent of the moon’s diameter covered by the umbra at 9:13 p.m.—all but the upper part of the moon’s disk is covered in deep shadow. The moon might then resemble a view of Mars with a bright northern polar cap. Twilight has ended in both locations, so the Milky Way should be visible, passing from Perseus rising in the north-northeast, through Cassiopeia, Cepheus, the neck of Cygnus the Swan within the Summer Triangle just east of overhead, and down to the steaming Sagittarius Teapot and the tail of Scorpius, low, just west of due south. It’ll be worth taking a trip to a dark place for this nearly total lunar eclipse, just to experience the full impression of how much darker the sky will get with the reduction in moonlight.
The umbral eclipse ends at 10:52 p.m., as the moon departs from the dark core of Earth’s shadow. The next total lunar eclipse visible from western U.S. will occur on the evening of June 25, 2029. During that event, the moon will rise at dusk already in total eclipse for observers in the western continental U.S.
Many of the sky events described above are illustrated on the Abrams Planetarium Sky Calendar. For $12 per year, subscribers receive quarterly mailings, each containing three months of calendars and evening star maps. Subscription info and a free printable sample are available at abramsplanetarium.natsci.msu.edu/sky-calendar/index.aspx.
Robert Victor was a staff astronomer at Abrams Planetarium at Michigan State University. He is now retired and enjoys promoting sky watching to folks of all ages in and around Palm Springs and East Lansing, Mich. Robert Miller, who provided the twilight charts, did graduate work in planetarium science and later astronomy and computer science at Michigan State University, and remains active in research and public outreach in astronomy.
