Venus, Saturn and Jupiter are all easy to see in August’s early evenings. Jupiter and Saturn, up in nearly all of August’s dark hours, reach their closest and brightest points of the year. The best chance to see a fourth planet—Mercury—comes late in the month; faint Mars is unlikely to be seen through haze and bright twilight glow.
If you enjoy a very thin crescent moon, watch at dawn on Aug. 7 and dusk on Aug. 9. The diminished moon should make for a great Perseid meteor shower this year, peaking on night of Aug. 11-12.
Finally, the blue moon on the night of Aug. 21-22 may attract little media attention—because of a mistake in a respected astronomy magazine many years ago.
In the evening sky: Three planets are well seen in all of August: Brilliant Venus, of magnitude -4, low in the west at dusk; Jupiter, of magnitude -2.9, rising in the east-southeast; and Saturn, of magnitude +0.2, some 19 degrees to the upper right of Jupiter.
Finding Mars is a challenge! At magnitude +1.8, it’s as faint as it ever gets, and is mired very low in bright twilight. You’ll need binoculars, very clear skies and an unobstructed view. Try for the red planet 11 degrees to the lower right of Venus on Aug. 1. On Aug. 9, look low, 17 degrees to the lower right of Venus, for a thin crescent moon, 37.5 hours after new. Binoculars may show Mars 3 degrees to the lower left of the 3 percent lunar crescent. That evening, Mars will set less than an hour after sunset, and Jupiter will rise less than a half-hour before Mars sets. Mars and Jupiter are just 2 to 3 degrees above opposite horizons 44 minutes after sunset on Aug. 9, so a sighting of all four planets simultaneously requires good low views toward the east-southeast and 10 degrees north of west.
Mercury (magnitude -0.5) emerges, passing only four arcminutes (1/15 of a degree) to the lower left of Mars on Aug. 18. A telescope will be required to split the tight, unequal pair. If you succeed in spotting the pair, then you can see five planets simultaneously! After Aug. 18, Mercury climbs 2 degrees higher in twilight by the month’s end, and is easier to find, though faded to magnitude -0.1. Find Mercury 20 to 16 degrees to the lower right of Venus Aug. 19-31, as both planets shift south of west.
It’s fun to see the two brightest planets—Venus and Jupiter—simultaneously, and track changes over several months. If you want to catch them at equal altitudes above the horizon, the time to look shifts closer to sunset as the month progresses: On Aug. 1, look 1 hour, 19 minutes, after sunset (when they are 4 degrees up—not easy with lots of mountains surrounding us); on Aug. 16, look 55 minutes after sunset (the planets are 9 degrees up); and on Aug. 31, look 33 minutes after sunset (nearly 14 degrees up).
Telescopic views: Watch Saturn’s rings for several nights before and after its opposition on Aug. 1. The rings are currently tipped 18 degrees from edgewise, and extend 42 arcseconds in width. Follow Titan, Saturn’s largest moon, in its 16-day orbit around the planet. Jupiter, at opposition on Aug. 19, displays parallel belts of clouds on a disk 49 arcseconds wide, with four bright satellites discovered by Galileo. Venus this month shows a tiny gibbous disk, 82 to 73 percent illuminated, and 13 to 15 arcseconds across. Venus will put on a fine show late this year. As it approaches Earth, Venus will swell to more than 60 arcseconds across and display spectacular crescent phases.
Other evening sights: Golden Arcturus begins the month high in the west-southwest at dusk, with Spica 33 degrees to its lower left. Look for the Summer Triangle of Vega, Altair and Deneb high in the eastern sky at dusk. In mid-August, a line from Vega to Altair, 34 degrees long, extended 31 degrees, locates Saturn. Antares, red supergiant heart of the Scorpion, is in the south to south-southwest at dusk. The Milky Way is spectacular in August at nightfall, on dates when a bright moon is absent: Aug. 1-11, and Aug. 26-31.
Follow the moon at dusk as it waxes from a thin crescent to full, Aug. 9-22. On Aug. 10, the 8 percent crescent is 5 degrees to the right of Venus; on Aug. 11, the 14 percent crescent is 9 degrees to Venus’ upper left.
As mentioned above, this should be an excellent year for the annual Perseid meteor shower. On the evening of Wednesday, Aug. 11, the 15 percent crescent moon sets just 3/4 of an hour after twilight ends, leaving the sky dark and moonless. The greatest number of “shooting stars” will occur during predawn darkness on Thursday, Aug. 12, until twilight begins, about 90 minutes before sunrise. Meteors can be seen anywhere in the sky; what the shower meteors have in common is their paths: All originate from a common radiant, in the constellation Perseus in the northeast, below the better-known “W” of Cassiopeia.
You might see small numbers of “Earth-grazer” meteors at nightfall on Aug. 11, when the radiant is low in the north-northeast, and meteors enter our atmosphere at a shallow angle. The count of meteors per hour will increase as the radiant rises higher, until the first light of dawn on Thursday. Expect some meteors, but not as many, on other mornings around the peak.
At dusk on Aug. 12 and 13, the moon is 8 degrees from Spica. On Aug. 16, the two-thirds gibbous moon is 5 degrees to the upper left of Antares. On Aug. 20 at dusk, the 98 percent moon is 5 degrees below Saturn. On Aug. 21, the moon at dusk is 5 degrees to the lower right of Jupiter. The full moon occurs the same night, at 5:02 a.m. on Sunday, Aug. 22. It’s also called a blue moon—the third full moon of a season with four full moons. What, you thought a blue moon meant the second full moon of a calendar month? Not so!
The moon passes just 4 degrees south of Jupiter about 3 a.m. on Aug. 22. Note Jupiter is still within 5 degrees of the “blue moon” as dawn begins to brighten on Sunday.
At dusk on Aug. 22, the 99 percent moon rises within 13 degrees to Jupiter’s lower left.
All the lunar and planetary groupings mentioned here are illustrated on the Sky Calendar. To subscribe for $12 per year (for three printed monthly issues mailed quarterly), or to view a sample issue, visit www.abramsplanetarium.org/skycalendar.
Morning planets: Jupiter, with Saturn to its lower right, is low in the southwest at dawn at the start of the month. The two giant planets drop lower each morning. Saturn is at opposition on the night of Aug. 1-2. Jupiter is at opposition on Aug. 19, as Earth passes between the sun and these planets.
Bright stars in morning: They are plentiful—the same stars which adorn the evening sky in January. The last to rise is Sirius, the Dog Star. Follow Orion’s belt down to the east-southeast horizon to catch Sirius rising in the morning twilight glow in August’s second week. Over many years, I have often ended my Perseid meteor vigil waiting for Sirius to appear before calling it a night. The moon wanes to a thin crescent through Aug. 7, and Aug. 22-Sept. 5. Watch the waning moon pass through Taurus (Pleiades, Aldebaran and horns) as a crescent Aug. 2-4 and again near last quarter phase, Aug. 29-31. Catch a beautiful crescent moon with earthshine among the stars of Gemini on Aug. 5 and 6 (near Pollux and Castor). On Aug. 7, catch the thin, 1 percent old crescent moon, 25.5 hours before new, rising 10 degrees below Pollux.
Catch the waxing gibbous moon near Saturn most of the night of Aug. 20-21. Catch the full “blue” moon near Jupiter throughout the night of Aug. 21-22.
Visualize: On Aug. 22, Regulus in Leo is invisible, on the far side of the sun. Our Spaceship Earth is heading toward a point 4 degrees south of the Pleiades star cluster in the morning sky, and away from a point 3 degrees west of Beta in the head of Scorpius in the evening sky. Ten days later, on Sept. 1, our planet is heading directly toward a point 5 degrees north of Aldebaran in morning sky, and directly away from a point 5 degrees north of Antares in the evening sky. As seen from above the northern side of the solar system, the revolutions of planets are counterclockwise around the sun. In the evening sky, the inner planet Venus moves faster than Earth, and will eventually catch up and overtake us, on its inside track, passing between Earth and the sun—and out of the evening sky, in early January 2022.
Earth is overtaking two slow-moving outer planets—Saturn and Jupiter—in August, causing them to climb higher into the southeast evening sky, and drop lower and exit the southwest morning sky. Earth’s faster motion as we overtake Saturn and Jupiter also causes them to appear to retrograde, or move westward, against the stars. Saturn retrogrades nearly 7 degrees, and Jupiter 10 degrees, for about 4-4 1/2 months, centered on the dates of their oppositions. Watch Jupiter pass 0.9 degrees north of fourth-magnitude Iota Aquarii on the night of Aug. 5-6, and 1.4 degrees north of third-magnitude Deneb Algedi (Delta Capricorni, tail of Sea-goat), on the evening of Sept. 16. In August, watch Saturn widen its distance west-southwest of Theta Capricorni from 3.9 to 6 degrees.
For the dedicated sky watcher: Look for the variable star Mira and the planet Uranus in the predawn sky this month. Look before twilight begins, on Aug. 2-19 and Aug. 30-Sept. 18, when bright moonlight doesn’t spoil the view. Mira, in the constellation Cetus, varies its brightness in an 11-month cycle. It usually brightens to about third magnitude, making it easy for the unaided eye to see for a couple of months. It typically fades to about magnitude 9, well below the reach of the unaided eye. Mira’s peak brightness this year is expected in mid-August. To find Mira, use the “V” head of Taurus (Aldebaran and the Hyades) as an arrow pointing toward 2.5-magnitude Alpha in the head of Cetus, and 7 degrees farther in the same direction to 4.1-magnitude Delta. (Don’t be distracted by brighter 3.5-magnitude Gamma nearby. It’s not on the line.) Continue another 6 degrees in the same direction Alpha to Delta extended—and there’s Mira!
Uranus is in the same part of the sky, about 20 degrees north-northeast of Mira. Use your binoculars to star-hop to Uranus from Mu on the northern edge of the head of Cetus, as follows: From 4.3-magnitude Mu Ceti, go 2.3 degrees due north to the 5.2-magnitude star 38 in Aries. Next, continue another 2.9 degrees due north to a 5.8-magnitude star, Omicron, in Aries. Uranus, comparable in brightness to Omicron in Aries in August, is in the middle of a compact isosceles triangle made of the stars Omicron, Sigma (magnitude 5.5), and Pi (magnitude 5.3) in Aries. Uranus begins to retrograde on Aug. 20, and remains very nearly a degree from Omicron all month. Uranus will pass only 9 arcminutes north of that star on the night of Oct. 11-12, prior to opposition on Nov. 4.
Robert Victor was a staff astronomer at Abrams Planetarium at Michigan State University. When the coast is clear, he looks forward to sky-watching sessions, hopefully in time for the fine display of three planets in the evening sky in autumn 2021. 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.