By Robert Victor and Robert D. Miller

These star maps, Sky Calendar, and detailed monthly summaries of sky events for August through October are intended to help teachers guide their students to a memorable start in naked-eye astronomy. Jupiter and Saturn appear near each other in the early evening for the rest of 2020 (and not again until 2040), and in December will have their closest pairing in nearly four centuries. (Students will have to wait until 2080 to see these giant planets as close to each other again.) Venus, Mars, and the spectacular star patterns of winter evenings shine in pre-dawn skies. Evenings in October, Mars will be closer and brighter than it will be again until 2035. And each month offers close pairings of the Moon with very bright planets.

Sky maps for twilight -- introduction

Sky events in August 2020

August’s planetary scene features giants in tandem in evening, Mars from late evening until dawn, Venus climbing high in dark pre-dawn, and four very close pairings of the Moon with the three brightest planets. The Perseid meteor shower, somewhat diminished by moonlight this year, peaks in predawn on August 12. Going camping? Best evenings for viewing the Milky Way from dark sites 1½  to 3 hours after sunset are August 9-21.

Bright Jupiter in SE to SSE at dusk and Saturn about 8° to its east make an attractive pair for viewing with unaided eye, binoculars, and telescope. Jupiter’s cloud belts and four bright satellites, and Saturn’s spectacular rings, always inspire at star parties.

In August 2020, Mars rises in late evening, about as Jupiter reaches its high point in the south. Mars’ tiny disk requires high magnification and good seeing (steady air) to reveal its surface details. I use my 6-inch reflecting telescope at 200-power with Mars high above the horizon, which occurs just before dawn this month. On the night of August 2, Mars passed perihelion, least distance from Sun. And on September 1, the South Pole of Mars is tipped at its greatest angle toward the Sun, initiating southern hemisphere summer. These events combine to make Mars’s South Polar Cap, mostly of frozen CO2, shrink rapidly in August and September, so look as soon as possible for the best views of this feature. While it’s still visible, the South Polar Cap appears as a small, bright white oval near the south end of Mars’s mostly rust-colored disk.

For resources on observing Mars during 2020, visit www.abramsplanetarium.org/msta/

In the morning sky, Venus rises more than 3 hours before the Sun and is well up in eastern sky as dawn brightens. Against star background in August, Venus shifts eastward about 1° per day. On August 2 it passed 1.8° S of 3rd-mag. Zeta Tauri, tip of Bull’s southern horn. Crossing through northernmost Orion into Gemini, Venus ends August within 9° S of 1st-mag. Pollux, Gemini’s brighter twin.

Mercury began month 29° lower left of Venus and within 7° S of Pollux. Speeding toward superior conjunction on far side of the Sun on August 17, Mercury is 36° lower left of Venus by August 8, when binoculars will be useful for spotting it low in bright twilight.

Also on August 8, in morning twilight, find Mars high in the south, 10° upper left of the waning gibbous Moon, 79 percent full.  Mars near Moon: In the late evening on August 8, as Jupiter reaches due south around 11 p.m., Moon and Mars will have just risen in the east, with the red planet only 1.7° upper left of the 73-percent Moon. Moon and Mars will appear closest, 1.4° apart center-to-center, just after 1 a.m. on August 9. They’re still just 2° apart at sunrise on August 9, with Mars to lower right of the 70-percent Moon.

On the night of Tuesday-Wednesday, August 11-12, the annual Perseid meteor shower reaches its peak. Even before twilight ends on Tuesday evening, shortly after 9 pm. locally or 1½ hours after sunset, some “earth-grazing” meteors might be seen. They will be long-lasting, because the meteoroids, leftover particles from previous passages of Comet Swift-Tuttle, will be entering the atmosphere on paths nearly parallel to the horizon. They’ll appear to radiate from the constellation Perseus, then rising in the NNE below the familiar “W” of Cassiopeia.

As the night progresses, our part of the Earth’s surface is turned ever more broadside to the incoming meteors, increasing the numbers seen, while the radiant (direction of the source) climbs higher in the northeastern sky. Throughout the darkness hours, meteors might light up anywhere in the sky, but the actual members of the shower, if their tracks are extended backward before the point where they lit up, will intersect at their common radiant, in upper Perseus. (The paths are actually parallel in space, but appear to converge in the distance.) By 11 p.m., Jupiter and Saturn will be 8° apart in the southern sky, and Mars will have risen in the east. Soon after midnight, the count will suffer a setback with the rising of a fat crescent Moon, but then will increase again for the rest of the night as the radiant continues its ascent.

A noted expert on meteor showers predicts that Earth will pass through a dense trail of comet particles around 3 a.m. PDT. If so, more meteors than usual will then be seen.

At first light of dawn on Wednesday,  August 12, the Pleiades star cluster, beautiful in binoculars, will appear 7° upper left of the Moon, and red-orange Aldebaran, follower of the Pleiades and eye of Taurus, the Bull, will be 11° to Moon’s lower left. Mars will then be approaching its high point in the south, while Venus will be gleaming in the east. Venus stands at greatest elongation, 46° from Sun, and shows as a “half moon” through a telescope.

On the following morning, August 13, the Moon will pass 4° north of Aldebaran. On the 14th, look for Venus 12° to Moon’s lower left, and red Betelgeuse, Orion’s shoulder, 16° to Moon’s lower right. Farther right and a little lower is blue-white Rigel, Orion’s foot. Midway between these colorful stars is the Hunter’s 3-star belt. Follow its line downward to Sirius, the Dog Star, just risen in ESE. Close to east and about the same distance above the horizon, look for Procyon, forming the nearly equilateral Winter Triangle with Sirius and Betelgeuse. If you catch Sirius soon after it rises, turn around to see the entire Summer Triangle of Vega, Deneb, and Altair in NW to W, before Altair sets, if there are no mountains blocking your view.

In mid-August, Mars is predicted to equal and then surpass Sirius in brightness, but it’s hard to make a fair comparison now, since Mars is high in the sky while Sirius is low. As Earth closes in until October, Mars nearly doubles its brightness each month in August and September.

Venus near Moon: The waning Moon continues eastward through the zodiac toward the Sun, and on the morning of August 15, an hour before sunrise, the 16-percent crescent appears within 4° north (upper left) of Venus. A pretty sight, with earthshine on Moon’s non-sunlit side! Keep track until sunrise, and you can get a daytime sighting of Venus with naked-eye or binoculars.

On August 16, the 9-percent crescent Moon appears in ENE, 5°-9° lower right of Pollux and Castor, the Gemini twins. The last easy old crescent Moon occurs on August 17, as a thin 3-percent sliver, low in ENE, 12° below Pollux.

The invisible New Moon occurs as it passes the Sun on August 18 at 7:42 p.m. PDT. Grab binoculars on the evening of August 19, about 30 minutes after sunset. From a site with an unobstructed view midway between W and WNW, you’ll get a chance to view a young crescent Moon, 2 percent full, only 4° up, and just over 24 hours old.

Follow the waxing Moon nightly one hour after sunset, August 20 through September 2. Note its phase, direction in the sky, and bright stars or planets nearby.  On August 22, the 22-percent crescent Moon passes within 6° N of Spica in WSW, in Virgo. On August 25, the 54-percent Moon, just past First Quarter phase (when it’s half full and 90 degrees east of the Sun) passes within 6° N of the red supergiant star Antares, heart of the Scorpion.

Jupiter near Moon: On August 28, just before sunset, use binoculars to try for a daytime sighting of Jupiter, within 2.3° upper left of the gibbous Moon in SE. One hour after sunset, Jupiter will be 2.4° almost directly above the 84-percent Moon in SSE. Saturn will be nearly 8° left of the Moon. That night Jupiter and Saturn are 8.3° apart, and the gap between them begins to close until their rare and spectacular conjunction just 0.1° apart, at dusk on Dec. 21, 2020. Until then, keep your eye on the giants!

On August 29, an hour after sunset, find Saturn and Jupiter 6° to 14° to the 91-percent Moon’s upper right.

Sky events in September 2020

Venus and Mars dominate the predawn. Jupiter and Saturn float in southern sky at nightfall. Mars doubles again in brilliance to outshine Jupiter, while its rising time shifts two hours earlier, into evening twilight.

Do you enjoy watching moonrise? Here’s your chance to see the Moon rise into view at a convenient time on several consecutive evenings: On September 1, the Full Moon rises shortly after sunset. During September 1-7, the Moon rises no more than 30 minutes later on each successive night. Notice Moon’s reddened color and flattened shape when it first appears above the horizon (both effects caused by Earth’s atmosphere); Moon’s decreasing phase from one day to the next; and the northward shift of its rising place from day to day. (Another chance to witness a string of early evening moonrises will occur October 1-7, beginning with the Harvest Moon of October 1.)

September at dusk: Jupiter is the bright “star” in SSE to S, with Saturn 8.3° to 7.4° to its east (left) in the course of the month. Blue-white Vega is nearly overhead, with nearby Altair and Deneb completing the Summer Triangle. Golden Arcturus is still well up in W, lower as month progresses. Reddish Antares, heart of the Scorpion, is in SSW to SW. Early in the month it’s still easy to see Spica very low in WSW. On September 14, try for brighter Mercury 10° lower right of Spica. Binoculars will be needed to follow them through their approach within 0.6° apart on September 21 and 0.8° apart on September 22. Fomalhaut, mouth of the Southern Fish, appears in SE by late in month.

Rising in early evening: Mars rises some 10° north of east within a quarter-hour after Jupiter reaches due south in early and mid-September, or at nearly the same time as Jupiter passes south late in the month. Mars surpasses Jupiter’s brightness in late September. To compare them fairly, wait until they’re at equal altitudes later in the evening, four hours after sunset around September 9, and three hours after sunset around September 30.

September at dawn: In the last days of August and first days of September, Venus, shining at mag. –4.3 to –4.1, stands at its highest in the eastern sky for this apparition. Moving just over 1° per day against background stars, Venus passes within 9° S of Pollux on August 31 and September 1. On September 5, the “Twin” stars Castor and Pollux form a straight line with Venus, 15° long.

Mars, doubling in brilliance from mag. –1.8 to –2.5 in September, now clearly outshines the brightest star, Sirius. Find the red planet in SW to W at dawn, lower as month progresses. Bright stars: The entire Winter Hexagon is now in fine view. Begin with its brightest member, Sirius, the Dog Star, in SE to SSE. Then, in clockwise order, find Procyon, Pollux and Castor, Capella nearly overhead, Aldebaran, Rigel, and back to Sirius. Orion’s shoulder, Betelgeuse, is inside the Hexagon. It forms the nearly equilateral Winter Triangle with the big and little dog stars, Sirius and Procyon. By the end of the first week of September, Regulus, heart of Leo, emerges low, north of east, and climbs higher daily in the morning twilight glow. By September 30, Venus pulls within 3° upper right of Regulus. (Don’t miss their close pairing in early October, described in next month’s account.) Deneb, last star of the Summer Triangle to set, is departing in NNW.

Follow Moon waning from full to a thin crescent on mornings of September 2-16. Look about one hour before sunrise to catch Moon about 3° upper left of Mars on September 6; near Aldebaran, eye of Taurus, on September 9; near Twins Pollux and Castor on September 12 and 13; about 6° lower left of Venus on September 14; and near Regulus on September 15. On September 16, look 45 minutes before sunrise to catch the last old crescent Moon, 3° up and 12° north of east. 

Follow the Moon waxing from thin crescent to full on evenings of September 18-October 1. On September 18, look 40 minutes after sunset to catch the thin 4-percent crescent very low in W to WSW. With binoculars, try for Mercury 5° to Moon’s lower left, and fainter Spica 4.5° to Mercury’s upper left. At the same stage of twilight on September 19, find a thicker 10-percent crescent Moon low in WSW, with Mercury 14° to its lower right, and Spica 3° to Mercury’s upper left. For binoculars, Mercury-Spica appear closest to each other, 0.6° apart, low in twilight on September 21. Spica is getting lower each evening (it will be on the far side of the Sun in the middle of next month), while Mercury edges only marginally higher for another week. The Moon is Full on October 1. For information on the close pairing of the Moon and Mars on the night of October 2-3, see Moon near bright planets, in the next section, Highlights for October 2020 and beyond.

On September 21 and 22, an hour after sunset, look for Antares 8°-9° from a fat crescent Moon. The Moon reaches First Quarter phase, half full and 90° east of the Sun, on September 23. On September 24, one hour after sunset, Jupiter appears about 3° to Moon’s upper left, with Saturn 7.7° east of Jupiter. Look again some 4hours later until they’ve nearly set, and you’ll find Jupiter only about 2° upper right of the Moon. On September 25, an hour after sunset, the Moon will be 4° lower left of Saturn and 11° from Jupiter.

Sky events in October 2020 and beyond

October presents two Full Moons, on the first and last day of month; Venus in a close pairing with Regulus on October 2 and 3; a close approach and peak brilliance of Mars; Jupiter closing in on Saturn; and four close pairings of the Moon with bright planets.

Mars, visible during nearly all of October’s nighttime hours, presents its closest and brightest (mag. –2.6) approach to Earth until 2035. Mars reaches its least distance of 38.57 million miles from Earth (light travel time: 3 min 27 sec) on October 6; and reaches opposition on October 13, as Earth overtakes the red planet. Mars even outshines Jupiter for most of the month.

Whenever Mars is at opposition, seasons in its N and S hemispheres are 95° ahead of Earth’s. So on October 13, 2020, it’s early summer in Mars’s S hemisphere, and little remains of the South Polar Cap, since most of its frozen CO2 has sublimated into the Martian atmosphere. But Mars’ most prominent dark feature Syrtis Major, first recorded in 1659 by Christiaan Huygens, is in good view for 12 consecutive nights, 36 minutes later nightly. The dusky triangular area passes most closely north of the center of the Martian disk on October 7 at 9:28 p.m. PDT; October 8 at 10:04 p.m.; October 9 at 10:40 p.m.; Oct 10 at  11:16 p.m.; October 11 at 11:52 p.m.; October 13 at 12:28 a.m.; October 14 at 1:04 a.m.; continuing about 36 min. later each morning until October 19 at 4:05 a.m., when Mars is getting low in WSW. Of those dates and times, Mars appears highest in S on the mornings of October 13 and 14, on the 6th and 7th nights of the 12-day series.  Resources for telescopic viewing of Mars. 

Also in the evening sky, Jupiter (mag. –2.4 to –2.2) and Saturn (+0.5 to +0.6) are paired in S to SSW at dusk, east of the Teapot of Sagittarius. In mid-October, our Spaceship Earth races away as each appears 90° E of Sun, first Jupiter on October 10-11, and Saturn on the following weekend. Both are shrinking in apparent size, Jupiter faster, because it’s closer. By October’s end, extent of Saturn’s rings begins to exceed Jupiter’s equatorial diameter. It’s a good month to witness eclipses of Jupiter’s Galilean moons away from the disk, and the shadow of Saturn cast upon its rings. With binoculars, note 2.1° by 1.1° kite-shaped asterism Territory of Dogs within 7° lower left of Saturn.

Countdown to their great conjunction: Jupiter-Saturn are 7° apart on October 7; 6° on October 21; 5° on Nov. 2; 4° on Nov. 13; and 3° apart on Nov. 23. Jupiter-Saturn will appear within a 1° field Dec. 12-29, and closest, 0.1° apart on Dec. 21, their tightest pairing between 1623 and 2080. Jupiter takes about 12 years to make one trip around the Sun, and Saturn takes nearly 30 years. So with each passing year, Jupiter progresses about 30°, and Saturn 12°, around the Sun, and Jupiter gains 18° on Saturn.  It therefore takes about 20 years for this 18° annual gain to accumulate to 360°, when Jupiter will overtake Saturn again. There will be conjunctions of Jupiter and Saturn in 2040 and 2060, but they’ll be just over 1.1° apart. We must wait until March 15, 2080 before we see another Jupiter-Saturn pairing as close as the one on Dec. 21, 2020.

Use our detailed constellation chart, October Evening Skies, to locate three planets and six stars of first magnitude or brighter. Binoculars give good views of the beautiful Pleiades star cluster and Andromeda Galaxy. Find the variable star Mira, expected at peak brightness in late September-early October A line from Alpha to Delta Ceti, 7° long, extended 6°, locates Mira. Compare Mira to Alpha Ceti (mag. 2.5), Gamma (3.5), and Delta (4.1). In mid-October, Uranus, mag. 5.7, forms a nearly isosceles triangle with Xi-2 and Mu Ceti, 4.3-mag. stars 4.5° apart in head of Cetus. In mid-October Uranus is nearly 6° from each. Neptune, mag. 7.8, is then 1.2° ENE of 4.2-mag. Phi Aquarii. Detailed finder charts for Neptune and Uranus showing helpful additional stars appear in the September and October issues of Sky & Telescope magazine, and are also available here and here. 

The brilliant morning “star” Venus (mag. –4), rises in E about 3 hours before Sun. Don’t miss the close pairings of Venus and Regulus on October 2 (0.5° apart) and October 3 (0.7° apart). Venus goes 1.2° per day, or 6° each 5 days, eastward against the stars. Observing Venus and Regulus daily September 28-October 7 an hour before sunrise should be quite engaging, as day-to-day changes will be easy to notice. With daylight saving time in effect, clock time of sunrise isn’t unreasonably early, so predawn sky watching can be done with little disruption. While you’re out on those mornings, follow brilliant Mars dropping a little lower day by day in WSW to W; the brightest star, blue-white Sirius in SSE to S; and the Moon, starting when it pops into view as Full low in W on October 1 an hour before sunrise. In next two weeks through October 15, watch Moon move west to east through the zodiacal band steeply inclined to the horizon, waning and passing, in order, Mars; Pleiades or Seven Sisters star cluster; Aldebaran, eye of Taurus; Castor and Pollux, twins stars of Gemini; Regulus, heart of Leo; and Venus. (See illustration for October 3-13 on October Sky Calendar.) Also visible in October’s morning sky is Orion, the Hunter, high in S to SSW. His striking 3-star belt points in one direction to the Dog Star, Sirius, and in the other, to Aldebaran and the Pleiades. Sky watchers in southern California can try for Canopus, shown on our morning twilight map and passing due south just 3 degrees above the horizon from the latitude of Los Angeles and Palm Springs. It’s the star next in brightness after Sirius and reaches its high point in the south 4 minutes earlier each day, some 22 minutes before the Dog Star.

Moon near bright planets: On night of October 2, shortly after 8:00 p.m. PDT, the Moon is 1¼ days past Full, and its center passes only 1.4° S (lower right) of Mars. Moon and Mars gradually spread apart for the rest of night, until sunrise on October 3, when you’ll find Mars about 4° lower right of Moon. On October 13 and 14, during the hour leading up to sunrise, Venus appears 7° to 8° from a waning crescent Moon. From sunset until an hour later on October 22, find Jupiter 5° upper right of a fat waxing crescent Moon, nearly half full, and Saturn within 4° to Moon’s upper left. From sunset until an hour later on October 29, find Mars about 5° to upper right of a nearly Full Moon.

On Saturday, October 31, the Full, so-called “Blue” Moon occurs at 7:49 a.m. PDT. This is the second Full Moon within the same calendar month (and so designated a “Blue Moon” by one definition) and smallest FM of this year. But by a prior definition, this is not a Blue Moon, and the next will actually be on August 22, 2021. The earlier definition of a Blue Moon is the third full moon of four within an astronomical season. There will be four Full Moons in summer 2021, on June 24, July 23 August 22, and September 20, so the third one, on August 22, will be a Blue Moon. 

The small apparent size of Halloween’s Full Moon (called a “micro-moon” in some media accounts) is a consequence of the Moon’s passage through the apogee of its orbit on October 30 at 12 noon PDT, 252,522 miles from Earth.

After October 31, the next Full Moon will occur on the morning of Monday, November 30, when part of the Moon will pass through the penumbra, or dusky outer portion of Earth’s shadow. At deepest eclipse, at 1:43 a.m. PST, the northern limb of the Moon will appear only slightly shaded. Aldebaran, red-orange eye of Taurus, will appear within 5° left of the Moon.

A much more impressive eclipse will occur on the morning of May 26, 2021. At 4:19 a.m. PDT, a colorful totally eclipsed Moon will be low in SW, and the red star Antares, heart of Scorpius, will be within 7° to its left.

Robert C. Victor originated the Abrams Planetarium Sky Calendar in October 1968, and has produced the October 2020 issue, included here. For subscription information and a sample of another month, visit Abrams Planetarium Sky Calendar. 

Evening and morning twilight sky maps by Robert D. Miller, who did graduate work in Planetarium Science and later astronomy and computer science at Michigan State University. He remains active in research and public outreach in astronomy.

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