By Robert C. Victor and Robert D. Miller

Introduction to sky maps for observing planets and bright stars at evening and morning mid-twilight

When organizing a first sky watching session for students, we suggest that you begin your outdoor session during evening twilight, so students can experience the joy of discovering and identifying the brighter stars as they first appear. Begin your session no later than one-half hour after sunset, or even earlier if the Moon or a bright planet is visible. You may wish to continue until the sky is dark enough for naked-eye observation of constellations of interest. Binoculars and telescopes can provide inspiring views of Moon, planets, double stars, star clusters such as the Hyades and the Pleiades, the Great Nebula in Orion’s sword, the Lagoon Nebula above the spout of the Teapot of Sagittarius, Andromeda Galaxy, and more.

If you also schedule a predawn session, you can start as early as 1¾ to 2 hours before sunrise, before morning twilight begins, to allow enough dark-sky time to observe a selection of constellations and deep sky objects. Continue long enough into twilight to watch some of the brighter stars disappear.

My friend and former colleague at Michigan State University, Mr. Robert D. Miller, has kindly created computer programs and provided us with monthly sky charts tracking daily locations of the five naked-eye planets and the 16 stars of first magnitude or brighter visible from latitude 34° north. Positions of the stars and planets are plotted each day at the moment the Sun is 9° below the horizon, which we have called “mid-twilight”. Locations of the planets are plotted as a separate dot for each day, with larger dots plotted weekly on the 1st, 8th, 15th, 22nd, and 29th day of the month. Star positions during the course of the month are plotted as continuous tracks, with all stars drifting westward (left to right on the charts) in the course of the month, owing to the Earth’s revolution around the Sun.

For latitude 34° N, the moment of evening mid-twilight during the course of the year occurs 39 to 47 minutes after sunset, and morning mid-twilight occurs a similar interval ahead of sunrise. For locations south of lat. 34° N, the same stage of twilight occurs closer to sunset and sunrise, and for locations farther north, twilights are longer.

Evening Twilight Star Map - August 2021

In August 2021, at evening mid-twilight, Venus shows little change in its altitude above western horizon, but shifts southward, from W toward WSW as month progresses. Jupiter first appears on ESE horizon in evening mid-twilight in first week, and climbs higher as month progresses. (Jupiter is at opposition to the Sun on August 19 and visible all night, from dusk until dawn. Note Saturn to Jupiter’s upper right at dusk. Saturn reached opposition on August 1.)

Bright stars of August evenings include the Summer Triangle of Vega, Altair, and Deneb climbing high in east. In mid-August 2021, a line from Vega to Altair, 34° long, extended 31° locates Saturn. Antares, heart of Scorpius, crosses from S to SSW. Bright Arcturus is in W, with Spica 33° to its lower left, sink lower in the course of the month. Note Venus closing in on Spica late in August. They’ll appear only 1.6° apart on Sept. 5.

Morning Twilight Star Map - August 2021

In the morning sky in August 2021, Saturn exits in the SW, with Jupiter following. The Summer Triangle is sinking in W to WNW, while the eastern sky is filling up with same bright stars we’ll see on winter evenings. On August mornings, the last of these to rise are the “Dog Stars”, Procyon in east and Sirius in ESE, in the second week. The two Dog Stars, plus Betelgeuse, Orion’s shoulder, comprise the Winter Triangle. Members of the Winter Hexagon, in clockwise order beginning with its brightest star, are Sirius; Procyon; the “Twin” stars Pollux and Castor just 4.5° apart; Capella; Aldebaran; Rigel; and back to Sirius, completing the polygon with Betelgeuse inside. 

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Sky Events, September-October 2021 and beyond

September 2021 provides sky watchers the opening act of an evening planetary display, increasingly spectacular for the rest of 2021. As this month begins, Earth has recently overtaken the two giant planets of our solar system, Saturn on August 1, and Jupiter on August 19. These are this year’s dates of their oppositions, when we see each planet by facing directly away from the Sun. A planet at opposition is near its closest to Earth, reaching maximum brightness, and visible all night, from dusk until dawn. After its opposition, a planet remains visible in the evening sky for about half a year or more.

Evening mid-twilight sky map - September 2021

On September 1 at evening mid-twilight, 40 minutes after sunset, Jupiter is prominent at mag –2.9 low in ESE, with Saturn at mag. +0.3 in SE, 17.5° to Jupiter’s upper right. The Sun is then 9° below the WNW horizon. On that date, our Spaceship Earth is racing directly away from a point 5° above the star Antares in SSW. This red supergiant star marks the heart of Scorpius, the Scorpion. An observer high above the northern side of our solar system would see the planets moving counter-clockwise in their orbits around the Sun. Compared to Earth, Jupiter and Saturn are slower-moving outer planets; since we overtook them in August, we are now leaving them behind.

The two planets interior to Earth’s orbit are now visible in the western sky at dusk: Venus is prominent at mag. –4 in WSW. You’ll need binoculars to spot zero-magnitude Mercury very low in bright twilight within 16° to Venus’ lower right. Both inner planets move faster than Earth, so will catch up and overtake us, Mercury passing inferior conjunction (between Earth and Sun) on October 9, and Venus doing so on Jan. 8.

Look within 5° upper left of Venus on September 1 as the sky darkens, and you’ll find Spica. Each day, Venus shifts position against the background stars by slightly more than one degree. On September 5, Venus will pass 1.6° upper right of Spica. The gap between them will widen to 4° by September 8, with Spica to the lower right of Venus.

A very thin crescent Moon may be visible on September 7. Using binoculars, look very early, about 30 minutes after sunset, for the 2-percent crescent very low in west 28° lower right of Venus. Next, find Mercury 14° to Moon’s upper left. Moon’s age is a tender 26 hours after New. Mercury is highest in twilight around this date.

The 6-percent Moon is much easier to spot on the next evening, September 8. Look 15° lower right of Venus, then find Mercury 5° lower left of Moon, and Spica 4° lower right of Venus.

The orientation of the lunar crescent reveals the location of the Sun below the WNW horizon. Imagine the crescent to be an arrowhead. The shaft of the arrow would point directly at the Sun. Note the crescent is tipped over to rest almost on its lower cusp (point), indicating the Sun is to the lower right of the crescent, rather than directly below it.

Dusk on Thursday, September 9 provides a spectacular view, of a 12-percent crescent Moon just 3°-4° upper right of Venus. Find Spica 5° below the Moon, and Mercury 10° lower right of Spica and 14° lower right of the Moon.

Dusk on September 10 finds Venus within 12° lower right of the Moon, and Mercury still 15° lower right of Venus. On September 11, locate Spica about midway between Venus and Mercury.

Dusk on September 10 finds Venus within 12° lower right of the Moon, and Mercury still 15° lower right of Venus. On September 11, locate Spica about midway between Venus and Mercury.

On September 12, the fat 42-percent lunar crescent appears about 3° above the red star Antares. On September 13, the 53-percent Moon is just over 90° (a quarter-circle) east of the Sun and so is just  past First Quarter phase, rather low in the southern sky around sunset. That same evening, Mercury reaches greatest elongation, nearly 27° east of Sun.

On September 16, the 83-percent waxing gibbous Moon is 4°-5° lower right of Saturn, while bright Jupiter passes 1.4° north of the 3rd-mag. star Delta Capricorni, or Deneb Algedi, tail of the Sea-goat.

On September 17, the Moon, 91-percent full in SE at dusk, is 5°-6° lower right of Jupiter. On the next evening, September 18, the 96-percent Moon appears 11° lower left of Jupiter.

On September 20, the fourth Full Moon of summer 2021 appears very low, about 8° S of east at evening mid-twilight. Also try for a difficult pairing, very low in WSW in bright twilight on September 20 and 21: Mercury passing within 1.5° south (lower left) of Spica. Using binoculars, try for the pair 19°-20° lower right of Venus.

Northern hemisphere’s autumn begins on September 22 at 12:21 p.m. PDT as the Sun, moving southward, crosses directly over the equator. Three months later, on Dec. 21, the Sun will reach its southernmost point, over the tropic of Capricorn, marking the solstice and the start of our winter season.

At dusk in September, the belt of zodiac constellations makes its lowest angle with the horizon. This year, the visible portion of the belt is marked by four bright planets, in order from west to east, Mercury, Venus, Saturn, and Jupiter. Also within that part of the zodiac are the first-magnitude stars Spica and Antares.

The southernmost part of the zodiacal belt, in the constellation Sagittarius, the Archer, is in the south around sunset in September. This causes the low angle that the zodiac makes with the horizon, the “tipped over” appearance of the waxing crescent Moon in the west, the positions of Venus and Mercury low at dusk this month, despite considerable elongations (angular distances from Sun) of 40° to 45° for Venus, and up to 27° for Mercury.

Another phenomenon resulting from the low angle of the zodiac to the horizon is the annual “Harvest Moon effect”,  early evening moonrises for several days after Full. During September 19-23, the Moon rises less than half an hour later each evening, and noticeably farther north each time. So, if you’d enjoy watching a string of daily moonrises without staying up very late, here’s your chance!

Besides the Moon and planets, bright stars visible at dusk include golden Arcturus, nearly halfway from horizon in west as September opens, and dropping lower as month progresses; Spica some 33° lower left of Arcturus; and the Summer Triangle of Vega, Altair, and Deneb reaching overhead. Blue-white Vega, its brightest member, passes directly overhead at lat. 38.8° N, within California! For the rest of 2021, a line from Vega to Altair, 34° long, extended 31° past Altair, locates Saturn. The alignment is most accurate in mid-August and again in late November-early December. In September, watch for Fomalhaut, mouth of the Southern Fish, rising 22°-23° lower left of Jupiter.

The morning sky -- no planets, but lots of bright stars!

Morning mid-twilight sky map - September 2021

After Jupiter sets in WSW before dawn, the “Dog Star” Sirius is the brightest star in the morning sky. In September’s morning mid-twilight, Sirius is found in SE to S. Trace out the huge Winter Hexagon of bright stars. Beginning with Sirius, its southernmost and brightest member, proceed clockwise through Procyon; the Twin stars Pollux and Castor 4.5° apart; Capella, the Mother Goat Star and northernmost member; Aldebaran, eye of Taurus the Bull and Follower of the Pleiades star cluster; Rigel, Orion’s foot; and back to Sirius. Betelgeuse, shoulder of Orion, lies inside the Hexagon.

A much more compact asterism is the almost equilateral Winter Triangle, consisting of Sirius, Procyon, and Betelgeuse.

On September 1, as Spaceship Earth races away from Antares in the evening sky, it’s heading toward a point about 5° above Aldebaran in the morning sky. The Sun is in Leo, below the ENE horizon. As we follow our orbit around the Sun during September, the stars will shift their positions westward, as shown on the morning chart. The Moon, just past Last Quarter, will appear near Aldebaran on the morning of Aug. 30. On September 2 and 3, the waning crescent Moon appears near the Twins, Pollux and Castor. On the morning of September 5, let the 3-percent, old crescent Moon low in ENE be your guide to Regulus, heart of Leo, just emerging from solar conjunction. Look for the star 6°-7° lower right of the thin, lunar crescent.

The Moon makes its next pass through the morning sky, waning from Full to a thin crescent, September 20-October 5. Watch the waning gibbous Moon pause midway between the Pleiades and Aldebaran on September 26. See a fat crescent Moon pass 3° south of Pollux on September 30, and a thin crescent pause 5°-6° lower left of Regulus on October 3. The last crescent will appear very low in east on October 5.

You’ll notice that the old crescent, with its cusps pointing upward, is oriented like a bowl on a table, signifying that the Sun is almost directly below it. Also, the Moon, near Last Quarter phase on September 28 and 29, is very high in the sky around sunrise, in Taurus and Gemini, the northernmost constellations of the zodiac.

Attractive gatherings of Moon with stars and planets, and planets with each other and with stars, are illustrated on the Sky Calendar. Subscribe for $12 per year for three monthly issues mailed quarterly, or view a sample copy at www.abramsplanetarium.org/skycalendar/

In the rest of 2021, Venus, Saturn, and Jupiter form an attractive lineup in the southwestern evening sky, anchored by brightening Venus. In late October, telescopes show Venus as a tiny “half-moon”. In November-December, Venus looms large as it approaches Earth, and displays ever thinner crescent phases, which can be resolved even with binoculars when the planet is observed during bright twilight, soon after sunset.

Planetary highlights for October 2021

The rest of 2021 will be wonderful for observing showpiece planets. Share the views, safely!

Evening mid-twilight sky map - October 2021

Evenings in October: Venus is low in SW at dusk, 3° higher at month’s end. Its setting time improves from 1.8 to 2.3 hours after sunset. Enjoy following Venus’ motion, now 1.1° to 1.0° daily against background stars. Watch it go 0.8° S of Delta, middle star in head of Scorpius, on October 9; and about 1.4° N of Antares on October 16.
 
Jupiter and Saturn, in SE to S at dusk, end retrograde, Saturn on October 10, and Jupiter on October 17, before turning east against the stars. Saturn’s turnaround on October 10 is 7.3° WSW of Theta Capricorni. Watch gap close until planet and star set in twilight in Jan. 2022. Jupiter’s pause on October 17 is 1.8° from 2.8-mag. Delta Capricorni, tail of Sea-goat, and 1.4° from 3.7-mag. Gamma. The two stars are 1¾° apart. Jupiter is equidistant from these stars on September 27 and Nov. 6. On October 24, Jupiter-Saturn are 15.4° apart, closer than they’ll be again until 2039.
 
Telescopic views: As Venus closes in on our home planet, its disk grows from 19” (arcseconds) to nearly 26” across this month, while illumination decreases from 62 percent to 48 percent. The best is yet to come! After Venus’ greatest elongation of 47° on October 29, next ten weeks until inferior conjunction on Jan. 8 will be fascinating! The crescent Venus will more than double in apparent size but get very thin, peaking in brilliance at mag. –4.9 midway through the ten weeks, in early December. Jupiter features cloud belts and four Galilean satellites, while Saturn displays rings tipped 19° from edge-on in October, the best view we’ll have for many years. Using higher magnification, look for Saturn’s shadow cast upon the rings at the northeast limb of the planet, giving the scene a 3-D appearance. In mid-October, Jupiter’s disk appears 44” across, slightly exceeding the 39” extent of Saturn’s rings.

Mornings in October: Mercury passes inferior conjunction on October 9 and attains mag. +1.0 in eastern morning sky on October 17. Brightening rapidly, Mercury attains mag. 0 by October 20, and mag. –0.7 by October 25, when it stands at a very favorable greatest elongation, 18° from Sun.

Morning mid-twilight sky map - October 2021

Events not to miss include these conjunctions of the Moon with the brightest planets, in evening sky unless otherwise noted.
Moon-Venus: August 10; September 9; October 9; November 7; December 6.
January 3, 13° apart in evening.
January 29 and 30, 13° apart in morning.
Moon-Jupiter: August 21, September 17, October 14, November 11, December 8, January 5, February 2.

A deep, nearly total eclipse of the Moon on the night of November 18-19:
Moon enters umbra: Thurs. November 18 at 11:18 p.m. PST.
Deepest eclipse, 97 percent in Earth’s shadow: 1:03 a.m. PST on Fri. November 19, with Moon just 5°-6° from the beautiful Pleiades star cluster.
Moon leaves umbra: Friday November 19 at 2:48 a.m. PST.

Mercury has a favorable morning apparition in late October through mid-November. Watch for these planet-star and planet-planet conjunctions in morning sky:
November 2: Mercury 4.1° N of Spica. Moon 2°-3° above Mercury and 5° N of Spica on next morning.
November 10: Mercury passes within 1.0° N of Mars. Departing Mercury introduces the emerging faint red planet into the morning sky.
December 27 and 28: Mars passes 4.5° N of its brighter similarly colored rival, Antares, whose name means “not Mars”.

Returning to the evening sky, Mercury emerges 15° lower right of Venus on December 20, or an easier-to-see 10° lower right of Venus on December 23.  On December 28, Mercury passes 4.2° S (lower left) of the soon-to-depart Venus. Mercury reaches a favorable greatest elongation from the Sun on January 6, attains its highest altitude in twilight on January 8, and approaches within 3.4° lower right of Saturn on January 12, before fading quickly through mag. +1.0 on January 15.

Here are monthly evening and morning mid-twilight sky maps for the rest of this year. Use them to  follow seasonal changes in positions of bright stars, and wanderings of the planets.

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Robert C. Victor was Staff Astronomer at Abrams Planetarium, Michigan State University. He is now retired and enjoys providing sky watching sessions for groups in and around Palm Springs.

Robert D. 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.       

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