Catch Mercury in the morning
Mercury springs into action in the last two weeks of October, offering its best morning appearance for 2021. Venus, by contrast, hangs low in the southwestern sky all month. Jupiter and Saturn dominate the evening sky, visible through midnight. And late-evening binocular views of Uranus and Neptune beckon more adventurous observers.
Let’s begin in the evening sky. Venus is visible soon after sunset, low in the southwest. It begins the month at magnitude –4.2 and brightens to –4.5 by Oct. 25. The planet lies in Libra for the first week of October and crosses into Scorpius
Oct. 7. Venus spends part of Oct. 15 crossing a small corner of Ophiuchus, before returning to Scorpius and passing 1.5° north of Antares on the 16th. It returns to Ophiuchus Oct. 21 and stays there through the end of the month.
Venus reaches its greatest elongation east of the Sun
Oct. 29, when it stands 47° from our star. However, this does little to increase the planet’s altitude after sunset, due to the low angle of the ecliptic to the western horizon. An hour after sunset, Venus is always between 7° and 11° high.
Any telescope reveals its changing appearance. On
Oct. 1, the planet subtends 19" and is 62 percent lit. The disk grows to 24" by Oct. 27 and reveals a lovely half-illuminated phase. By Halloween, the disk spans 26" and is less than 50 percent lit.
One observational curiosity is the moment the phase reaches exactly 50 percent. Venus’ observed phase often differs from the calculated phase. Called Schröter’s effect, the onset of dichotomy might be observed up to a week before Oct. 27. The likely culprits at the heart of this effect are the dimness of the central part of the terminator combined with the scattering of light in the venusian atmosphere. When do you observe Venus as 50 percent lit?
As soon as the sky is dark, look low in the south for two bright objects. The westernmost is Saturn, followed by Jupiter
16° farther east. Both are in Capricornus the Sea Goat.
Saturn’s motion against the background stars grinds to a
halt as its retrograde path ends Oct. 10 and it resumes eastward motion. The planet’s movement is barely noticeable by eye. Saturn shines at magnitude 0.4 in early October and dims by
0.1 magnitude in the latter half of the month. On Oct. 13, Saturn stands about 6° northeast of a waxing gibbous Moon.
The giant is a stunning sight in a telescope, with the best viewing in the few hours after sunset. The magnificent ring system, tilted by 19° to our line of sight, is clearly on view around the 17"-wide planetary disk. The northern face of the ring system is now visible. Over the next few years, the rings will become narrower, revealing more of the planet’s southern hemisphere. The brightest ring is Ring B, separated from the duskier Ring A by the Cassini division. Ring C, or the Crepe ring, is quite thin. You should be able to see the planet through it.
Saturn’s yellowish disk reveals few details. Rare storms do pop up, so watch for small white spots that might develop into something larger. An equatorial belt is often visible under good seeing conditions.
The brightest moon of Saturn, magnitude 8.5 Titan, is easy to see through any telescope. It appears north of the planet Oct. 5 and 21, and south of the planet Oct. 13 and 29.
A trio of 10th-magnitude moons orbit closer than Titan.
Tethys, Dione, and Rhea are quite easy to spot. More difficult is Enceladus, shining near magnitude 12. It lies close to the bright edge of the rings.
Iapetus reaches superior conjunction Oct. 10, then progresses east, reaching greatest elongation Oct. 29. Its darker face has turned earthward, dimming it to 12th magnitude.
It stands 8' east of Saturn, 12 times farther than Tethys, also east of the rings the same night.
Saturn sets about 2 A.M. local time on Oct. 1 and just before midnight by Halloween.
Jupiter shines brilliantly in eastern Capricornus all month, dimming slightly from magnitude –2.6 to –2.5. Its retrograde path slows to a halt Oct. 18, a week after Saturn. Attentive skywatchers will see Jupiter’s position relative to Deneb Algedi change slightly this month. It lies 1.8° northwest of the star Oct. 1 and moves to a point 2.1° northwest by Oct. 18. It then returns to within 1.9° of Deneb Algedi by Oct. 31. On Oct. 14 and 15, the waxing gibbous Moon lies nearby. The planet sets about 3:10 A.M. local time on Oct. 1, and by 1:15 A.M. on Oct. 31.
Jupiter is best viewed beginning in late twilight — when low-contrast views of its atmosphere are a sight to behold — and for a few hours into the evening, before it sets in the west after midnight. Any telescope reveals wonderful detail, from the planet’s pair of dark equatorial belts to more subtle features to their north and south. Every few days, the Great Red Spot is also visible.
Observers can also catch the four Galilean moons. Their resonant orbits mean some events repeat during the month. For example, at 9:48 P.M. EDT on
Oct. 5, Io comes out of eclipse by Jupiter’s shadow, which stretches east of the planet. Europa is occulted by Jupiter’s western limb 26 minutes later, at 10:14 P.M. EDT.
These events repeat
Oct. 12/13, with a nice addition: Before Io comes out of eclipse, Callisto enters eclipse from an unusual location — farther from Jupiter than Io’s position at reappearance. The reason is that at Callisto’s greater distance from Jupiter, the planet’s shadow extends farther east. Callisto begins to disappear at 11:11 P.M. EDT, followed by Io’s reappearance at 11:43 P.M. EDT. Such events highlight the geometry of the jovian system. An hour later, at 12:40 A.M. EDT on the 13th, Europa dips behind Jupiter’s western limb.
The Io/Europa pair performs a similar act early on Oct. 20. On Oct. 28, Io reappears from eclipse 6" northwest of Europa, just minutes after 10 P.M. EDT.
Neptune is a month past opposition and visible most of the night in Aquarius the Water-bearer. Binoculars will show the magnitude 7.7 planet well. Neptune begins the evening in the southeastern sky and reaches its highest point above the southern horizon before local midnight.
October opens with Neptune less than 4° east of magnitude 4 Phi (ϕ) Aquarii. The gap shrinks to 3.3° by Oct. 31. Look roughly 6.5° south of the Circlet in Pisces for a triangle of
6th-magnitude field stars; Neptune is west of this group all month. You can also use a pair of 6th-magnitude stars east of Phi in same binocular field of view. Draw a line from Phi through the midpoint of a line connecting these stars, and you’re heading right for Neptune. The ice giant stands 7° west of a gibbous Moon Oct. 17.
Neptune currently spans 2". A telescope at high magnification on a steady night of seeing will show its bluish-green disk.
Uranus is approaching opposition, which occurs next month, and is visible nearly all night. In early October, wait a few hours after sunset to view it. Uranus lies about 16° west of the Pleiades, in a sparse region of southern Aries. It should be easy to spot at magnitude 5.7, especially with a handy nearby star to guide you. Uranus begins the month close to Omicron (ο) Arietis, which shares the planet’s current magnitude. On Oct. 1, they are 23' apart. By Oct. 10, Uranus is less than 10' to the star’s north. Uranus continues westward and lies a Full Moon’s width west of Omicron by Oct. 24.
Omicron is 2.9° due north of 38 Arietis, which itself is 2.3° due north of 4th-magnitude Mu (μ) Ceti. Look for Mu in 7x50 binoculars before midnight and place it in the lower right of your field of view. 38 Arietis should be near the center of the field of view. If you’re looking around midnight, instead place Mu at the bottom of your field of view. From Mu, move your binoculars to place 38 Arietis in the lower right and look for a 6th-magnitude double star — one of those “stars” is Uranus. Check each night and you’ll easily spot the planet’s motion.
Mercury reaches inferior conjunction with the Sun Oct. 9. You might imagine it will next cross in front of the Sun, but not this month. Such transits only occur when inferior conjunction is in May or November, when the planet’s orbit crosses the ecliptic. Nov. 13, 2032, is the next time this happens.
Mercury skips into the morning sky soon after conjunction and is well placed before sunrise. The high angle of the ecliptic at dawn makes this the planet’s best morning apparition for Northern Hemisphere observers this year. Mercury reaches greatest elongation (18°) Oct. 25. This is far shy of the possible 27°, due to the fact that Mercury reached perihelion just a week earlier, on the 19th. Observers with a clear eastern horizon might catch Mercury that day, shining at magnitude 0.4 and only 3.5° high an hour before sunrise.
Two days later, Mercury is much easier to spot. By Oct. 21, it’s magnitude 0 and nearly 5° high an hour before sunrise. A week later, on Oct. 28, it has brightened to magnitude –0.7, an easy object with the unaided eye. Mercury brightens by another 0.1 magnitude through the end of the month and remains clearly in view each morning, even as its elongation from the Sun slowly diminishes.
Mars is too close to the Sun to observe this month. It returns to the morning sky by December.
Martin Ratcliffe is a planetarium professional with Evans & Sutherland and enjoys observing from Wichita, Kansas. Alister Ling, who lives in Edmonton, Alberta, is a longtime watcher of the skies.