A diagram of Jupiter in August 2021. | SkyNews
Jupiter, August 2021 (Chris Vaughan, Starry Night Education)

Planets at a Glance: August 2021

This month, watch as speedy Mercury nears Mars on August 18, while Jupiter and Saturn continue their dance through the southern sky.

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For those of you who like to listen to your stories, click below to hear Susan Trishel Monsøn reading Chris Vaughan’s summary of the planets movements this month.

Mercury

A diagram of Mercury in August 2021. | SkyNews
Mercury, August 2021 (Chris Vaughan, Starry Night Education)

On August 1, Mercury will be at superior conjunction with the Sun and unobservable until about one week later, when the speedy planet will reappear in the northwestern sky after sunset. This will be the best apparition of the year for Southern Hemisphere observers. Unfortunately, Mercury’s position close to a shallow evening ecliptic will make this its worst appearance of the year for mid-northern latitude dwellers, but they can try to glimpse the planet sitting low over the west-northwestern horizon during a brief period commencing at about 8:30 p.m. local time. On August 18, the rapid eastward orbital motion of Mercury will carry it less than 9 arcminutes south of eight times fainter Mars, allowing both planets to share a blurry eyepiece view through a backyard telescope. Mercury will halve in brightness during August, from magnitude -1.1 to -0.1. Viewed in a telescope, the speedy planet will wane in illuminated phase from 95-per-cent on August 10 to 74-per-cent illuminated at month’s end. Meanwhile, the planet will exhibit a mean apparent disk diameter of 5.5 arc-seconds. A slim, 12-hours-old crescent Moon will shine a few finger widths to the right (celestial northwest) of Mercury on August 8.

Venus

A diagram of Venus in August 2021. | SkyNews
Venus, August 2021 (Chris Vaughan, Starry Night Education)

Extremely bright Venus will continue to hold court in the western sky after sunset throughout August, but its position hugging the steeply tilted evening ecliptic will prevent the magnitude -3.95 planet from climbing very high — or from shining in a dark sky — for observers in the Northern Hemisphere. A near-vertical ecliptic in the Southern Hemisphere will allow the planet to shine in total darkness. At mid-northern latitudes, Venus will set at about 10 p.m. local time on August 1 and 45 minutes earlier on August 31. On August 10, Venus will cross into Virgo. That night, the young crescent Moon will shine a palm’s width to its right (or 6 degrees to the celestial northwest). Viewed through a telescope during August, Venus will show a gradually waning gibbous phase and an apparent disk diameter that grows from 12.7 to 15 arc-seconds.

Mars

A diagram of Mars in August 2021. | SkyNews
Mars, August 2021 (Chris Vaughan, Starry Night Education)

Magnitude 1.8 Mars, travelling prograde eastward in Leo all month long, will become increasingly difficult to glimpse as it sinks deeper into the western post-sunset skyglow every night. When the month begins, Mars will set at about 9:45 p.m. local time. That time will advance to 8:30 p.m. at month’s end. Mars’ tiny 3.6 arc-second apparent disk diameter will be difficult to resolve in a telescope because it will be shining through so many air masses of atmosphere. On August 18, eight times brighter Mercury will pass 9 arc-minutes to the south of Mars, allowing both planets to share a blurry eyepiece view through a backyard telescope. (For safety, use optics only after the Sun has completely set.) On August 9, the young crescent Moon will sit several finger widths to the upper right (or 3.7 degrees northwest) of Mars.

Jupiter

A diagram of Jupiter in August 2021. | SkyNews
Jupiter, August 2021 (Chris Vaughan, Starry Night Education)

Bright, white Jupiter will be well-placed for observing all night long during August. The planet will be travelling retrograde westward across the stars of Aquarius until it crosses into Capricornus on August 18. Yellowish Saturn will shine 19 degrees to Jupiter’s right (or celestial east). Unfortunately, the low summertime ecliptic will prevent those planets from climbing very high up the southern sky. When Jupiter reaches opposition on August 20, the gas giant will be 373.1 million miles, 600.4 million kilometres or 33.3 light-minutes from Earth, and it will shine at its maximum brightness of magnitude -2.88 for 2021. Because Jupiter is approaching perihelion in January 2023, the planet will sport a generous, 49 arc-seconds-wide disk at this year’s opposition. Views of Jupiter in amateur telescopes will show its equatorial bands, and the Great Red Spot will appear every second or third night. Around opposition, Jupiter and its four large Galilean satellites frequently eclipse and occult one another, and cast their round, black shadows on the planet, both singly and in pairs. The full Moon will shine less than 5 degrees below (celestial south of) Jupiter on August 21.

Saturn

A diagram of Saturn in August 2021. | SkyNews
Saturn, August 2021 (Chris Vaughan, Starry Night Education)

Yellowish Saturn will be also be ideally positioned for viewing all night long during August. It will be travelling retrograde westward across the stars of Central Capricornus – with much brighter Jupiter positioned 19 degrees to its left (east). Unfortunately, the low summertime ecliptic will prevent either planet from climbing very high up in the southern sky. On August 2, Saturn will reach opposition. At that time, the ringed planet will be at a distance of 830.6 million miles, 1.337 billion kilometres, or 74.3 light-minutes from Earth. While planets at opposition always look their brightest, Saturn’s peak magnitude 0.18 will be enhanced by the Seeliger effect, backscattered sunlight from its rings. In a telescope at opposition Saturn will show an apparent disk diameter of 18.6 arc-seconds, and its rings will subtend 43.3 arc-seconds. Saturn’s rings will be tilting more edge-on to us every year until the spring of 2025. This year they are already closed enough for Saturn’s southern polar region to extend beyond them. Opposition is also the best time to view Saturn’s moons with a backyard telescope in a dark sky. The nearly full Moon will sit several finger widths below Saturn on August 20.

Uranus

A diagram of Uranus in August 2021. | SkyNews
Uranus, August 2021 (Chris Vaughan, Starry Night Education)

During August, Uranus will be best observed during the second half of the night. On August 1, it will rise at 12:20 a.m. local time and climb more than halfway up the southeastern sky before dawn. By month’s end Uranus will be rising before 10:30 p.m. local time and will culminate over the southern horizon, two-thirds of the way up the sky, at around 5 a.m. local time. On August 20, Uranus will cease its motion across the stars of southern Aries in preparation to commence a westward retrograde loop that will last until January, 2022. That night, the blue-green, magnitude 5.75 planet will be surrounded by the fifth-magnitude stars Sigma, Omicron, Pi and Rho Arietis — creating a distinctive asterism for anyone viewing Uranus in binoculars. The waning gibbous Moon will pass two finger widths below (or 1.5 degrees to the celestial south) of Uranus on August 28.

Neptune

A diagram of Neptune in August 2021. | SkyNews
Neptune, August 2021 (Chris Vaughan, Starry Night Education)

During August, the distant planet Neptune will be observable during most of the night while it travels slowly retrograde westward through northeastern Aquarius. Look for the blue, magnitude 7.83 planet positioned a slim palm’s width to the left (or 5.5 degrees to the east) of the fourth-magnitude stars Psi, Xi and Phi Aquarii. On August 1 Neptune will rise at about 10:25 p.m. and then culminate halfway up the southern sky around 4 a.m. local time. At month’s end, two weeks before its opposition, Neptune will rise shortly after sunset, at 8:25 p.m. local time.

Chris Vaughan is a science writer, geophysicist, astronomer, planetary scientist and an “outreach RASCal.” He writes Astronomy Skylights, and you can follow him on Twitter at @astrogeoguy. He can also bring his Digital Starlab portable inflatable planetarium to your school or other daytime or evening event. Contact him through AstroGeo.ca to tour the universe together.

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