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Uranus' Menagerie of Major Moons

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Uranus' Menagerie of Major Moons A Digital Supplement to Astronomy Insights Astronomy Magazine © 2019 Kalmbach Media Meeting the Ice Giants September 2019 • Astronomy.com WWW.ASTRONOMY.COM 1 Visible to the naked eye Visible with binoculars Visible with a telescope RISING MOON I A terrific trio of terraced craters SKY THIS MONTH THE WAXING CRESCENT MOON offers a THE SOLAR SYSTEM’S CHANGING LANDSCAPE AS IT APPEARS IN EARTH’S SKY. Theophilus, Cyrillus, and Catharina BY MARTIN RATCLIFFE AND ALISTER LING smorgasbord of fantastic views. From the slen- derest of arcs to First Quarter phase, Luna shows off rolling seas bordered by craters with towering OBSERVING peaks and aprons of debris blasted out during the impacts that created them. The best detail September 2019 HIGHLIGHT Theophilus appears along the terminator, where the Sun is NEPTUNE reaches its 2019 peak September 9/10, when the rising and encroaches on the dark lunar night. planet glows at magnitude 7.8 Let’s focus on the scene the evening of and spans 2.4" when viewed September 4. If you look just south of the lunar through a telescope. equator, three magnificent craters will grab your Neptune peaks attention. Northernmost Theophilus has the Mädler most classic shape of the trio: It showcases a Cyrillus sharp, almost perfectly circular rim with a dra- matic multiple peak at its center. Because it is Catharina N in Aquarius that follow close on their heels, much bigger and deeper than the crater Mädler Neptune’s can be a riveting experience. immediately to its east, Theophilus’ walls have distinct color slumped into terraces that appear most notice- E The giants of the You won’t have to strain at shows up in With Jupiter on display for solar system take all to see Jupiter. The giant amateur scopes, only a few hours each evening, able on its western flank. These three large craters form a stunning group when center stage this month. While planet shines brilliantly in though the however, only a limited number You can make some educated guesses as to the Sun first lights them up the evening of September 4. cloud features the relative ages of lunar features even without CONSOLIDATED LUNAR ATLAS/UA/LPL; INSET: NASA/GSFC/ASU Jupiter and Saturn stand out southern Ophiuchus. It stands that Voyager 2 of events are visible. Your first every September evening and 25° high in the south-southwest saw in 1989 are good look comes September 4, the tools of a geologist. Theophilus must be deserve the lion’s share of your an hour after sunset in early out of reach. when Io transits Jupiter. The younger than neighboring Cyrillus to the south- Full Moon, this topographic detail vanishes. NASA/JPL attention, Neptune ranks a September and 20° high in the innermost moon first touches west because it overlays the latter crater’s ragged Mädler seems to shelter the terrain to its east close third as it reaches peak southwest at the same time as the planet’s limb at 8:04 p.m. rim. Although Cyrillus also has a complex peak from this debris, hinting at its intermediate age. and slumped walls, the impacts of smaller objects visibility. The distant world the month closes. During the EDT, and its shadow follows The southernmost member of the crater trio over the ages have degraded them. Also note is Catharina. Once upon a time, it must have skims past a relatively bright same period, Jupiter fades from at 9:21 p.m. Both moon and that the debris apron surrounding Theophilus been sharp-featured like Theophilus, but billions star, making it easier to find magnitude –2.2 to –2.0. This shadow take about 130 minutes partially fills Cyrillus. The apron’s rough texture of years of bombardment have erased the cen- than usual. Uranus lies farther makes the planet some 20 times twilight, when the planet is Jupiter reveals a wealth of to transit the jovian disk. is obvious at this phase. Under a high Sun at tral peak and left its walls lower and softer. east and makes a worthy morn- brighter than Antares, the red high enough to avoid the poor detail through any scope. Even Giant Ganymede’s shadow ing object. Finally, Mercury and supergiant star that lies 8° to seeing conditions that typically an untrained eye will notice crosses Jupiter’s north polar Venus appear in evening twi- Jupiter’s lower right. The First interfere with objects near the two dark equatorial belts, one region starting at 11:22 p.m. light at the end of the month. Quarter Moon stands 4° to horizon. This gives you a cou- on either side of a brighter zone EDT on September 5. Although Catch the false dawn Let’s start our tour in the Jupiter’s right and 7° above ple of hours of prime observing coinciding with the equator. the gas giant is setting along the METEOR WATCH I evening sky with the toughest Antares on September 5. in early September and at least The gas giant’s disk measures East Coast, observers farther targets. Both Mercury and Jupiter appears best through an hour of good views as the 37.4" across the equator and west will get nice views of the The zodiacal light shines on autumn mornings Solar system dust particles cast an eerie glow into the predawn sky in Venus make fleeting appear- a telescope starting in late month winds down. 35.0" through the poles at mid- shadow’s 153-minute transit. late September and early October. ances in bright twilight as month, an obvious difference Jupiter’s four bright moons BARRY BURGESS September draws to a close. On once you know to look for it. all orbit in the planet’s equato- A distant world’s close encounter the 30th, Venus stands 1° above Lower-contrast details pop rial plane, so they usually line the western horizon 30 minutes into view along the edges of up. But this symmetry breaks or zodiacal light, on after sunset. Gleaming at mag- N the equatorial belts during down once in a while. Watch September mornings. nitude –3.9, it shows up only PISCES moments of steady seeing. the evening of September 19, This faint, cone-shaped glow has a broad base in because it shines so brightly. Jupiter’s fast spin drives these and you’ll see Ganymede pass Leo and tapers as it climbs You might see magnitude –0.2 features. The giant planet’s 30" due north of Callisto. through Cancer and Gemini Mercury one binocular field to equatorial regions rotate once Saturn lies 30° east of into Taurus. The glow arises Venus’ upper left. Jupiter and trails some two every 9 hours 50 minutes, tak- from sunlight reflecting off The inner planets fare poorly E φ ing five minutes less than at hours behind its sister world. fine dust particles in the because the ecliptic — the path Neptune higher latitudes. This lag is great news for Saturn plane of the solar system, of the Sun and planets across Any telescope also reveals watchers because it places the which is why it aligns with the sky — makes a shallow Jupiter’s four bright moons. All planet at its highest in the south the ecliptic. To see the light, angle to the western horizon AQUARIUS χ λ four typically show up against as darkness falls and keeps it on you need to observe from a after sunset from mid-northern ψ2 the blackness of space near the view past midnight. Saturn lies dark site shortly before twi- latitudes. Thus, the angular ψ1 planet, but occasionally one or in Sagittarius, just south of the THE ECLIPTIC’S LOW ANGLE — the ecliptic stands nearly light begins. Catch the false separation of a planet from the ψ3 1° more hides in plain sight as it constellation’s Teaspoon aster- to the western horizon after sun- straight up from the eastern hori- dawn’s ethereal glow on Sun translates mostly into dis- crosses in front of the planet. ism. The magnitude 0.4 ringed set, which keeps Mercury and zon before sunrise. This solar sys- moonless mornings, which tance along the horizon and not Neptune lies in eastern Aquarius at opposition the night of September 9/10, Watching these transits, and world shines nearly four times Venus immersed in bright twilight tem geometry affords observers run from September 27 to into altitude. just 6' from 4th-magnitudeASY-SM0919_10 Phi (ϕ) Aquarii. ALL ILLUSTRATIONS: ASTRONOMY: ROEN KELLY the far easier shadow transits — Continued on page 42 all month, has a favorable flip side great views of the false dawn, October 11 this year. 2 ASTRONOMY INSIGHTS • SEPTEMBER 2019 Visible to the naked eye Visible with binoculars Visible with a telescope RISING MOON I A terrific trio of terraced craters SKY THIS MONTH THE WAXING CRESCENT MOON offers a THE SOLAR SYSTEM’S CHANGING LANDSCAPE AS IT APPEARS IN EARTH’S SKY. Theophilus, Cyrillus, and Catharina BY MARTIN RATCLIFFE AND ALISTER LING smorgasbord of fantastic views. From the slen- derest of arcs to First Quarter phase, Luna shows off rolling seas bordered by craters with towering OBSERVING peaks and aprons of debris blasted out during the impacts that created them. The best detail September 2019 HIGHLIGHT Theophilus appears along the terminator, where the Sun is NEPTUNE reaches its 2019 peak September 9/10, when the rising and encroaches on the dark lunar night. planet glows at magnitude 7.8 Let’s focus on the scene the evening of and spans 2.4" when viewed September 4. If you look just south of the lunar through a telescope.
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