Saturn’s Bounty Saturn’s spectacular rings and fascinating moons make it a favorite among observ- ers. See inside for an observing guide, as well as S&T articles on the rings, Titan, and the rest of the bizarre satellite family. OBSERVING Sun,OBSERVING Moon & Planets May’s Sun, Moon & Planets May’s Amazing Planet Trio The three brightest planets form an extremely tight group at dusk. Sun, Moon & Planets Saturn is well placed for viewing almost On May 19th, Jupiter, Venus, and Mer- upper right of Venus. The trio is most all night throughout May. And a remark- cury form a line less than 13° long. Mer- compact on the evening of May 26th in able planetary drama unfolds in the last cury is only about 3° high a half hour after the Americas, when the three planets fit third of the month: Venus, Jupiter, and sunset, so you may need binoculars to spot within a circle less than 2½° wide. This Mercury converge into a very tight knot it even though it shines at magnitude –1.4. is also when Jupiter and Mercury are low in the evening twilight. Venus is about 4° upper left of Mercury, closest. On May 27th, Venus and Jupiter and Jupiter 9° upper left of Venus. are side by side 1¼° apart with Mercury DUSK The line shortens each evening as less than 22/3° above them. Then on May May begins with Jupiter high and Venus Mercury appears higher and Jupiter lower. 28th, Venus and Jupiter, the two brightest low in the west-northwest shortly after From May 24–29, Venus, Jupiter, and planets, are only 1° apart. sunset. Venus sets in bright twilight, just Mercury form a “trio” — a temporary Compare the disks of the three planets ¾ hour after the Sun. Jupiter, a huge 27° gathering of three celestial objects that fits in a telescope while they’re a trio: Jupiter is to Venus’s upper left, remains visible long within a circle 5° or less wide. The event 32″ wide and fully lit, Venus 10″ wide and after the sky is dark. But Jupiter appears takes place fairly low in bright twilight, about 96% lit, while Mercury’s gibbous about ¾° lower each evening, while Venus but it’s readily visible because all three disk alters subtly from 5.7″ and 80% lit to creeps a little higher, as they head toward planets are very bright. Mercury fades 6.2″ and 68% lit. Try to catch the planets conjunction on May 28th. just a trace, from magnitude –0.9 to –0.7, immediately after sunset, while they’re as Venus and Jupiter form lovely configu- while Venus and Jupiter hold steady at high as possible above the horizon. rations with the crescent Moon on May magnitude –3.9 and –1.9, respectively. For All good things must come to an end. 10–12, as shown below. skywatchers around latitude 40° north, all On May 31st the three planets are again Mercury is invisible for the first half of three planets are more than 6° above the in a diagonal line, but now it’s expand- May, passing through superior conjunc- west-northwest horizon a half hour after ing — Venus is 4° lower right of Mercury tion behind the Sun on May 11th. But it sunset throughout the trio. and Jupiter 3½° lower right of Venus. And joins the evening planet scene just eight Venus and Mercury appear closest Jupiter, falling out of the gathering, starts days later. together on May 24th, with Mercury 11/3° setting less than an hour after the Sun. These scenes are Capella Dusk, May – Dusk, May always drawn for near minutes after sunset minutes after sunset the middle of North America (latitude 40° north, longitude Moon 90° west); European May observers should Capella move each Moon Jupiter ° AURIGA Betelgeuse symbol a quarter of the way toward the Moon one for the previ- May ous date. In the Far β Tau East, move the Moon Aldebaran Use binoculars for ζ Tau halfway. The blue 10° Jupiter very young Moon! scale bar is about Mercury Moon the width of your fist May Venus Venus at arm’s length. For clarity, the Moon is Rigel shown three times its Looking West-Northwest Looking West-Northwest Looking Northwest actual apparent size. 1 sky & telescope To see what the sky looks like at any given time and date, go to SkyandTelescope.com/skychart. Fred Schaaf December solstice See skypub.com/may2013planets for Venus Mars an animation of the planets’ changing Mercury configurations. March equinox Sun Sept. equinox DUSK THROUGH MOST OF NIGHT Earth Saturn was at opposition on April 28th, June solstice so in May it remains bright, bigger than usual in telescopes, and visible almost all night long. The planet shrinks and Jupiter dims just a little, ending May at magni- Uranus tude +0.3. But it climbs high earlier in Saturn the night. At dusk on May 31st Saturn is Neptune already well up in the south-southeast, shining about one-third the way up the sky. Saturn retrogrades out of Libra into Pluto easternmost Virgo this month. Its separa- tion from Spica decreases from 15° to 13°. ORBITS OF THE PLANETS The curved arrows show each planet’s movement DAWN during May. The outer planets don’t change position Uranus, in Pisces, and Neptune, in over the Pacific, but the path of annularity enough in a month to notice at this scale. Aquarius, are highest at dawn; see skypub. crosses northwestern Australia, and the com/urnep for finder charts. partial phase is visible in most of Mela- star Beta Scorpii in the southeastern U.S. Pluto, in Sagittarius, is highest around nesia, Australia, and New Zealand. See and elsewhere; see page 52. the beginning of morning twilight; next skypub.com/may2013eclipses for details on The Moon is a very thin crescent just month’s issue will contain finder charts. this and the lunar eclipse described below. lower left of Venus on May 10th very low Mars rises too soon before the Sun to The Moon undergoes an extremely in the west-northwest a half hour after be observed. slight penumbral eclipse on the American sunset. It passes Jupiter on the 11th and night of May 24–25, probably undetect- 12th. The waxing gibbous Moon is just SUN AND MOON able even by the most sensitive scientific to the right of Spica on the American The Sun undergoes an annular eclipse instruments. But there’s a consolation that evening of May 21st and near Saturn on on May 9–10. The eclipse is visible mostly night: the full Moon occults the double the 22nd. ✦ Dusk, May Dusk, May Dusk, May minutes after sunset minutes after sunset minutes after sunset AURIGA Capella Capella AURIGA AURIGA Capella β Tau Mercury Mercury β Tau Mercury ζ Tau Jupiter Venus Venus β Tau Jupiter Venus ζ Tau Jupiter ζ Tau Looking West-Northwest Looking Northwest Looking Northwest Looking Northwest SkyandTelescope.com 2 Ringworld Revelations By Matthew S. Tiscareno Saturn’s rings are arguably the flattest structure known to science. From end to end they would reach from Earth to the Moon, yet they’re hardly taller than a troupe of acrobats stand- ing on one another’s shoulders. By analogy, if the rings were as thin as a sheet of paper, they would stretch 3 kilometers (2 miles) across. All four giant planets in our solar system have rings, but only Saturn has a bright and mas- sive ring system. Is this uniqueness a matter of chance, or something more? What can Saturn’s rings tell us about the origin of our solar system and similar processes elsewhere in the galaxy? 3 Sky & Telescope Cassini’s high-resolution camera took 126 images of Saturn and its rings on October 6, 2004, which scientists assembled into this natural-color mosaic. All images are courtesy the Cassini Imaging Team, NASA/JPL, and the Space Science Institute unless otherwise credited. Revelations Saturn’s rings are reluctantly giving up their secrets to Cassini’s prying eyes. Sky & Telescope 4 pringworld revelations These questions, and more, motivated the development of NASA’s Cassini spacecraft, which was launched in 1997 and entered Saturnian orbit in July 2004. Along with map- ping methane lakes on Titan’s surface, catching Enceladus spewing icy geysers, and tracking storms in Saturn’s atmo- sphere, Cassini has conducted extensive studies of the ring system and its environment. These observations have shed new light on the nature of the ring particles, how they in- teract with one another and with the planet’s moons, and the role of fine dust within the ring system. And, as always with exploration, Cassini’s revelations are prompting us to ask even more questions, as mysteries we didn’t know about come to our attention. Saturn’s main rings are divided into the outermost A ring, the dense and bright B ring, and the inner, more deli- cate C ring. The Cassini Division between the A and B rings is not empty but contains tenuous material analogous to the C ring. The F ring is a wild and woolly narrow loop just outside the A ring, while three dusty, diaphanous rings are designated D (inside C), G (outside F), and E (outside G). Moons and Disks Saturn’s rings provide our only way to get up close and per- Even though their scales and origins differ, spiral arms arise in both sonal with an astrophysical disk. Astronomers have found Saturn’s rings and galaxies. Resonant effects from larger, more distant many nearby stars encircled by young gas disks or slightly moons generate spiral density waves in the rings, where particles bunch older dust disks; presumably our solar system once looked up in a rhythmic progression.