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Mon06272016

Last updateSun, 30 Aug 2015 2pm

Astronomy

Now that the Earth has overtaken all the bright outer planets (with the final one being Saturn, reaching opposition on June 2), we can conveniently follow them at dusk for several months: Jupiter until start of September, Saturn until Thanksgiving, and Mars until end of May 2017.

Venus passes behind the sun on June 6, and by middle of July will emerge into our early evening sky very low in the bright west-northwest twilight glow shortly after sunset. Binoculars will help you spot Venus very low in twilight in its first weeks, and can reveal Mercury nearby from mid-July until late August. That’ll bring the total to all five bright planets visible simultaneously!

Our chart depicting the sky at evening mid-twilight follows positions of naked-eye planets and stars of first magnitude or brighter in the sky when the sun is 9 degrees below the horizon, roughly 45 minutes hour after sunset.

Jupiter starts high in the southwest in early June 2016, with faint Regulus, heart of Leo, preceding it as they move toward the western horizon.

Red Mars starts out unusually bright while low in southeast in early June, attracting attention to the triangle it forms with nearby Saturnand the red twinkling star Antares, heart of the Scorpion. The triangle crosses through south in late July-early August, and morphs into a spectacular, compact straight line in south-southwest on Aug. 24, as Mars passes directly between Saturn and Antares.

The Summer Triangle of Vega, Deneb and Altair rises in the eastern sky in June, ascending to nearly overhead in August. Two other bright stars, Arcturus and Spica, are highest in the southern sky in June, moving into west and west-southwest by the end of August.

With the unaided eye and binoculars, follow the motions of planets against the background stars. In mid-May, Mars passed closely north of a second-magnitude star, Delta Scorpii, the middle and brightest member of a nearly vertical slightly curved line of three stars in the head of Scorpius, to the west of Antares. Mars retrogrades, or goes westward, until ending nearly 10 degrees west of Delta Sco on June 29. Mars barely moves for several evenings, and then resumes eastward or direct motion, passing closely south of Delta Sco on Aug. 9.

Jupiter is moving eastward against the background. Note the first-magnitude stars Regulus in Leo, west of Jupiter, and Spica in Virgo, well to Jupiter’s east.

Follow the moon: Its day-to-day change in position against the stars, averaging 13 degrees per day, is much more noticeable than that of the planets. Watch the moon change from a thin crescent to just past full June 6-20, July 5-20 and Aug. 4-18, while passing the planets and the bright stars plotted on our evening twilight charts.

Jupiter’s dark equatorial cloud belts and up to four of the bright satellites discovered by Galileo offer a pleasing view for small telescopes. Even binoculars can show the moons; the best chances to see at least Callisto and Ganymede, the two outermost, come on June 11 and 12 (outermost Callisto and brightest Ganymede are both west, or to the lower right, of Jupiter); June 22 (both widely east, or to the upper left, of Jupiter) and June 29 (Callisto is widely west, Ganymede widely east). Using a medium-sized telescope with higher magnification, if atmospheric conditions are steady, try to see the Great Red Spot shortly after 9 p.m. on June 6, 18 and 30.

Mars, still showing a nice disk after its opposition and close approach of late May, displays surface detail! In June 2016, it is still late winter in Mars’ southern hemisphere—the southern spring equinox (or northern autumnal equinox) will occur on July 4. Although the south pole of Mars is then on the Martian terminator (day-night boundary), that pole is now tipped 15 degrees out of Earth’s view. Yet the surrounding southern polar cap of frozen carbon dioxide (or its overlying cloud cover) is near its maximum extent, reaching halfway from pole to equator, far enough to spill over the southern limb. So whenever you look at Mars early this summer, you’ll see a bright southern edge to Mars’ disk.

The most prominent dark marking on Mars, Syrtis Major, will be in fine position when it lies astride the pole-to-pole centerline of Mars at June 24 at 8:44 p.m.; June 25 at 9:21 p.m.; June 26 at 9:59 p.m.; June 27 at 10:36 p.m.; June 28 at 11:13 p.m.; June 29 at 11:51 p.m.; and on the night of June 30 (the morning of July 1) at 12:28 a.m. (Note these opportunities occur 37-38 minutes later each night, because Mars takes more than 24 hours to complete one rotation.) South of Syrtis Major lies Hellas Basin, the deepest crater on Mars. Its surface is often covered by clouds or frost, which would add to the bright area of the polar cap extending over the southern limb of Mars’ disk.

Enjoy Saturn’s rings this summer! They’re now tipped from edge-on by at almost the greatest angle possible. Using as high of a magnification as your telescope and atmospheric conditions allow (I often use 200x with my 6-inch reflector), try for the Cassini Division dividing the outer A ring from the broader, brighter B ring. This narrow gap is nearly two-thirds of the way from the inner edge of B toward the outer edge of A. Look also for Titan, Saturn’s largest moon, taking 16 days to complete one circuit around the planet. Its nearly circular orbit is almost nine times as large as the outer edge of Saturn’s A ring, in nearly the same plane as the rings, now tipped 26 degrees from edge-on, and so appears as an ellipse. Wherever Titan is in its orbit, it’s always about four ring diameters from the nearer ring edge, if you choose the diameter of the ring system passing through Saturn’s center and pointing toward and away from Titan.

Titan will appear farthest east of Saturn on June 9 and 25, and July 11, and farthest west on June 17, July 3 and July 19. We are seeing the north face of the rings and of Titan’s orbit, so we’ll see Titan at inferior conjunction (the near side of Saturn) passing south of the planet four days after greatest elongation east, and at superior conjunction (far side of Saturn) four days after it appears farthest to the west.

The Astronomical Society of the Desert hosts monthly star parties at Sawmill Trailhead (elevation 4,000 feet). For dates, with map and directions to the site, visit www.astrorx.org.

Sky’s the Limit Observatory in Twentynine Palms offers star parties most Saturday evenings, except when moon is close to full. For details, visit www.skysthelimit29.org.

Robert C. Victor was a staff astronomer at Abrams Planetarium at Michigan State University. He is now retired and enjoys providing skywatching opportunities for school children in and around Palm Springs. Robert D. Miller, who provided the twilight charts and the planet-orbit 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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