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29 Aug 2019

September Astronomy: Follow the Fainter-Than-Normal Moon as Autumn Arrives (Finally!)

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From Sagittarius to Gemini and back, the moon swings! And the crescent rocks!

Enjoy watching moonrises? The harvest moon on Friday the 13th is the first of a half-dozen moonrises in a row taking place in the early evening, through Sept. 18. Meanwhile, Jupiter shines steady and brightest at dusk, and Sirius, the “Dog Star,” twinkles brightest at dawn.

Overnight on Sept. 22—actually at 12:50 a.m., Monday, Sept. 23—the sun is directly over Earth’s equator, marking the start of autumn for residents of the Northern Hemisphere. On the date of an equinox, the sun rises in the east, and sets in the west 12 hours later. (Well, this is not precisely true, because of the way sunrise and sunset are defined—when the top of the solar disk, rather than its center, appears on an ideal, flat horizon; refraction by our atmosphere uplifts the sun’s disk and lengthens the day by several minutes.)

The moon travels around the Earth in an orbit inclined 5 degrees to the Earth’s orbital plane, traveling through the same belt of the zodiac and returning to the same stars after only 27.3 days, the sidereal period of the revolution of the moon. So in less than four weeks, the moon’s rising and setting places on the horizon and height at mid-path range from southern to northern extremes and back.

On Saturday, Sept. 7, the moon “rides low,” reaching the southernmost part of its 27.3-day trip around the zodiac, in the constellation Sagittarius, the Archer. From Palm Springs, the moon rises nearly 28 degrees south of east at 3:10 p.m. It reaches its highest point in south at 8:16 p.m., only 33 degrees above the horizon.

As the moon passes south on Sept. 7, look for Saturn 5 degrees to its left; Jupiter 23 degrees to the moon’s right; and the third-magnitude star Kaus Borealis, northern star of the Archer’s bow, within 3 degrees to Luna’s lower right. September’s southernmost moon sets nearly 28 degrees south of west later that night.

Might we call the lunar event of Sept. 7-8 a “Lunastice”? There are two solstices each year, when the “sun stands still” directly over the Tropic of Capricorn and the Tropic of Cancer at the southern and northern extremes of the sun’s annual journey around the zodiac. But the moon takes just 27.3 days to make the same circuit. That’s about two days less than the moon’s 29.5-day cycle of phases, so each successive time the moon “rides low” (or “rides high,” or returns to the same star background) occurs 2.2 days earlier in its phase cycle. Since the moon makes just more than 13 trips around the zodiac annually, there are 26 or 27 “Lunastices” each year, spaced at intervals averaging just less than two weeks.

After Sept. 7, the moon rises farther north, getting later each day. By Sept. 11, the moon rises 19 degrees south of east at 6:04 p.m., some 55 minutes before sunset, so it’s still possible from some places in the Coachella Valley to view sun and moon simultaneously. On Sept. 12, the moon rises 15 degrees south of east at 6:37 p.m., just 20 minutes before sunset. For the next five nights through Sept. 17, moonrise occurs only about a half-hour later on each successive night. Short daily delays in the time of moonrise, significantly less than the long-term average of 50 minutes per day, always happen around the full moon near the beginning or autumn, and is called the Harvest Moon Effect.On Friday, Sept. 13, the full moon rises 10 degrees south of east at 7:07 p.m., just 11 minutes after sunset.

This month’s full moon also happens to be the most distant of this year. It’s also the faintest of all the full moons (except for those eclipsed by Earth’s shadow)—not only because of its distance, but also because it passes 5 degrees south of the point opposite the sun and reflects less of its light back toward Earth. (Look up “opposition effect.”) By Sept. 15, the moon rises about one degree north of east at 8:05 p.m., and by the 17th, 12 degrees north of east at 9:04 p.m. September’s northernmost moonrises occur on Saturday evening, Sept. 21, at 11:44 p.m., and on the next night, Sunday, Sept. 22—actually Monday morning, Sept. 23, at 12:40 a.m., some 27 degrees north of east. On both occasions, the moon “rides high” in Gemini, passing within 12 degrees south of overhead on Sept. 22 at 7:04 a.m., and on Sept. 23 at 8:01 a.m., setting 7 hours and 20 minutes later.

Follow the moon at dusk through Sept. 14, and again Sept. 29-Oct. 13. Follow the moon at dawn Sept. 13-27. Note the “tipped bowl” orientation of the waxing lunar crescents in the western sky at dusk through Sept. 3 and Sept. 29-Oct. 2, preceding the nearly first-quarter (half) moons found low in the southern sky around sunset on Sept. 5 and on Oct. 4 and 5. Note the “upright bowl” waning lunar crescents in the eastern sky at dawn Sept. 25-27, following the nearly half-moon close to last-quarter phase found high in the southern sky around sunrise on Sept. 21 and 22. Best views of zodiacal light (reflected off comet and asteroid dust in the plane of the solar system) can be had from dark places free of light pollution just before start of morning twilight, 90 minutes before sunrise, Sept. 27-Oct. 11. All these observations are related!

The only planets easy for the unaided eye to see in September are both conveniently placed in the southern sky in the early evening: Bright Jupiter, in the south-southwest at dusk; and Saturn, in the south-southeast to south, 29 to 26 degrees east (left) of Jupiter. Telescopes show Jupiter’s disk, now just over 0.01 degrees in apparent diameter, usually with two dark equatorial belts, and the four bright Galilean satellites. Since their orbits lie nearly in plane of Jupiter’s equator, and Jupiter’s south pole is tipped more than 2 degrees toward Earth for rest of 2019, it’s often possible to mentally work out whether a satellite of Jupiter is on the near or far side of its orbit. Currently, if a moon is displaced north of a line through Jupiter’s center parallel to its equator, then that moon must be on the near side of Jupiter. If displaced south, it must be on far side of its orbit. Jupiter’s equator will appear edge-on in early 2021, and so, nearly, will the orbits of the satellites, making them appear in a straight line.

Saturn’s rings span slightly wider than Jupiter’s apparent size. Saturn’s north pole is now tipped toward Earth by more than 25 degrees, giving us our best view of the rings until 2030, when the south pole will be tipped toward us by a similar angle. Before then, the rings will be edgewise to the Earth and sun during spring 2025. Maximum tilt of rings, nearly 27 degrees with north face visible, occurred in 2017; the maximum with south face visible will occur in 2032.

There are no naked-eye planets in September’s morning sky, but the brightest star visible is Sirius, in the southeast to east-southeast as dawn brightens. Confirm by noting that the three-star belt of Orion—between bluish Rigel marking his foot and reddish Betelgeuse his shoulder—points directly to Sirius.

For a preview of the wonderful gatherings of the moon and planets in 2019-2020, visit www.abramsplanetarium.org/msta. Abrams Planetarium publishes a monthly Sky Calendar with an evening sky map. The calendar illustrates many of the present and future events described above. Subscriptions are $12 per year at www.abramsplanetarium.org/skycalendar for three printed issues mailed quarterly.

Check the website of the Astronomical Society of the Desert at www.astrorx.org for dates and times of our evening star parties at two locations: Sawmill Trailhead, our high-altitude site (elevation 4,000 feet), will have its next monthly sessions starting at dusk on Saturday, Aug. 31 and Sept. 28. Monthly sessions resume Saturday, Oct. 5, at our primary, more accessible location, the Visitor Center of the Santa Rosa and San Jacinto Mountains National Monument (on Highway 74, within 4 miles south of Highway 111 in Palm Desert).

But don’t wait until then! You can pre-register for one of the Stargazing Parties offered several times each month at the Rancho Mirage Library Observatory. Visit www.ranchomiragelibrary.org/observatory.html; click on Stargazing Parties, then sign up for their eNewsletter to receive registration information. If you go farther afield, check out the Sky’s the Limit Observatory and Nature Center in Twentynine Palms, at www.skysthelimit29.org.

Wishing you clear skies!

Robert C. Victor was a staff astronomer at Abrams Planetarium at Michigan State University. He is now retired and enjoys providing sky watching opportunities for a variety of groups in the Coachella Valley. Robert D. Miller 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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