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29 Sep 2020

October Astronomy: The Month Brings Us a Blue Moon—on Halloween, No Less!

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October presents two full moons, on the first and last day of month, as well as Venus in a close pairing with Regulus before dawn on Oct. 2 and 3; a close approach and the peak brilliance of Mars; Jupiter closing in on Saturn; and four close pairings of the moon with bright planets.

Mars, visible during nearly all of October’s nighttime hours, presents its closest and brightest (peak magnitude -2.6) approach to Earth until 2035. Mars reaches its least distance of 38.57 million miles from Earth (light travel time: 3 minutes, 27 seconds) on Oct. 6, and reaches opposition on Oct. 13, as Earth overtakes the red planet. Mars even outshines Jupiter for most of month.

On Oct. 13, it’s early summer in Mars’ southern hemisphere, and little remains of the south polar cap, since the warmth of the sun has caused most of the cap’s frozen carbon dioxide to sublimate into the Martian atmosphere. But Mars’ most-prominent dark feature, Syrtis Major, first recorded in 1659 by Christiaan Huygens, is in good view for 12 consecutive nights, 36 minutes later nightly. The dusky triangular expanse of dark volcanic basaltic rock passes most closely north of the center of the Martian disk on Oct. 7 at 9:28 p.m.; Oct. 8 at 10:04 p.m.; Oct. 9 at 10:40 p.m.; and continuing about 36 minutes later daily until Oct. 19 at 4:05 a.m., when Mars is getting low in the west-southwest. Of those dates and times, Mars appears highest in the south on the mornings of Oct. 13 and 14. Find more resources for the telescopic viewing of Mars at www.abramsplanetarium.org/msta.

Also in the evening sky, Jupiter (magnitude -2.4 to -2.2) and Saturn (+0.5 to +0.6) are paired in the south to south-southwest at dusk, east of the Teapot of Sagittarius. In mid-October, our Spaceship Earth races away as each appears 90 degrees east of the sun—first Jupiter, on Oct. 10-11, and then Saturn, on the following weekend. Both are shrinking in apparent size—Jupiter faster, because it’s closer. By October’s end, the extent of Saturn’s rings begins to exceed Jupiter’s equatorial diameter. It’s a good month to notice the shadow of Saturn cast upon its rings. With binoculars, note the kite-shaped asterism Territory of Dogs, 2.1 by 1.1 degrees, within 7 degrees to the lower left of Saturn.

Countdown to their great conjunction: Jupiter and Saturn are7 degrees apart on Oct. 7; 6 degrees on Oct. 21; 5 degrees on Nov. 2; 4 degrees on Nov. 13; and 3 degrees apart on Nov. 23. Jupiter-Saturn will appear within a degree Dec. 12-29, and will be just 0.1 degree apart on Dec. 21, their tightest pairing between 1623 and 2080. Jupiter takes about 12 years to make one trip around the sun, and Saturn takes nearly 30 years. So with each passing year, Jupiter progresses about 30 degrees, and Saturn 12 degrees, around the sun—so Jupiter gains 18 degrees on Saturn. It therefore takes about 20 years for this annual gain to accumulate to 360 degrees, when Jupiter will overtake Saturn again. There will be conjunctions of Jupiter and Saturn in 2040 and 2060, but they’ll still be about 1.1 degrees apart.

The brilliant morning “star” Venus (magnitude -4) rises in the east about three hours before the sun. Don’t miss the close pairings of Venus and the star Regulus on Oct. 2 (0.5 degrees apart) and Oct. 3 (0.7 degrees apart). Regulus marks the heart of Leo, the Lion. Venus goes 1.2 degrees per day, or 6 degrees each 5 days, eastward against the stars. Observing Venus and Regulus daily through Oct. 7 an hour before sunrise can be quite engaging, as the changes from one day to the next will be easy to notice.

While you’re out on those mornings, follow brilliant Mars, dropping a little lower day by day, in the west-southwest to west; the brightest star, blue-white Sirius, in the south-southeast to south; and the moon, starting when it pops into view as full, low in the west on Oct. 1, an hour before sunrise. In next two weeks through Oct. 15, watch the moon move west to east through the zodiacal band steeply inclined to the horizon, waning and passing, in order, Mars; the Pleiades, or Seven Sisters, star cluster; Aldebaran, eye of Taurus; Castor and Pollux, twin stars of Gemini (in mid-October, Spaceship Earth is racing toward these twin stars); Regulus, heart of Leo; and Venus. Also visible in October’s morning sky is Orion, the Hunter, high in the south to south-southwest. His striking three-star belt points in one direction to the Dog Star, Sirius, and in the other, to Aldebaran and the Pleiades. Local sky-watchers can try to see Canopus, shown on our morning twilight map and passing due south, just 3 degrees above our horizon in the Coachella Valley. It’s the star next in brightness after Sirius and reaches its high point in the south four minutes earlier each day, some 22 minutes before the Dog Star.

The moon near bright planets: On night of Oct. 2, shortly after 8 p.m., the moon is 1 1/4 days past full, and its center passes only 1.4 degrees south (to the lower right) of Mars. The moon and Mars gradually spread apart for rest of night, until sunrise on Oct. 3, when you’ll find Mars about 4 degrees to the lower right of the moon. On Oct. 13 and 14, during the hour leading up to sunrise, Venus appears 7 to 8 degrees from a waning crescent moon. From sunset until an hour later on Oct. 22, find Jupiter 5 degrees to the upper right of a fat waxing crescent moon, nearly half full, and Saturn within 4 degrees to the moon’s upper left. From sunset until an hour later on Oct. 29, find Mars about 5 degrees to the upper right of a nearly full moon.

On Saturday, Oct 31, the full and so-called “blue” moon occurs at 7:49 a.m. This is the second full moon within the same calendar month, and so designated a “blue moon” by one definition; it’s the smallest full moon of this year. But by a prior definition, this not a blue moon, and the next will actually be on Aug. 22, 2021. The earlier definition of a blue moon is the third full moon of four within an astronomical season. There will be four full moons in the summer of 2021—on June 24, July 23, Aug. 22 and Sept. 20, so the third one, on Aug. 22, will be a blue moon.

The small apparent size of Halloween’s full moon (called a “micro-moon” in some media accounts) is a consequence of the moon’s passage through apogee, the most distant point in its orbit, on Oct. 30 at noon, 252,522 miles from Earth.

The Abrams Planetarium’s monthly Sky Calendar now has more than 3,000 subscribers nationwide. I originated the Sky Calendar in October 1968 and produced the October 2020 issue. For more information about Sky Calendar and a sample, visit www.abramsplanetarium.org/skycalendar.

To check for eventual resumption of star parties hosted by the Astronomical Society of the Desert, visit the club’s website at www.astrorx.org. As of now, the parties have been cancelled through the end of the year.

Wishing you clear skies!

Robert Victor was a staff astronomer at Abrams Planetarium at Michigan State University. He is now retired and enjoys providing informal sky watching opportunities for folks in and around Palm Springs. 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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