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Notes for the Season features information on meteor showers, space exploration events, and astronomy-related anniversaries from October through December 2017.
Sunrise Solar Noon Sunset
October 1 7:06 am PDT 12:59 pm PDT 6:52 pm PDT
November 1 7:36 am PDT 12:53 pm PDT 6:11 pm PDT
December 1 7:07 pm PDT 11:59 am PDT 4:51 pm PDT
(Times are for San Francisco, CA, and will vary slightly for other locations.)
Notes for Fall
Like Earth, Saturn has a constant axial tilt that—as seen from Earth—causes the planet to teeter back and forth each time it orbits the Sun. The most obvious sign of this is our view of the rings, as we see them tip up and down every 29 years, alternately presenting their northern or southern face. On October 1, Saturn's rings are tilted their maximum 27 degrees from our line of sight, offering the most wide-open view of their northern face. The last time the rings were seen this open was in 2003, when we saw the southern face, and the next time will be in 2032, when we'll see the southern face again.
The Space Age was born on October 4, 1957, with the launch of Sputnik I from the Soviet Union. The first artificial satellite to circle Earth, Sputnik was about the size of a beachball and in the intervening years has been followed by thousands of satellites, the largest being the football field-sized International Space Station, and the smallest a new generation of ice-cube size "Sprites." According to the Union of Concerned Scientists, at any one time, there are approximately 1,500 operational satellites circling the planet, but counting decommissioned satellites, spent rocket boosters, and trackable chunks of debris (including a runaway toolbox from the International Space Station), there are more than 13,000 objects up there. On any clear, dark night, patient skywatchers can catch at least a few satellites orbiting silently overhead in the first hour or two of the night.
On October 9, 1933, dragon fire filled the night sky as the Draconid meteor shower produced a storm of impressive proportions, with estimated rates of up to 6,000 meteors per hour, radiating from the stars of Draco the Dragon. Active October 6-10 and peaking on the 9th, this shower is highly variable, producing displays from as modest as 1-2 per hour to dramatic storms (>1000 per hour), as in 1933 and 1946, both years that corresponded to the periodic return of the shower's parent comet, 21/P Giacobini-Zinner, which rounds the sun every 6.6 years. The comet's next return is expected in September 2018, which leads some observers to forecast at least elevated numbers of meteors during the shower's peak, which coincides next year with a new Moon.
Uranus at opposition on October 19. Discovered in 1781 by Sir William Herschel, who first thought it was a comet, Uranus is arguably visible to the unaided eye, but only to observers with excellent eyesight and under ideal, dark conditions. Opposition places the planet opposite the Sun in the sky, so it rises at sunset—on this occasion against the stars of the faint Zodiacal constellation Pisces the Fishes. This is also when Earth and Uranus are closest together (though still separated by two billion miles), causing Uranus to appear at its brightest. It's located just inside the left-hand (or northern) side of the "V" formed by the stars of Pisces, about a third of the way up from the point. In binoculars and telescopes, it looks like a distinctly-greenish point standing out prominently in comparison to the stars in the background.
The Orionid meteor shower peaks on October 20. It usually produces as many as 35 meteors per hour under ideal conditions, and these seem to radiate from the club of Orion the Hunter. This year's peak coincides with a new Moon, which is about as ideal as observing conditions get, excluding a trip to the high desert or mountains to get away from city lights. This is one of two showers caused by dust particles shed from the nucleus of Halley's Comet, as Earth intersects the comet's orbit twice each year—in May (resulting in the Eta Aquarid meteors) and October.
Active November 14-20 and peaking on November 17, the Leonid meteor shower has been responsible for some of the most spectacular meteor storms in history, defined as displays with rates of 1000 or more meteors per hour—about 100 times the usual rate. But those occasions usually coincide with the return of the shower's parent comet, 55/P Tempel-Tuttle, which has a 33-year orbital period. The regularity of the storm was determined in 1864 and accurately predicted for 1866, even though the progenitor comet remained unknown—but not for long. The comet was discovered in 1867 and after its orbit was determined, it was quickly associated with the annual displays. About every 33 years since then, astronomers have predicted elevated meteor rates, and in 1966, perhaps the greatest meteor storm of modern times was seen from the western U.S., during which an estimated 50 meteors per second were counted during a 15-minute burst. In 1999, a rate of 3,000 per hour was estimated. This year...not so much—even with the new Moon, don't expect more than the usual 10-15 per hour away from city lights, radiating from the backward question-mark that represents the head of Leo the Lion.
Will the Geminid meteors put on a good show this year? The shower's peak on December 13-14 comes just a few days before new Moon, when the Moon is a thin, waning crescent that rises shortly before dawn. This means it's not at such a bright phase that its light will interfere much with viewing. The Geminids seem to radiate from the vicinity of the constellation Gemini the Twins, which rises in the early hours of the evening and so is visible for most of the night. With the shower's radiant rising about an hour after sunset, the geometry of Earth's rotational position as it encounters the Geminid dust stream presents the opportunity to catch an "Earth-grazing" meteor. This is a rare meteoroid that skims horizontally across the top of the atmosphere early in the evening, leaving a long trail across the sky. Closer to dawn is when more meteors will be seen associated with the shower, although as the radiant rises higher into the sky, the meteors will seem short and swift—expect to see about 50 per hour away from city lights. The shower made itself known in the 1800s, but its source was unknown until 1983, when an asteroid was found matching its orbit. Subsequent observations established that the asteroid, named 3200 Phaethon, is actually a strange variety of "rock comet" that ejects dust rather than ice).
The "shortest day of the year" for the northern hemisphere occurs on December 21, also known as the winter solstice, generally recognized north of the equator as the start of winter. Not that the day is any less than 24 hours long—Earth's rate of rotation hasn't changed. It's just that our planet's axial tilt causes the north pole to be tipped farthest away from the Sun at this time. In the northern hemisphere, this means the Sun rises and sets at its farthest points south and follows a short, low arc across the sky, spending the least amount of time over the horizon (that's where the "shortest day" comes from, so it should really be "shortest daylight"). In the southern hemisphere, where seasons are reversed, December 21 is the summer solstice.
Asteroid discoveries made October-December
2620 Santana (1980), 214476 Stephencolbert (2005), 3623 Chaplin (1981), 9007 James Bond (1983), 15907 Robot (1997), 125071 Lugosi (2001), 115561 Frankherbert (2003), 10221 Kubrick (1997), 12426 Racquetball (1995), 9951 Tyrannosaurus (1990), 8080 Intel (1987), 4749 Ledzeppelin (1989), 11548 Jerrylewis (1992), 19367 Pink Floyd (1997), 55555 DNA (2001), 342843 Davidbowie (2008).
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