Sky's Up July-September 2017 - Page 36

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Frequent events with Jupiter and infrequent events among its satellites
It is worth noting that with steady skies and a medium-size telescope , observers from Earth can see a variety of phenomena involving Jupiter and / or its four , large Galilean satellites . Three of the four satellites regularly participate in all three phenomena with Jupiter . Callisto has event seasons , like eclipse seasons on Earth , and is sometimes out of place for these . During Callisto ’ s seasons , all four of the Galilean satellites mutually affect the view from Earth of each other . The common phenomena are transits of two types , eclipses and occultations . A search of the internet or in monthly astronomy magazines will provide times for all of these events when Jupiter is in the night sky . Just be sure to correct the given time to your time zone . Transits are viewed on the illuminated , day side of Jupiter . The shadows of the satellites crossing Jupiter ’ s illuminated hemisphere are more easily seen . An intense black dot appears surrounded by the bright clouds . Over time , the dot moves as the shadow-casting satellite continues around its orbit . A careful observer using a good telescope under steady conditions will be able to see the satellite , itself , in transit , leading or trailing the shadow spot . ( For a few days over the months Jupiter is in the night sky , the satellite may actually overlap its shadow or be found directly above or below it .) Eclipse disappearances and reappearances around Jupiter can be observed with binoculars . They are just like lunar eclipses here on Earth , but Jupiter ’ s shadow is much darker . The satellite is visible near the planet , then disappears . Alternatively , a satellite may appear rather suddenly after it emerges from Jupiter ’ s shadow . It ’ s very difficult to see a satellites disappearance and later reappearance because Jupiter itself hides one event or the other depending on the relative positions of Jupiter , Earth and Sun . During the few days mentioned in the paragraph above , a satellite would be seen to merge with Jupiter and disappear . It would reappear as a growing on the other side of the planet and then separate from Jupiter ’ s cloud tops . During periods when only an eclipse disappearance
COURTESY OF NASA / JPL / University of Arizona This true-color composite frame , made from narrow angle images taken on Dec . 12 , 2000 , by the Cassini spacecraft , captures Io and its shadow in transit against the disk of Jupiter . The distance of the spacecraft from Jupiter was 19.5 million kilometers ( 12.1 million miles ). The image scale is 117 kilometers ( 73 miles ) per pixel .
is visible , the satellite will emerge from occultation behind Jupiter . During periods when only an eclipse reappearance is visible , the satellite will disappear into occultation behind Jupiter . During Callisto ’ s eclipse seasons ( approximately every six years ), all four of the Galilean satellites can cast shadows on each other and transit or occult each other . ( The correct word to use depends on the sizes of the involved satellites and who is doing what to whom as seen from Earth .) The durations of these events are typically a few minutes but can , occasionally be much longer if the geometry is right . It is worth noting that , when the geometry is right , similar interactions are also possible among all the other outer planets and their satellites , though the frequency of the seasons is generally less . For each planet , the event seasons are twice per orbit around the Sun . For long orbits , there is a long wait until the next season .
Sky ’ s Up 19
Totality is a sensory feast By DR. MIKE REYNOLDS Guest Contributor IMAGES COURTESY OF Dr. Mike Reynolds Above, observers watch as the Moon’s shadow blankets the daytime landscape in surreal darkness during a total solar eclipse on July 22, 2009. Since I’ve had the privilege of seeing 18 total solar eclipse, people often ask me what to look for or what to do during the eclipse. My answer is always the same: take it all in. Totality is truly an all-senses experience. Do not get stuck behind a camera, telescope or even a pair of binoculars during the entire totality. Yes, the beautifully- eclipsed sun, doing its celestial dance with the moon, is an alluring sight! Yet there is much more for your senses to try to take in. As the partial eclipse progresses towards totality, you will note that shadows are becoming sharper. Look for small partially-eclipsed suns on the ground around trees, as the leaves make pinhole projectors. Or make your own pinhole projector — cardboard with holes poked in it or Right, the diamond ring appears just before totality begins during a total solar eclipse on July 11, 2010. 36 Sky ’ s Up even a spaghetti colander works great. About 10 minutes before totality, you will probably be able to see the planet Venus. You’ll also note that the temperature is dropping, and even a perfectly-clear sky will look like clouds on the horizon towards the northwest. That’s the rapidly-approaching moon’s shadow. As totality approaches, you will see colors — like a sunset or sunrise — begin in the northwest horizon and then circle the horizon. You might also glimpse those elusive shadow bands, dancing on the ground. The sun diminishes rapidly until a brilliant spot is left called the diamond ring. Then… totality! The sun’s corona, or crown, extends from the now-eclipsed star. Through a telescope or binoculars you will probably see small reddish spots COURTESY OF Dr. Mike Reynolds around the sun-moon spectacle. These Venus (near the right edge of the frame) makes a daytime appearance during a total are prominences. The corona shows solar eclipse on April 8, 2005. fine, almost-hair-like structure. As you look up, you’ll see more stars; maybe even Regulus really beautiful to heartbreak, heaven and hell in the same sky. Absolute silence reigned. No human being spoke. No bird close to the eclipsed sun. twittered. Even sighing of the surf breathed into utter repose, Look around you – take it all in. The total eclipse is one and not a ripple stirred the leaden sea. of nature’s special events. And we are honored to be able One human being seemed so small, so helpless, so slight a to take it all in. For as quickly as the total eclipse began, it part of all this strangeness and mystery! It was as if the hand is over it seems. Another diamond ring signals the end of of Deity had been visibly laid upon space and worlds, to totality as the celestial sun and moon dance moves toward allow one momentary glimpse of the awfulness of creation. the end. Hours might have passed — time was annihilated; and Totality is hard to capture in word or photo; perhaps the yet when the tiniest globule of sunlight, a drop, a needle- best-written account is that of Mabel Loomis Todd regarding shaft, a pinhole, reappeared, even before it had become the the August 9, 1896, total solar eclipse: slenderest possible crescent, the fair corona and all color “Then an instantaneous darkness leaped upon the world. in sky and cloud withdrew, and a natural aspect of stormy Unearthly night enveloped all. twilight returned. Then the two minutes and a half in memory With an indescribable out-flashing at the same instant the seemed but a few seconds — a breath, the briefest tale ever corona burst forth in mysterious radiance. But dimly seen told.” through thin cloud, it was nevertheless beautiful beyond — Mabel Loomis Todd, Chapter 29, “The Eclipse,” description, a celestial flame from some unimaginable Corona and Coronet: Being a narrative of the Amherst heaven. Simultaneously the whole northwestern sky, nearly E ͔)ѥѼ)5ȸ)ϊé͍ȴ)Ѽѡ镹Ѡ݅́ݥѠɥхѱ䁉ɥ)免) ɽа)Ѽ͕ٔѡMջéѽх͍Ʌѥ)Ʌɽ́ݡɥѕՑ́ͱѱ䁑ɭȰ(Ѡ՝а)́եȁ՝фɽͽمЁٽ))))!̸QݕЁͽѡݕЁ͡)ȸ5I幽́ͅ܁́Ёѽхͽȁ͔5ɍܰ)啱ܸ(!́յɽ́ѥ͕ٕ́ѽх)1Ёչ͕аЁ݅́ѽͽȁѕɥQͽȁ͕̃Lѕ쁽͕٥ɽ͕ȸ)ɽɍɽЁѥݕݥѠѡɥ)I幽ϊd͕مѥ́ѽɅ́ٔՉ͡)ձ̰ݡɔȁɕѠȁѡȁх)յɽ̰́Ցѡ=͕͕ٔ́)ѡ́ѥх)ɽ丁!́ѡͽѥ1չȁAх)]ЁЁٔɕՑѼѡ͡ɥٕ)=͕ٕϊd͔ ɑѽȁAɽͽȁɽ䁅)ͅɅѡݡݽɱPݕɐѼɽȰ)ɥMхє )M䃊d)U(