Cherry

来源: BlogBus 原始链接: http://www.blogbus.com:80/blogbus/blog/diary.php?diaryid=152629 存档链接: https://web.archive.org/web/20040807214210id_/http://www.blogbus.com:80/blogbus/blog/diary.php?diaryid=152629

Cherry <<<Miss you | Home | i knew>>> The Leonids Introduction―The Leonids Made Easy What are the Leonids? The Leonids are a meteor shower. They are called the Leonids because they appear to radiate out of the constellation Leo. A meteor, sometimes referred to as a "shooting star," is a particle from space. Its typical size ranges from that of a grain of sand to that of a pea. A meteor appears when it enters Earth's atmosphere and burns up high overhead. Meteors can be seen on any night, but Earth enters clouds of particles several times each year and the result is a meteor shower. What Do the Leonids Look Like? All meteors appear as brief streaks of light moving a short distance across the sky. Some meteors move slow and some move fast. Here is a video of a bright, slow meteor. Note that a streak persisted for a little while after the meteor vanished. This "streak" is called the train and is basically a trail of glowing dust left in the wake of the meteor. The Leonids are fast meteors and they leave lots of trains. They enter Earth's atmosphere traveling at speeds of over 158,000 miles per hour (mph). For comparison an Indy race car can reach a top speed of about 250 mph, the fastest jet has a top speed of 2190 mph, and an orbiting spacecraft has an average speed of 20,000 mph. Besides being fast, the Leonids usually contain a large number of very bright meteors. The trains of these bright meteors can last from several seconds to several minutes. Where Do the Leonids Come From? Most if not all meteor showers are produced by comets. In the case of the Leonids the parent comet is named Tempel-Tuttle and it makes an appearance in our skies every 33 years. Comets are composed of ice and dust. Every time a comet approaches the sun the ice melts and dust is released. Eventually the dust spreads completely around a comet's orbit, but the highest concentration of dust is still close to the comet, since it gets replenished every 33 years. When Earth passes through the dense cloud of dust the result is a spectacular meteor shower or a meteor storm. Meteor storms produce several thousand meteors per hour. When Do the Leonids Occur? Every November 17-18 Earth crosses the orbit of comet Tempel-Tuttle and the Leonids become visible. An observer with clear, dark skies can see 10 or 15 Leonid meteors every hour on that morning. Unless an observer is living at a very high northern latitude (generally within the Arctic Circle), the Leonids are only visible during the morning hours. Since Tempel-Tuttle passed closest to the sun in February of 1998, the years following were expected to produce very strong displays. Displays of 250-300 per hour occurred in 1998, 3700 per hour occurred in 1999, and 480 per hour occurred in 2000. During the mornings of November 17 and 18 observers anywhere in the Northern Hemisphere should see greater than normal displays. Start watching sometime after about 11:30 p.m. local time (because of Earth's rotation, local time literally means "your time"). The radiant will still be about an hour from rising, but you will have the opportunity to see the "Grazers". These are Leonids that are not dropping down into Earth's atmosphere, but are instead grazing the atmosphere. These appear as reddish meteors that advance from east to west across a large part of the sky. In 1998 there were reports that several east coast television stations were receiving calls that rockets were seen crossing the sky. These "rockets" were in fact the Leonid grazers. As the radiant rises near 12:30 a.m. local time, the Leonids will travel shorter distances across the sky as they drop down into the atmosphere. Their color will also change to white and blue-white. Some of the brightest will actually appear slightly greenish. How Do You Observe the Leonids? Click here for more information on how to observe the Leonid meteor shower, including a chart showing where to watch in the sky. The Orbit of the Leonids The orbit of the Leonid stream is shown as the green ellipse in the image below. The orbit extends all the way to the orbit of the planet Uranus and particles within the stream take about 33 years to make one trip around the sun. The stream is produced by the periodic comet Tempel-Tuttle . The Leonids This represents the view from mid-northern latitudes at about 2:00 a.m. local time around November 19. Because of Earth's rotation, this view will roughly be the same for every mid-northern latitude location in the world. The red line across the bottom of the image represents the horizon. This does not necessarily indicate the time of maximum. For this information, click here . (Image produced by the Author using SkyChart III 3.5 and Adobe Photoshop 5.5.) Observing the Leonids The point from where the Leonid meteors appear to radiate is located within the constellation Leo and is referred to as the radiant. The radiant is located in the western portion of that constellation in what is commonly referred to as the "sickle" or "backwards question mark." The following charts will help you find it from both the Northern Hemisphere and Southern Hemisphere: Northern Hemisphere This represents the view from mid-northern latitudes at about 2:00 a.m. local time around November 19. Because of Earth's rotation, this view will roughly be the same for every mid-northern latitude location in the world. The red line across the bottom of the image represents the horizon. This does not necessarily indicate the time of maximum. For this information, click here . (Image produced by the Author using SkyChart III 3.5 and Adobe Photoshop 5.5.) Southern Hemisphere This represents the view from mid-southern latitudes at about 3:00 a.m. local time around November 19. The red line across the bottom of the image represents the horizon. Once again this does not necessarily indicate the time of maximum. For this information, click here . (Image produced by the Author using SkyChart III 3.5 and Adobe Photoshop 5.5.) To best observe the Leonids wear appropriate clothing for the weather. Lie outside in a reclining lawn chair with your feet pointing towards the east (the general direction of the radiant). Do not look directly at the radiant, but at the area above and around it. The Leonids can be observed into morning twilight. Other minor meteor showers will be going on at the time and stray meteors, more commonly called sporadics, will frequently be seen that do not belong to a meteor shower. When you see a meteor mentally trace it backwards and if you arrive at the "sickle" of Leo it is probably a Leonid. Information for Advanced Observers The Radiant The duration of this meteor shower covers the period of November 13-20. Maximum currently occurs on November 17 (solar longitude=235.3 deg), from an average radiant of RA=153 deg, DEC=+22 deg. Although the maximum hourly rate typically reaches 10-15, this shower is most notable for producing greatly enhanced activity every 33 years---events that are associated with the periodic return of comet Tempel-Tuttle. During these exceptional returns, the Leonids have produced rates of up to several thousand meteors per hour. The Leonids are swift meteors, which are best known for producing many exceptionally bright meteors that leave a high percentage of persistent trains. The radiant's daily motion is +1.0 deg in RA and -0.4 deg in DEC. The Orbit The orbit of the Leonid stream, based on the orbits of 12 meteors photographed during the early 1950s and the orbits of 3 meteors detected by radar during the early 1960s, is as follows: AOP AN i q e a 173.1 235.4 161.0 0.983 0.901 9.956 The 1965 orbit of comet Tempel-Tuttle , according to Yeomans' 1981 study published in Icarus is AOP AN i q e a 172.6 234.4 162.7 0.982 0.904 10.272 The Leonids's Pictures 1998 Philip Freeman of the Boeing Astronomy Club was observing the shower in Illinois and obtained back to back photos, each of which contained one Leonid. The two photos were exposed 10 minutes apart around 2:45 a.m. CST on November 17 (8:45 UT on the 17th). The 35mm camera was mounted on a Meade telescope with a clock drive so that the images contained identical star fields. The images were subsequently scanned in and merged in photoshop to obtain the image you see here. If both meteors are traced backwards, i.e. extended toward the left, their intersection illustrates the radiant location. Mark Jones of the St. Louis Astronomical Society took this photo on the morning of November 17 from a site in Missouri. A four-hour, all-sky photo taken at Astronomical Observatory Modra in Slovakia on the morning of the 1998 Leonid maximum. The original image showed 156 meteors. The faintest meteors shown were about magnitude -2. A slightly larger version of the image can be found here Paulo M. Raymundo took the two above photos from Reaiche Observatory (Bahia, Brazil). For the top image, he wrote, "Dramatic photograph showing a -13 magnitude fireball only partially captured on the frame. Although the picture shows blue sky, the photo was actually taken at night. The fireball was so bright that the night sky was briefly illuminated like daytime. Photo taken ... at 07:17 UT on 17 November 1998. Exposure lasted for 18 seconds. 50mm lens and Kodak Ektapress Plus 1600." For the bottom image he wrote, "This photo shows the residual smoke train left by the fireball in the previous photograph. It persisted for well over 2 minutes. The belt and sword of Orion are visible in the upper right. Photo taken ... on 17 November 1998. Exposure ~30 seconds. 50mm lens and Kodak Ektapress Plus 1600." Raymundo has further images on his web site 2001 This brilliant Leonid meteor is seen lighting up the dark desert skies of the California desert area of Joshua Tree National Park. This meteor is seen near the Pleiades. It was one of thousands seen with 2-3 meteors being seen per second at the peak of the show. Photo taken with a 35mm camera with a 50mm lens on a tripod. Copyright by Wally Pacholka / wally@AstroPics.com / more of Pacholka's photos are at http://www.AstroPics.com Paulo M. Raymundo (Salvador, Brazil) saw a -9 magnitude fireball near Capella at 06:07 UT on November 18, which left a persistent train that remained visible for over ten minutes with the naked eye. The animated sequence of photos shows the evolution of the smokey train drifting in high-altitude winds. Leonid Artwork Eric S. Young, an artist and graphic designer, has been an amateur astronomer for years. Besides observing at the telescope and taking an occasional photograph, he has also captured some of his most memorable astronomical observations through the use of sketches, paintings, and computer illustrations. He is also illustrating the covers for my Cometography series of books being released by Cambridge University Press. Featured below are some of his Leonid inspirations. Other samples of his artwork can be found at his web site . "The Night the Stars Fell" The morning of November 13, 1833, the sky burned with ribbons of fire. Hundreds of thousands of meteors lit up the night over northeastern America. The incredible spectacle stirred emotions, and the resulting quantity of observations later led astronomers to a better understanding of "falling stars." My picture imagines the scene from a Sioux encampment in North Dakota. The time is 2:40 am. "Leonid Over Mt. Locke " 1999. Computer illustration A fiery Leonid meteor blazes over the dome of the 82-inch Otto Struve Telescope at McDonald Observatory. Still used atop the summit of Mt. Locke in Fort Davis, Texas, when dedicated in 1939 this was the 2nd largest telescope in the world. "Fireworks" 2001. Digital illustration. Every 33 years, the Leonid meteor shower puts on a great show somewhere over the the world. My friend, Gary Kronk, began this image with a digital photo of his wife, Karen, in their backyard in daylight. He added the starfield showing Leo rising. Gary turned it over to me next, and I added the house, trees, lawn, and meteors. "Lake Scene" 2001. Digital illustration. (The newest illustration. No text available) ** cherryoney Post 2004-04-25 12:19 Quote(Trackback0) Tonight JiuZhaiGou Colorado Canyon Drift Classroom TV show _^_^ Happy Birthday Naughty Boy My Computer Pinky and the Brain nothing too big My Site MissLiu