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During August, our planet passes through debris strewn around the orbit of comet 109P/Swift-Tuttle. The particles, or meteoroids, encountered tend to be small, typically about the size of a grain of sand. Although on parallel orbits, a perspective effect makes it look like the meteor trails produced emanate from a small region of sky. What is being described here is a meteor shower and the apparent point of origin is known as the shower radiant. Perseids Meteor Shower - Blink and You Might Miss It 04:53 This particular shower produces meteors from July 23 to Aug 23 from a radiant drifting slowly in position from the W-shaped constellation of Cassiopeia, eastwards into Camelopardalis. Perseid radiant movement The apparent movement of the Perseid radiant between July and August. The best rates will occur on night from Aug 11/12 to 13/14 The number of meteors produced is low throughout much of the period. However, during the nights centred around August 12, we pass through the densest part of the stream and the number of meteors increases. The radiant is in Perseus when this happens so this is known as the Perseid meteor shower. Perseid meteor shower 2016 reaching its peak over California, USA 01:18 How to get the best view A meteor shower’s activity is expressed by its zenithal hourly rate or ZHR. At its peak on August 12-13, the Perseid shower will have a ZHR of 80-100 meteors-per- hour. Bright events such as this Perseid fireball occur when larger meteroids, typically around the size of a grape, vaporise in Earth’s atmosphere Credit: pete lawrence This normalised value is calculated on the assumption that the radiant is directly overhead, sky conditions are perfect and that you can see the whole sky in one go. In reality few of these conditions are ever met and the visual rate experienced is significantly lower than the published ZHR. A bright Perseid photographed in a partly cloudy sky illuminated by bright moonlight Credit: pete lawrence However, a bright Moon will interfere this year. The best viewing strategy will be to find as dark a location as possible and centre your view approximately two-thirds up the sky. Look in any direction where the Moon cannot be seen. Fainter trails will be lost to moonlight but bright ones should still be visible. The Great American Eclipse This year’s Perseid maximum occurs less than two weeks before a total eclipse of the Sun set to be seen along a 71-mile (115km) wide track which crosses the United States of America. A second contact diamond ring photographed from MS Boudicca in the middle of the Norwegian Sea in March 2015 Credit: pete lawrence Dubbed the ‘Great American Eclipse’, this event takes place on Aug 21 and is likely to be the most observed and photographed total solar eclipse in history. Nasa plots the path Credit: Nasa The conditions leading to a particular eclipse virtually repeat 18 years 11.3 days later, a period known as the Saros cycle, and this month's event belongs to Saros series 145. The last eclipse in this series was that which crossed the Cornish peninsular in August 1999. This time the UK’s experience won’t as dramatic and only a small portion of the Sun’s disc will be covered by the Moon. UK eclipse circumstances for August 21 2017 Partial eclipse timings for locations across the UK. Note that the Moon’s disc has been shown off the Sun’s disc for clarity in the graphic. In reality, the only portion of the Moon’s disc that can be seen is that which is passing in front of the Sun. The eclipse starts around 19:40 BST. Maximum eclipse is around 20:02 BST when 3.5 per cent of the Sun’s disc will be obscured. The eclipse ends at 20:26 BST. These values are for the centre of the UK and vary slightly with location. The Sun also sets before the eclipse ends from some locations. View safely and never look at, or point any instrument at the Sun unless you do so through a certified solar safety filter. The Summer Triangle The Moon is new at the point a total solar eclipse occurs and for those more interested in the night sky, this is the ideal time to get outside and enjoy the darkness. The three bright stars that form the Summer Triangle dominate the view high to the south as night falls during August. Altair rotates so quickly that it bulges at the equator Credit: pete lawrence The most southerly star in the triangle is Altair, the brightest in Aquila the Eagle and notable because it appears flanked by two dimmer stars; Tarazed above and Alshain below. Altair is known to be a fast rotating star, completing one rotation every 8.9 hours. Its equatorial region spins so fast at 240km/s that Altair bulges at the waist. The bright star Altair flanked by Tarazed and Alschain form a useful navigational beacon for locating Sagitta the Arrow, Delphinus the Dolphin and Equuleus the Foal Credit: pete lawrence North of Altair is a small but distinctive pattern of stars representing Sagitta the Arrow. In mythology the immortal Titan Prometheus took it upon himself to give mankind the gift of fire without permission from Zeus. As punishment, Prometheus was chained to a mountain where he was attacked by Aquila. The eagle tore into the Titan’s flesh and ate his liver. Being immortal it grew back leaving Prometheus to suffer the same ordeal repeatedly. Eventual release came when Hercules killed Aquila with the arrow Sagitta. Located east-southeast of Sagitta is distinctive Delphinus the Dolphin. This doesn’t immediately resemble a dolphin until you realise Delphinus is supposed to represent just the bottle-nose and head of the creature. Continue further east-southeast and you’ll arrive at four very faint stars representing Equuleus the Foal. Here any similarity between the pattern and its namesake requires a huge leap of imagination! Night sky - August 2017 Night sky August 2017 This chart shows how the sky will appear at 1am BST on Aug 1, midnight BST on Aug 15 and 11pm BST on Aug 31. The planets are shown along with the location and phase of the Moon at five-day intervals. The Moon is full on Aug 7. The stars are shown as circles; the larger the circle the brighter the star. The hazy area represents the Milky Way. Orientate the chart by holding it in front of you rotated so the compass bearing at the bottom matches the direction you’re facing. The bottom of the chart then reflects your horizon with the middle of the chart representing the view directly above your head. Solar eclipse 2017, in pictures
Original Article: Night sky – August 2017: shooting stars and America eclipsedNEXT ARTICLE
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