Tuesday, 30 June 2015

Venus meets Jupiter on the longest day of the year - June 30!!

1. VERY close conjunction of Venus and Jupiter.
2. An extra 'Leap Second' will be added on June 30.
 Venus the brilliant "Evening Star", has been shining gloriously in the evening twilight for the past few months. Although it's a planet, it's so far away that it looks just like a star to the naked eye, and in fact the Ancient Greeks called the planets 'wandering stars'. It's by far the brightest starlike object in the night sky.
   The second brightest 'star' is the giant planet Jupiter, and their orbits have gradually been bringing Venus and Jupiter ever closer together as we see them from Earth.
Although it is so big that it could contain 1,000 planet Earths, it is so far away that it also appears like a point of light - bright, but not as bright as Venus which is much closer.
   On Tuesday evening Jupiter and Venus will appear much less than a Moon diameter apart - only 1/3 of a degree! In a telescope, you would be able to see them easily in the same field of view! You will need a clear horizon to the west, as they will be quite low down. Look low in the W twilight about 10.45 pm, and you'll easily see Venus. If you have binoculars you should just be able to spot fainter Jupiter just above and slightly to the left of Venus, but if just using your eyes, you may need to wait until about 11 p.m. or a little later before the sky gets dark enough to see Jupiter.
   Even though they appear so close together, actually Venus is a 'mere' 48 million miles away, whereas Jupiter is 565 million miles away: they just happen to lie in the same line of sight.
   From 1 July on they will gradually appear to separate, and sink lower in the evening twilight.
2. AN EXTRA SECOND WILL BE ADDED TO 30 JUNE, making it the longest day of the whole year. The time we use in our clocks, watches, computers, satnav etc is absolutely constant: it's called UTC (Universal Time, Co-ordinated), or informally 'International Atomic Time'. And we now measure that time more accurately than the daily spin of Planet Earth, which used to be taken as constant, giving us our 24h day.
   But actually, the Earth's rotation speed is NOT constant, and it varies very slightly, at the moment it is VERY slowly slowing down, for a number of reasons. So in order to keep clock time in synch with the daily time given by the spin of the Earth, an extra second is added when needed, either at the end of June, or the end of December. So the world's official clocks, which would usually read 23h 59m 59s to 00h 00m 00s will instead read 23h 59m 59s, 23h 59m 60s, 00h 00m 00s.
     The main factors which slow down the Earth's spin are climate variations, melting polar ice, El Nino effects, tectonic movements and changes in ocean currents.
   This will be the 4th leap second added since the year 2000.
1. Officially, a day is exactly 86,400 seconds long. But in practice, for the last few decades at least, it has been about 86,400.002 seconds long. The extra 2 thousands of a second may not seem much, but cumulatively the difference adds up, and when the clocks are out of step by almost a second, the adjustment is made.
2. UTC is measured by the very regular electromagnetic transitions in Cesium atoms. These give an accuracy of 1 second in 1,400,000 years!
3.  Some have expressed concern that inbuilt computer clocks which read UTC will have some sort of a 'hiccup' at this time: it is unlikely, but if anyone is concerned you might want to make sure your computer is off at the time.
4. The Irish Astronomical Association is a registered charity dedicated to promoting interest in, and information about, astronomy and space and related topics. It is the oldest and largest astronomical society based in N. Ireland, and the largest amateur astronomy society in Ireland.
5. The IAA runs public events throughout the year, including free public lectures at QUB, and public outreach events in various locations throughout Northern Ireland, details of which are available on the website: www.irishastro.org and http://www.bbc.co.uk/thingstodo
Terry Moseley
President and PR Officer,
Irish Astronomical Association

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