Post by Taxigirl on Jun 8, 2004 9:12:43 GMT
news.bbc.co.uk/1/hi/sci/tech/3784293.stm
The black disc of Venus moves over our star
The planet Venus is making a very rare passage across the face of the Sun - an event not witnessed since 1882.
The so-called transit of Venus began at about 0620 BST (0520 GMT) with the planet's tiny black disc edging over the bright limb of our star.
The phenomenon continues for about six hours, ending around 1224 BST.
Experts have warned the public about the dangers of looking directly at the Sun and have urged observers to use a safe method such as pinhole projection.
"Indirect projection is the method we recommend as the safest way to observe the Sun," said Robert Walsh, senior lecturer at the University of Central Lancashire's (UCLAN) Centre for Astrophysics.
"In this way, the light from the Sun is projected on to a screen, reducing the brightness to safe levels. Looking directly at the Sun for even a brief moment with the naked eye or any sort of optical equipment - telescope, binoculars, camera - could result in possible loss of sight.
"Also, dark sunglasses or smoked glass are inadequate to protect your eyes and should not be used."
Special arrangement
UCLAN is leading the transit co-ordination in the UK. It has put useful tips for safe viewing on its special Venus website (www.transit-of-venus.org.uk).
The simplest technique is to put a pinprick in one piece of card and position it so that the Sun's rays can pass through the hole and focus on a white card held behind.
Alternatively, there are many astronomy websites which will carry pictures during the day, and for UK TV viewers, BBC One is broadcasting special programmes presented by Adam Hart-Davis.
Venus transits occur approximately four times in 243 years, more precisely in pairs of events separated by about eight years and these pairs are separated by about 120 years.
The reason for these long intervals lies in the fact that the orbits of Venus and the Earth do not lie in the same plane and a transit can only occur if both planets and the Sun are situated exactly on one line.
This has happened only six times in the telescopic age: in 1631, 1639, 1761, 1769, 1874 and 1882. So, the next will be in 2012, but the one after that will be in 2117.
This year's event can be seen best across Europe, most of Africa and much of Asia.
Very slow turn
The transits have had huge significance in the past, as they were used by scientists to work out the Sun-Earth distance - and hence to get a proper scale for the Solar System.
Today their value is more in their uniqueness. "It's an extremely rare astronomical event," said Gordon Bromage, a professor of astronomy at UCLAN.
"If you're thinking of observing this from the UK in its entirety, for the whole six hours - the last time that was possible was 1283. The next time it will happen is 2247. We intend to make the most of it."
The planet itself is soon to come under close scrutiny from Europe's Venus Express probe. The spacecraft, due to be launched next year, will study the planet's thick atmosphere and look for any volcanic activity that might still persist today.
It is an extraordinary world. Its thick, yellowish clouds contain sulphuric acid. The atmospheric pressure at the surface is equivalent to that at a depth of 900m in the Earth's oceans.
And because Venus rotates so slowly on its axis, its year (225 Earth days) is actually longer than its day.
"In a lot of ways, Venus can be considered as Earth's hellish twin because it has evolved in a quite different way to Earth," said Dr Andrew Coates, a mission scientist on Venus Express.
"It has a runaway greenhouse effect which has caused the temperature on the surface to be one of the hottest in the Solar System at 460C."
METHOD 1: Cut two holes in card for binocular eye-pieces. Push card onto binoculars and fix with tape. Block one lens (with lens cap, for example). Holding binoculars at waist height, angle towards Sun - do not look through them. Move binoculars around until card shadow minimised on viewing surface (piece of white card best). During six-hour transit, move set-up to follow path of Sun across sky, for example by mounting it on tripod or chair.
METHOD 2: Take piece of stiff card and pierce with pin. Hold it up and, looking away from the Sun, adjust angle of card until shadow minimised. Pinhole will project image of Sun into middle of shadowed area. Place another piece of card under shadow and adjust distance to get best picture - more distance gives larger but fainter image. Hole must be clean and as small as possible.
WARNING: Never observe the Sun with the naked eye or telescope, camera or other optical device. Doing so will seriously damage eyesight and may lead to permanent blindness.
The black disc of Venus moves over our star
The planet Venus is making a very rare passage across the face of the Sun - an event not witnessed since 1882.
The so-called transit of Venus began at about 0620 BST (0520 GMT) with the planet's tiny black disc edging over the bright limb of our star.
The phenomenon continues for about six hours, ending around 1224 BST.
Experts have warned the public about the dangers of looking directly at the Sun and have urged observers to use a safe method such as pinhole projection.
"Indirect projection is the method we recommend as the safest way to observe the Sun," said Robert Walsh, senior lecturer at the University of Central Lancashire's (UCLAN) Centre for Astrophysics.
"In this way, the light from the Sun is projected on to a screen, reducing the brightness to safe levels. Looking directly at the Sun for even a brief moment with the naked eye or any sort of optical equipment - telescope, binoculars, camera - could result in possible loss of sight.
"Also, dark sunglasses or smoked glass are inadequate to protect your eyes and should not be used."
Special arrangement
UCLAN is leading the transit co-ordination in the UK. It has put useful tips for safe viewing on its special Venus website (www.transit-of-venus.org.uk).
The simplest technique is to put a pinprick in one piece of card and position it so that the Sun's rays can pass through the hole and focus on a white card held behind.
Alternatively, there are many astronomy websites which will carry pictures during the day, and for UK TV viewers, BBC One is broadcasting special programmes presented by Adam Hart-Davis.
Venus transits occur approximately four times in 243 years, more precisely in pairs of events separated by about eight years and these pairs are separated by about 120 years.
The reason for these long intervals lies in the fact that the orbits of Venus and the Earth do not lie in the same plane and a transit can only occur if both planets and the Sun are situated exactly on one line.
This has happened only six times in the telescopic age: in 1631, 1639, 1761, 1769, 1874 and 1882. So, the next will be in 2012, but the one after that will be in 2117.
This year's event can be seen best across Europe, most of Africa and much of Asia.
Very slow turn
The transits have had huge significance in the past, as they were used by scientists to work out the Sun-Earth distance - and hence to get a proper scale for the Solar System.
Today their value is more in their uniqueness. "It's an extremely rare astronomical event," said Gordon Bromage, a professor of astronomy at UCLAN.
"If you're thinking of observing this from the UK in its entirety, for the whole six hours - the last time that was possible was 1283. The next time it will happen is 2247. We intend to make the most of it."
The planet itself is soon to come under close scrutiny from Europe's Venus Express probe. The spacecraft, due to be launched next year, will study the planet's thick atmosphere and look for any volcanic activity that might still persist today.
It is an extraordinary world. Its thick, yellowish clouds contain sulphuric acid. The atmospheric pressure at the surface is equivalent to that at a depth of 900m in the Earth's oceans.
And because Venus rotates so slowly on its axis, its year (225 Earth days) is actually longer than its day.
"In a lot of ways, Venus can be considered as Earth's hellish twin because it has evolved in a quite different way to Earth," said Dr Andrew Coates, a mission scientist on Venus Express.
"It has a runaway greenhouse effect which has caused the temperature on the surface to be one of the hottest in the Solar System at 460C."
METHOD 1: Cut two holes in card for binocular eye-pieces. Push card onto binoculars and fix with tape. Block one lens (with lens cap, for example). Holding binoculars at waist height, angle towards Sun - do not look through them. Move binoculars around until card shadow minimised on viewing surface (piece of white card best). During six-hour transit, move set-up to follow path of Sun across sky, for example by mounting it on tripod or chair.
METHOD 2: Take piece of stiff card and pierce with pin. Hold it up and, looking away from the Sun, adjust angle of card until shadow minimised. Pinhole will project image of Sun into middle of shadowed area. Place another piece of card under shadow and adjust distance to get best picture - more distance gives larger but fainter image. Hole must be clean and as small as possible.
WARNING: Never observe the Sun with the naked eye or telescope, camera or other optical device. Doing so will seriously damage eyesight and may lead to permanent blindness.