Post by wheelspinner on Jun 6, 2012 3:14:47 GMT -5
Apart from being a spectacular and rare astronomical event, the transit of Venus across the sun holds a special place in Australian history. It was to observe the transit that Captain James Cook sailed to Tahiti in HMS Endeavour in 1769. When that part of his mission was complete, he sailed on to explore the South Pacific for evidence of what was then rumoured to be the Great South Land. And the rest, as they say, is history.
_________________
It's T-Day for Venus
Melrose High student's (front) Liam Saillare and behind (L to R) Paul Nilsson, Alex Hughes, Tristan Khaw and Oliver Cross are all part of the school science program that is looking forward to the transit of venus. Photo: Colleen Petch
PETER SPINKS
Weather permitting, from 8.16 this morning you will be able to watch, with the aid of a telescope, an impish black dot for more than six hours as it meanders its way across the sun’s glowing face.
The transit of Venus, one of the most keenly anticipated events on the astronomical calendar, will not happen again for a long time – the year 2117, to be exact – because the Earth, Venus, and the sun rarely line up in a row as their orbits intersect.
In our solar system, a transit means a planet passes between the Earth and the sun, says Swinburne University astronomer Associate Professor Sarah Maddison. "This can only happen with Mercury and Venus."
Transits of the first rock from the sun, Mercury, are relatively common – about 13 occur every century – but they are hard to detect because of Mercury’s small size, Professor Maddison explains. "Transits of Venus are much rarer, but more interesting because of Venus' larger size."
With an equatorial diameter of 12,104 kilometres, Venus is almost as big as the Earth. As a result, the humble but clear black dot of Venus can be made out as it transits the sun.
Every 584 days, Venus and the Earth are aligned in the same direction towards the sun, known as a conjunction.
"The orbital plane of Venus around the sun is tilted by 3.4 degrees from that of the Earth's orbital plane, so Venus and the Earth don’t perfectly line up with the sun every conjunction," Professor Maddison says. For a transit to occur, the transiting planet needs to be in the same plane as the Earth's orbit about the sun.
The pattern of Venus' transits repeats every 243 years. Specifically, a pair of transits occurs eight years apart separated by 121.5 years and 105.5 years. So, since their discovery, there have been just six transits of Venus: in 1639, 1761, 1769, 1874, 1882 and 2004 – and now 2012. The next one is due in the year 2117.
Potted history
By 1627, with accurate tables of planetary motions and a new understanding of orbits, Johannes Kepler had determined his laws of planetary motion.
Realising that Mercury and Venus can pass in front of the sun, as observed from the Earth, he predicted the 1631 transit of Venus. "But the 1631 Venus transit occurred at night time in Europe – so no one saw it," Professor Maddison laughs.
The first successful observation of a predicted transit of Venus was made by a young English astronomer-mathematician, Jeremiah Horrocks, in December 1639.
In 1663, another astronomer-mathematician, James Gregory, suggested that, by observing the next Venus transit from different locations, the Earth-sun distance could be calculated.
Once this distance was known, the scale of the solar system could be estimated using Kepler's third law. "And knowing the scale is important in understanding how the solar system formed," she notes.
Australia and the transit
The transit of Venus has special significance for Australia. Britain commissioned Captain James Cook to sail to Tahiti aboard the Endeavour to witness the historic astronomical event in 1769, in part to calculate the Earth-sun distance, known as an astronomical unit.
Sailing on later to discover the "great south land", Cook and his crew of scientists and sailors charted, and claimed for George III, the east coast of what later came to be known as New Holland and, eventually, Australia.
Atmospheric detection
Another scientific result to come from the transit of Venus was the discovery of its atmosphere.
The 1761 transit was observed by more than 150 scientists, including Russian astronomer Mikhail Lomonosov. He noticed that, instead of Venus appearing as a very black disc sliding into the bright edge of the sun, for a brief moment between first and second contact its dark edge was surrounded by a halo of light.
"This is what you would expect if the sun’s rays were refracted, or bent, through the planet's atmosphere," Professor Maddison says.
Using modern spectroscopy, astronomers use a transit to study the Venusian atmosphere's altitude and chemistry. This reveals a gaseous envelope, almost entirely of carbon dioxide, which traps the sun’s heat like a greenhouse, raising temperatures to an average of 450 degrees.
The dense cloud cover, up to 70 kilometres high, reflects much of the sun's rays back into interplanetary space. This largely explains why, when viewed from the Earth, Venus is almost as bright as Jupiter – the sky's fourth brightest object.
Other worlds
The transit technique can be used to detect atmospheres of extra-solar planets, hundreds of which have been found.
"If the orbit of a planet around its host star is exactly aligned between Earth and that star, we notice a regular dimming of the stellar light each time the planet passes in front of – or transits – its host star," Professor Maddison explains.
Astronomers then conclude that an exoplanet orbits the star and can calculate its relative size and distance from the parent star. "The planet’s atmosphere can be studied at infrared and other wavelengths, to detect atoms and molecules such as hydrogen, carbon, sulphur and carbon dioxide."
These, in turn, provide clues as to whether these distant worlds might harbour life of some sort.
Planet in profile
Venus, among the least hospitable of places in the solar system, has thick clouds of sulphuric acid, a very dense atmosphere and searing temperatures.
Baking under a dull orange sky, its rolling, rocky volcanic plains cover more than three-quarters of the planet's surface, believed to be little more than 700 million years old.
Venus has about 1600 volcanoes, distributed more or less randomly. Whether or not they are still erupting lava or molten rock is less certain.
Maat Mons, one of the biggest volcanoes, rises about eight kilometres above the mean surface level. Lava flows at its base extend for hundreds of kilometres across the surrounding plains.
So far, about 900 impact craters, ranging from almost two kilometres to about 280 kilometres across, have been found. "There are so few craters because of the thick atmosphere – making it hard for meteorites to survive their passage to the surface – and because ancient craters would have been wiped out by widespread lava flows," Professor Maddison says.
Yet many ancient craters are still intact, despite the bouts of volcanism that once resurfaced the planet.
Venus fact file
Venus, the brightest celestial object after the sun and moon, frequently appears in the twilight sky as the evening or morning star (Hesperus and Lucifer to the ancient Greeks and Romans) – even though it’s a planet, not a star.
A day on Venus is longer than its year because the planet spins on its axis far slower than it orbits the sun.
As with Mercury, Venus has no moon, though its phases are similar to those of our moon.
Venus has nearly 100 times Earth’s atmospheric pressure, about that experienced at an ocean depth of one kilometre or so.
The orb spins in the opposite direction to Earth – and most of the other planets, for that matter. So the sun on Venus rises in the west and sets in the east.
Links
Learn other ways to watch today's transit at: www.transitofvenus.org.
A transit factsheet can be found at: www.astronomy.org.au/ngn/engine.php?SID=1000011&AID=100326
Find out what the Hubble Space Telescope will be doing during today at: archive.stsci.edu/proposal_search.php?id=12537&mission=hst
Watch NASA's live-stream of the once-in-a-lifetime transit at: venustransit.nasa.gov/transitofvenus/
Visit The Age education archive at: education.theage.com.au/content/science/gjj5ew
Please send bright ideas for new topics to pspinks@theage.com.au
www.theage.com.au/technology/-1zu7q.html
_________________
It's T-Day for Venus
Melrose High student's (front) Liam Saillare and behind (L to R) Paul Nilsson, Alex Hughes, Tristan Khaw and Oliver Cross are all part of the school science program that is looking forward to the transit of venus. Photo: Colleen Petch
PETER SPINKS
Weather permitting, from 8.16 this morning you will be able to watch, with the aid of a telescope, an impish black dot for more than six hours as it meanders its way across the sun’s glowing face.
The transit of Venus, one of the most keenly anticipated events on the astronomical calendar, will not happen again for a long time – the year 2117, to be exact – because the Earth, Venus, and the sun rarely line up in a row as their orbits intersect.
In our solar system, a transit means a planet passes between the Earth and the sun, says Swinburne University astronomer Associate Professor Sarah Maddison. "This can only happen with Mercury and Venus."
Transits of the first rock from the sun, Mercury, are relatively common – about 13 occur every century – but they are hard to detect because of Mercury’s small size, Professor Maddison explains. "Transits of Venus are much rarer, but more interesting because of Venus' larger size."
With an equatorial diameter of 12,104 kilometres, Venus is almost as big as the Earth. As a result, the humble but clear black dot of Venus can be made out as it transits the sun.
Every 584 days, Venus and the Earth are aligned in the same direction towards the sun, known as a conjunction.
"The orbital plane of Venus around the sun is tilted by 3.4 degrees from that of the Earth's orbital plane, so Venus and the Earth don’t perfectly line up with the sun every conjunction," Professor Maddison says. For a transit to occur, the transiting planet needs to be in the same plane as the Earth's orbit about the sun.
The pattern of Venus' transits repeats every 243 years. Specifically, a pair of transits occurs eight years apart separated by 121.5 years and 105.5 years. So, since their discovery, there have been just six transits of Venus: in 1639, 1761, 1769, 1874, 1882 and 2004 – and now 2012. The next one is due in the year 2117.
Potted history
By 1627, with accurate tables of planetary motions and a new understanding of orbits, Johannes Kepler had determined his laws of planetary motion.
Realising that Mercury and Venus can pass in front of the sun, as observed from the Earth, he predicted the 1631 transit of Venus. "But the 1631 Venus transit occurred at night time in Europe – so no one saw it," Professor Maddison laughs.
The first successful observation of a predicted transit of Venus was made by a young English astronomer-mathematician, Jeremiah Horrocks, in December 1639.
In 1663, another astronomer-mathematician, James Gregory, suggested that, by observing the next Venus transit from different locations, the Earth-sun distance could be calculated.
Once this distance was known, the scale of the solar system could be estimated using Kepler's third law. "And knowing the scale is important in understanding how the solar system formed," she notes.
Australia and the transit
The transit of Venus has special significance for Australia. Britain commissioned Captain James Cook to sail to Tahiti aboard the Endeavour to witness the historic astronomical event in 1769, in part to calculate the Earth-sun distance, known as an astronomical unit.
Sailing on later to discover the "great south land", Cook and his crew of scientists and sailors charted, and claimed for George III, the east coast of what later came to be known as New Holland and, eventually, Australia.
Atmospheric detection
Another scientific result to come from the transit of Venus was the discovery of its atmosphere.
The 1761 transit was observed by more than 150 scientists, including Russian astronomer Mikhail Lomonosov. He noticed that, instead of Venus appearing as a very black disc sliding into the bright edge of the sun, for a brief moment between first and second contact its dark edge was surrounded by a halo of light.
"This is what you would expect if the sun’s rays were refracted, or bent, through the planet's atmosphere," Professor Maddison says.
Using modern spectroscopy, astronomers use a transit to study the Venusian atmosphere's altitude and chemistry. This reveals a gaseous envelope, almost entirely of carbon dioxide, which traps the sun’s heat like a greenhouse, raising temperatures to an average of 450 degrees.
The dense cloud cover, up to 70 kilometres high, reflects much of the sun's rays back into interplanetary space. This largely explains why, when viewed from the Earth, Venus is almost as bright as Jupiter – the sky's fourth brightest object.
Other worlds
The transit technique can be used to detect atmospheres of extra-solar planets, hundreds of which have been found.
"If the orbit of a planet around its host star is exactly aligned between Earth and that star, we notice a regular dimming of the stellar light each time the planet passes in front of – or transits – its host star," Professor Maddison explains.
Astronomers then conclude that an exoplanet orbits the star and can calculate its relative size and distance from the parent star. "The planet’s atmosphere can be studied at infrared and other wavelengths, to detect atoms and molecules such as hydrogen, carbon, sulphur and carbon dioxide."
These, in turn, provide clues as to whether these distant worlds might harbour life of some sort.
Planet in profile
Venus, among the least hospitable of places in the solar system, has thick clouds of sulphuric acid, a very dense atmosphere and searing temperatures.
Baking under a dull orange sky, its rolling, rocky volcanic plains cover more than three-quarters of the planet's surface, believed to be little more than 700 million years old.
Venus has about 1600 volcanoes, distributed more or less randomly. Whether or not they are still erupting lava or molten rock is less certain.
Maat Mons, one of the biggest volcanoes, rises about eight kilometres above the mean surface level. Lava flows at its base extend for hundreds of kilometres across the surrounding plains.
So far, about 900 impact craters, ranging from almost two kilometres to about 280 kilometres across, have been found. "There are so few craters because of the thick atmosphere – making it hard for meteorites to survive their passage to the surface – and because ancient craters would have been wiped out by widespread lava flows," Professor Maddison says.
Yet many ancient craters are still intact, despite the bouts of volcanism that once resurfaced the planet.
Venus fact file
Venus, the brightest celestial object after the sun and moon, frequently appears in the twilight sky as the evening or morning star (Hesperus and Lucifer to the ancient Greeks and Romans) – even though it’s a planet, not a star.
A day on Venus is longer than its year because the planet spins on its axis far slower than it orbits the sun.
As with Mercury, Venus has no moon, though its phases are similar to those of our moon.
Venus has nearly 100 times Earth’s atmospheric pressure, about that experienced at an ocean depth of one kilometre or so.
The orb spins in the opposite direction to Earth – and most of the other planets, for that matter. So the sun on Venus rises in the west and sets in the east.
Links
Learn other ways to watch today's transit at: www.transitofvenus.org.
A transit factsheet can be found at: www.astronomy.org.au/ngn/engine.php?SID=1000011&AID=100326
Find out what the Hubble Space Telescope will be doing during today at: archive.stsci.edu/proposal_search.php?id=12537&mission=hst
Watch NASA's live-stream of the once-in-a-lifetime transit at: venustransit.nasa.gov/transitofvenus/
Visit The Age education archive at: education.theage.com.au/content/science/gjj5ew
Please send bright ideas for new topics to pspinks@theage.com.au
www.theage.com.au/technology/-1zu7q.html
