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Comet Ison viewing may be at its best
Comet may soon be lost in sun's glare and might not survive Nov. 28 solar encounter
Emily Chung CBC News
Posted:Nov 15, 2013 3:09 PM ET
Last Updated:Nov 15, 2013 4:10 PM ET
http://www.cbc.ca/m/touch/news/story/1.2428280
This weekend, comet Ison should be visible with binoculars before dawn, just above the star Spica in the constellation Virgo. This image of the comet was captured using the 0.8m Schulman telescope at Mount Lemmon SkyCenter at the University of Arizona on Oct. 8.
This weekend, comet Ison should be visible with binoculars before dawn, just above the star Spica in the constellation Virgo. This image of the comet was captured using the 0.8m Schulman telescope at Mount Lemmon SkyCenter at the University of Arizona on Oct. 8. Adam Block/Mount Lemmon SkyCenter/University of Arizona
A sudden brightening of the comet Ison this week means now might be your best chance to see the beautiful green "sungrazer," as NASA is warning the comet may not last.
"If you want to see Ison with your own eyes, do it now," said a blog post on the website of NASA's Comet Ison observing campaign Friday. "We can not and do not guarantee that it will survive the next few weeks and become naked-eye visible in our night skies."
'With a good pair of binoculars on your deck, if you know where to look in the sky, you will see it.'- Robyn Foret, Royal Astronomical Society of Canada
This week, the comet suddenly had an "outburst" — an event where it jumped dramatically in brightness — so that it may now already be visible with the naked eye in some places.
"It's naked eye if you're deep, deep in the country, the sky is pristine and clear and your eyes are fantastically good," said Robyn Foret, chair of the education and public outreach committee for the Royal Astronomical Society of Canada, in an interview from Calgary Friday.
"That said, with a good pair of binoculars on your deck, if you know where to look in the sky, you will see it."
Right now, astronomers recommend looking for the comet just above the star Spica in the constellation Virgo in the southeast sky, just before dawn.
Comets are often described as "cosmic snowballs," as they are made of frozen gas, rock and dust. When their orbits bring them close to a star such as the sun, they heat up, causing some of the gas and dust to spew from the surface, forming a "coma," or atmosphere, that makes them look like fuzzy balls in the sky.
As they get closer to a star and heat up even more, water ice and other heavier gases start to evaporate, making them look bigger and brighter. Sometimes, the coma may expand into a long, beautiful tail pointing away from the sun.
Not as bright as expected
Comet Ison is currently closer to the sun than the Earth is and is getting nearer the sun each day. The "sungrazer" — the name given to comets that pass very close to the sun, becoming very bright -— is expected to brush past our nearest star a distance of 1.6 million kilometres on Nov. 28. That distance is just a little bit bigger than the sun's own diameter.
When Ison was first detected near the orbit of Jupiter last year, astronomers predicted it might become one of the brightest comet ever seen. However, so far it has not brightened as much as expected over the course of its sunward journey.
Comet Ison
Following an 'outburst' this week, Comet Ison may be visible to the naked eye in rural areas under clear skies. Tenho Tuomi of Lucky Lake, Sask., captured this image of Comet Ison on Nov. 12. (Tenho Tuomi/Royal Astronomical Society of Canada)
As the comet approaches the sun, it is expected to keep brightening. But that doesn't mean it will be easier to see.
"Because it's moving closer to the sun, it moves closer and closer to where the sun is in the sky," Foret said.
While it is now visible a fair distance above the horizon before dawn, it will get lower and lower in the sky each day, until it is too close to the sun to view safely.
"If you accidentally look at the sun through a pair of binoculars, it can have lifelong negative effects on your vision," Foret warned.
'Handful more days'
NASA's Comet Ison Observing Campaign says you have "maybe a handful more days" to see the comet before it gets lost in the sun's glare.
If the comet survives its close encounter with the sun on Nov. 28, it will be at its brightest in the month of December, as it moves away from the sun and may be "spectacular" at that point, Foret predicts.
But comets sometimes break up into pieces as they get close to the sun, and Foret said it wouldn’t be surprising if Ison does just that.
In fact, NASA suggests it's possible that the comet's recent brightening may have been caused by the comet's solid core fragmenting.
"Given that ISON's nucleus is shrouded in such a tremendous volume of light-scattering dust and gas right now, it will be almost impossible to determine this for at least a few day," said the NASA blog post.
If the comet's core is already breaking up, it is unlikely that any piece will survive the comet's close approach to the sun, since a chunk would have to be at least 200 metres wide to survive, NASA says. It estimates that as of this week, the comet was estimated to be 500 metres to two kilometres in diameter.
That means those who want to see Comet Ison shouldn't wait.
Foret said comets are always worth looking for, even if you've seen a comet before, because they all look different, especially their tails.
"Sometimes, comets appear to have two tails; sometimes, they'll have what looks like a tail in front of comet."
He himself hasn't had a chance to see Ison yet.
"I'm in Calgary, it's that time of year," he said. "It's very nice and cloudy in the east the past few days. But we're hopeful we'll get a break."
如何拍攝ISON彗星？
美國太空總署(ＮＡＳＡ)拍到的ISON彗星
大家拍攝過彗星沒有？還未親身看過的話，機會來了！２０１３年１１月至２０１４年１月，ISON彗星很大機會在地球上空閃亮登場。ISON彗星被國際科學光學監測網（International Scientific Optical Network，簡稱：光科網 ISON）位於俄羅斯的0.4米反射鏡發現，因而被冠以光科網的英文簡稱ISON。至於它的天文編號則為C/2012 S1。
ISON軌跡
拍攝ISON最佳時機
ISON有何特別？國際小行星中心的自動化程序顯示其最大亮度可達到-14等，比滿月（-12.74等）還光亮。數字越小表示光度越大。何時會到達最大亮度？就是ISON最接近地球的時候，即是２０１３年１２月２６日。其實從今年１１月起至明年１月，它的亮度已可透過肉眼直接觀測。
這顆不是ISON，但卻清楚表現出彗核、彗髮和彗尾。
彗星的結構
要把彗星拍攝得出色，必需對它的特徵和構造有所理解，才能充分把它的細節表露無遺。
彗星由彗核、彗髮和彗尾組成。彗核和彗髮被合稱為彗頭。
彗核
彗核就是頭部最光的區域。由於彗核的質量低，彗核通常都是不規則的形狀，而非球形。
彗髮
當接近太陽時，太陽的熱力會使彗核物質熔解並昇華為氣體，這些包圍彗核周圍的雲狀物就是彗髮。注意：彗髮不包括彗尾。
彗尾
彗尾是由太陽的輻射壓和太陽風施加在彗髮上而形成的，因此它的方向永遠背向太陽。
拍攝彗星
光圈
許多初次拍攝星體的人都會收小光圈，以圖把星宿拍成球體，最後當然什麼也拍不到。它的實體是不會被拍到的，只有它的光芒會被我們看見。就算是哈勃（Hubble)望遠鏡也不能把彗星拍成立體的球體。既然星光只是二維影像，天學攝影師都會用最大光圈來捕捉較微弱的光線。
ISO
彗星有條長長的尾巴，但末端是非常暗淡的。使用高ISO可以拍出更長的尾巴。由於ISON的光度和月亮相近，就算使用ISO400也可拍到彗核，但彗尾會短一點。使用ISO1600則可拍得更長的尾部，但會有較多噪點。大家要自行取捨尾巴的長度和噪點。
快門
為了讓彗星的暗部更明亮，可以曝光5-20秒（視乎當時的光度，因大氣和煙霧會影響彗星光度）。不要曝光太久，如數分鐘，否則彗星會變成星軌。
三腳架
不用多說，大家都知道三腳架是不可或缺的工具。
方向
所有星宿都是東起西落，ISON彗星也不例外。入夜後，ISON會向東北方向慢慢升起。深夜時，只要抬頭便能看見。
from Starry Night Skies Photography
廣角鏡
廣角鏡下，彗星可以點綴優美的景色。
from space.com
遠攝鏡
遠攝鏡則可捕捉彗星的細節。
相片來源：
１. NASA Solar System Exploration - Comet Gallery
２. Starry Night Skies Photography
３. Space.com
Comet may soon be lost in sun's glare and might not survive Nov. 28 solar encounter
Emily Chung CBC News
Posted:Nov 15, 2013 3:09 PM ET
Last Updated:Nov 15, 2013 4:10 PM ET
http://www.cbc.ca/m/touch/news/story/1.2428280
This weekend, comet Ison should be visible with binoculars before dawn, just above the star Spica in the constellation Virgo. This image of the comet was captured using the 0.8m Schulman telescope at Mount Lemmon SkyCenter at the University of Arizona on Oct. 8.
This weekend, comet Ison should be visible with binoculars before dawn, just above the star Spica in the constellation Virgo. This image of the comet was captured using the 0.8m Schulman telescope at Mount Lemmon SkyCenter at the University of Arizona on Oct. 8. Adam Block/Mount Lemmon SkyCenter/University of Arizona
A sudden brightening of the comet Ison this week means now might be your best chance to see the beautiful green "sungrazer," as NASA is warning the comet may not last.
"If you want to see Ison with your own eyes, do it now," said a blog post on the website of NASA's Comet Ison observing campaign Friday. "We can not and do not guarantee that it will survive the next few weeks and become naked-eye visible in our night skies."
'With a good pair of binoculars on your deck, if you know where to look in the sky, you will see it.'- Robyn Foret, Royal Astronomical Society of Canada
This week, the comet suddenly had an "outburst" — an event where it jumped dramatically in brightness — so that it may now already be visible with the naked eye in some places.
"It's naked eye if you're deep, deep in the country, the sky is pristine and clear and your eyes are fantastically good," said Robyn Foret, chair of the education and public outreach committee for the Royal Astronomical Society of Canada, in an interview from Calgary Friday.
"That said, with a good pair of binoculars on your deck, if you know where to look in the sky, you will see it."
Right now, astronomers recommend looking for the comet just above the star Spica in the constellation Virgo in the southeast sky, just before dawn.
Comets are often described as "cosmic snowballs," as they are made of frozen gas, rock and dust. When their orbits bring them close to a star such as the sun, they heat up, causing some of the gas and dust to spew from the surface, forming a "coma," or atmosphere, that makes them look like fuzzy balls in the sky.
As they get closer to a star and heat up even more, water ice and other heavier gases start to evaporate, making them look bigger and brighter. Sometimes, the coma may expand into a long, beautiful tail pointing away from the sun.
Not as bright as expected
Comet Ison is currently closer to the sun than the Earth is and is getting nearer the sun each day. The "sungrazer" — the name given to comets that pass very close to the sun, becoming very bright -— is expected to brush past our nearest star a distance of 1.6 million kilometres on Nov. 28. That distance is just a little bit bigger than the sun's own diameter.
When Ison was first detected near the orbit of Jupiter last year, astronomers predicted it might become one of the brightest comet ever seen. However, so far it has not brightened as much as expected over the course of its sunward journey.
Comet Ison
Following an 'outburst' this week, Comet Ison may be visible to the naked eye in rural areas under clear skies. Tenho Tuomi of Lucky Lake, Sask., captured this image of Comet Ison on Nov. 12. (Tenho Tuomi/Royal Astronomical Society of Canada)
As the comet approaches the sun, it is expected to keep brightening. But that doesn't mean it will be easier to see.
"Because it's moving closer to the sun, it moves closer and closer to where the sun is in the sky," Foret said.
While it is now visible a fair distance above the horizon before dawn, it will get lower and lower in the sky each day, until it is too close to the sun to view safely.
"If you accidentally look at the sun through a pair of binoculars, it can have lifelong negative effects on your vision," Foret warned.
'Handful more days'
NASA's Comet Ison Observing Campaign says you have "maybe a handful more days" to see the comet before it gets lost in the sun's glare.
If the comet survives its close encounter with the sun on Nov. 28, it will be at its brightest in the month of December, as it moves away from the sun and may be "spectacular" at that point, Foret predicts.
But comets sometimes break up into pieces as they get close to the sun, and Foret said it wouldn’t be surprising if Ison does just that.
In fact, NASA suggests it's possible that the comet's recent brightening may have been caused by the comet's solid core fragmenting.
"Given that ISON's nucleus is shrouded in such a tremendous volume of light-scattering dust and gas right now, it will be almost impossible to determine this for at least a few day," said the NASA blog post.
If the comet's core is already breaking up, it is unlikely that any piece will survive the comet's close approach to the sun, since a chunk would have to be at least 200 metres wide to survive, NASA says. It estimates that as of this week, the comet was estimated to be 500 metres to two kilometres in diameter.
That means those who want to see Comet Ison shouldn't wait.
Foret said comets are always worth looking for, even if you've seen a comet before, because they all look different, especially their tails.
"Sometimes, comets appear to have two tails; sometimes, they'll have what looks like a tail in front of comet."
He himself hasn't had a chance to see Ison yet.
"I'm in Calgary, it's that time of year," he said. "It's very nice and cloudy in the east the past few days. But we're hopeful we'll get a break."
如何拍攝ISON彗星？
美國太空總署(ＮＡＳＡ)拍到的ISON彗星
大家拍攝過彗星沒有？還未親身看過的話，機會來了！２０１３年１１月至２０１４年１月，ISON彗星很大機會在地球上空閃亮登場。ISON彗星被國際科學光學監測網（International Scientific Optical Network，簡稱：光科網 ISON）位於俄羅斯的0.4米反射鏡發現，因而被冠以光科網的英文簡稱ISON。至於它的天文編號則為C/2012 S1。
ISON軌跡
拍攝ISON最佳時機
ISON有何特別？國際小行星中心的自動化程序顯示其最大亮度可達到-14等，比滿月（-12.74等）還光亮。數字越小表示光度越大。何時會到達最大亮度？就是ISON最接近地球的時候，即是２０１３年１２月２６日。其實從今年１１月起至明年１月，它的亮度已可透過肉眼直接觀測。
這顆不是ISON，但卻清楚表現出彗核、彗髮和彗尾。
彗星的結構
要把彗星拍攝得出色，必需對它的特徵和構造有所理解，才能充分把它的細節表露無遺。
彗星由彗核、彗髮和彗尾組成。彗核和彗髮被合稱為彗頭。
彗核
彗核就是頭部最光的區域。由於彗核的質量低，彗核通常都是不規則的形狀，而非球形。
彗髮
當接近太陽時，太陽的熱力會使彗核物質熔解並昇華為氣體，這些包圍彗核周圍的雲狀物就是彗髮。注意：彗髮不包括彗尾。
彗尾
彗尾是由太陽的輻射壓和太陽風施加在彗髮上而形成的，因此它的方向永遠背向太陽。
拍攝彗星
光圈
許多初次拍攝星體的人都會收小光圈，以圖把星宿拍成球體，最後當然什麼也拍不到。它的實體是不會被拍到的，只有它的光芒會被我們看見。就算是哈勃（Hubble)望遠鏡也不能把彗星拍成立體的球體。既然星光只是二維影像，天學攝影師都會用最大光圈來捕捉較微弱的光線。
ISO
彗星有條長長的尾巴，但末端是非常暗淡的。使用高ISO可以拍出更長的尾巴。由於ISON的光度和月亮相近，就算使用ISO400也可拍到彗核，但彗尾會短一點。使用ISO1600則可拍得更長的尾部，但會有較多噪點。大家要自行取捨尾巴的長度和噪點。
快門
為了讓彗星的暗部更明亮，可以曝光5-20秒（視乎當時的光度，因大氣和煙霧會影響彗星光度）。不要曝光太久，如數分鐘，否則彗星會變成星軌。
三腳架
不用多說，大家都知道三腳架是不可或缺的工具。
方向
所有星宿都是東起西落，ISON彗星也不例外。入夜後，ISON會向東北方向慢慢升起。深夜時，只要抬頭便能看見。
from Starry Night Skies Photography
廣角鏡
廣角鏡下，彗星可以點綴優美的景色。
from space.com
遠攝鏡
遠攝鏡則可捕捉彗星的細節。
相片來源：
１. NASA Solar System Exploration - Comet Gallery
２. Starry Night Skies Photography
３. Space.com
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