６月６日早６时０８分（美国东部时间６月5日晚６时０4分）一场激动人心的“金星凌日”盛宴将在天空上演。这一罕见天象两次一组，两次相距８年，两组之间相距１００多年。上一次“金星凌日”是２００４年６月８日，下次要等到２１１７年１２月１１日，距今１０５．５年。因此，对大多人来说这将是观看“金星凌日”的最后一次难得的机会，千万不要错过。

    请看新华网和CBC的有关报道。

http://news.xinhuanet.com/tech/2012-06/03/c_112103530.htm

6日天宇上演“金星凌日” 天文专家指点观“小日食”

资料图片：2006年6月8日发生金星凌日天文现象，这是在宁夏境内观测到的凌始内切景象。 新华社记者 余福卿摄

新华网 天津６月３日电（记者帅安宁、周润健）６月６日，一场激动人心的天空盛宴将上演，这就是“金星凌日”。那么，对此次罕见的天象奇观该如何观测？怎样才能拍摄出效果绝佳的照片？如何与“小日食”合影呢？

我国天文教育专家、天津市天文学会理事赵之珩介绍说，金星将影子投在太阳圆面上，缓慢地从明亮的日面上经过，这种天象称为“金星凌日”。由于日面被金星遮挡，因此，“金星凌日”也被看做是一次“小日食”。这一罕见天象两次一组，两次相距８年，两组之间相距１００多年。上一次“金星凌日”是２００４年６月８日，下次要等到２１１７年１２月１１日，距今１０５．５年。

６月６日上演的“金星凌日”，我国各地都可以看到，其中在东部可见到完整凌日过程，西部地区则可见太阳“带凌而出”。

“金星凌日”分为“凌始外切”“凌始内切”“最小角距离”“凌终内切”和“凌终外切”５个阶段。赵之珩提示说，本次“金星凌日”整个过程持续６个多小时，但“凌始外切”和“凌终外切”时间在全国各地略有不同，前者在６时０８分至６时１１分之间，后者在１２时４８分到１２时５２分之间。因此，各地公众在观测时要提前做好相应的案头工作。

为了更好地观测本次“金星凌日”，赵之珩建议公众要在早上６点之前起床，提前赶到能够看到日出的地点，争取看到从“凌始外切”到“凌终外切”的全过程。

赵之珩提醒说，观看“金星凌日”时，公众一定要使用滤光片，不可裸眼观看，电焊工头盔上的滤光片是首选，也可使用专用太阳观测镜等工具，每看一次不可超过１０秒钟，否则会伤害眼睛。

爱好天文摄影的公众要提前准备好相机，数码或胶片的都可以，并提前选好能看到日出地平线的地点。在拍摄时，一定要在相机镜头前加上滤光片，采用适当的曝光量。“可用长焦镜头摄影，每隔１０分钟－１５分钟拍摄一张，入凌和出凌（开头和结尾）期间可以连续拍摄。如能选取一些有特色的背景物，比如高塔、古建筑、树木等，将更有观赏和纪念价值。”赵之珩说。

天文专家表示，今年是太阳活动高峰年，日面上经常有太阳黑子出现。如有太阳黑子，最好把“金星凌日”与太阳黑子拍摄在同一张照片上，这将是难得的天文摄影佳作。

http://www.cbc.ca/news/technology/story/2012/05/16/f-venus-transit.html

June 5 alignment of Earth, sun and Venus won't happen again for 105 years

By Kazi Stastna,

CBC News

Posted: Jun 1, 2012 5:34 AM ET  Last Updated: Jun 1, 2012 3:06 PM ET

Venus transits across the sun as seen over Hong Kong on June 8, 2004. There have only been seven such transits since 1631. The eighth will take place on June 5, 2012, and will be the last chance that people alive today will have to witness the rare celestial event. The next transit won't occur until 2117. (Bobby Yip/Reuters)

If you happen to glance at the sun in the early evening next Tuesday and notice a black dot moving across it, fear not, that's not dust in your eye or an early sign of glaucoma — it's Venus.

Our nearest planetary neighbour will be passing between the Earth and the sun starting at 6:04 p.m. ET on June 5 and will be aligned in such a way that its passage will be visible with the naked eye. The transit, as it's known, will last about six hours, but in most parts of Canada, it will be visible for only a few hours before sunset.

"The view is best the farther west you go, but, actually, it's pretty good for most of Canada," said Mike Reid, a lecturer in the department of astronomy at the University of Toronto and a public outreach co-ordinator at the university's Dunlap Institute for Astronomy and Astrophysics.

Eastern Australia, Asia will have best vantage points

The best view of the transit will be in the middle of the Pacific Ocean, but people in eastern Australia, New Zealand, Japan, Philippines, Korea, parts of China and Russia and Pacific nations like Papua New Guinea will also be able to see Venus's entire journey across the sun — although for them it will be occurring the morning of June 6.

Europeans, and people in eastern Africa, the Middle East, India and places in between will have to get up very early to catch a glimpse of the transit. West Africa, Portugal and parts of Latin America and Spain will miss it altogether.

For people on Earth to be able to see Venus transiting the sun, the planet has to cross the plane of Earth's orbit at the precise time that Earth, Venus and the sun are lined up.

That doesn't happen very often because most of the time when Venus crosses Earth's orbital plane, Earth is somewhere else in its orbit, so there is no direct sight line from Earth to Venus and the Sun

"Earth orbits the sun slower than Venus. If they both orbited in the same plane, like runners running around a track, then every time Venus 'lapped' Earth, there would be a transit," Reid said.

"However, Venus's orbit is 'tilted' relative to Earth's orbit, so the only way Venus and Earth can line up with the sun is if Venus laps Earth exactly when Venus happens to be passing through the plane of Earth's orbit.

"That only occurs at two points in Venus's orbit (the two points where the ring of Venus's orbit 'punctures' Earth's orbit). The odds of Venus happening to be at one of those two points when it 'laps' Earth are small."

Children in Bombay watch the June 8, 2004, transit of Venus. Special eye protection must be worn when looking directly at the sun to avoid eye damage. (Arko Datta/Reuters)

When transits do occur, they occur in pairs, with each transit in a pairing separated by eight years, and then not again until 105.5 or 121.5 years later. The last transit was in June 2004, and after this year's transit, the next one won't occur until December 2117. (Learn why the timing of transits follows this pattern.)

"It's very similar to the reason why we don't see a solar eclipse every time there is a new moon," Reid said. "You'd think that every time the moon comes between Earth and the sun, it would block out the sun, but sometimes, it's a bit above the sun, sometimes it's a bit below the sun."

Mercury also occasionally aligns with the Earth and the sun, and does so more frequently than Venus, but its transits can't be seen with the naked eye, because Mercury is much smaller and farther away from Earth than Venus.

Transits used to measure distance to sun

This will be the eighth transit of Venus since the invention of the telescope in 1609. The earliest record of a transit we have is from 1639 (the transit prior to that, in 1631, was known about but not visible in Europe).

The 1639 transit of Venus was documented by a young amateur astronomer in Lancashire, England, named Jeremiah Horrocks (sometimes spelled Horrox) and his friend William Crabtree. Horrocks used his observations to measure the diameter of Venus and to obtain a crude estimate of the distance between the Earth and the sun.

In later years, astronomers, most notably Edmond Halley, refined the method of using transits to estimate the distance to the sun by measuring the differences in how long the transit appeared to take depending on where the observer was located on Earth and applying the principles of the displacement effect known as parallax.

Getting a precise measurement of the distance preoccupied many researchers in the 18th and 19th centuries and was in part the motivation behind Captain James Cook's first expedition to the South Pacific. Cook was tasked with observing the 1769 transit of Venus by Britain's Royal Society and did so from Tahiti.

Today, the distance between the Earth and the sun, referred to as the astronomical unit, is well known and can be measured using radar, and transits have instead begun serving another purpose: they are helping astronomers locate planets outside our solar system, known as exoplanets.

When these planets pass between Earth and the star they are orbiting, they block out some of the light the star is emitting, causing it to dim slightly. By observing the nature of this dimming, astronomers can learn about the planet causing it.

"You look for the same pattern of dimming to repeat over and over again, and you can deduce from that what kind of planet it is, how far from the star it is, whether it's a big planet or a small planet, and now, we're getting to the point where we can measure some elements of the atmospheric composition of the planet," Reid said.

Public viewings planned across Canada

To mark this year's transit of Venus, events are being held across Canada at which the public will be able to observe the transit safely and learn more about the phenomenon.

The Slooh Space Camera, a robotic telescope that transmits live images of space online, will have real-time feeds of the transit from solar telescopes in Australia, Japan, New Zealand, Hawaii, Norway, Arizona and New Mexico. Other sites, including those of the San Francisco-based Exploratorium museum and the group Astronomers Without Borders, will also carry live webcasts of the event.

Children in Vienna watch an image of the sun and Venus projected onto a cardboard box on June 8, 2004. Simple devices like pinhole projectors can be made to view the transit indirectly. (Heinz-Peter Bader/Reuters)

Users of Apple and Android smartphones can download a free transit of Venus app created by Astronomers Without Borders and ESRI, a company that works with geographic information system, or GIS, technologies. The app, available at iTunes App Store and Android Market, allows users to find out when and where the transit will be visible, follow transit-related tweets, download photos and videos and learn about the history of the transit.

In Toronto, the Dunlap Institute for Astronomy and Astrophysics will be holding a public transit viewing at Varsity Stadium, starting at 5:30 p.m. ET. It will distribute 5,000 pairs of specially treated glasses that will allow people to look directly at the sun without damaging their eyes and will have telescopes on hand equipped with solar filters that show the sun in greater detail than can be seen with the naked eye.

Astronomers will be present to answer questions and provide information about transits and their use in contemporary astronomy. Participants will also be able to view live streams of the transit from around the world.

The Institute has distributed another 43,000 pairs of "transit glasses" to universities and Royal Astronomical Society of Canada chapters across the country, and similar glasses are being sold through Sky News magazine and on various websites.

Eye protection needed

For those who can't get their hands on a pair of glasses, Reid suggests crafting a simple device like a pinhole projector to project an image of the sun onto the ground or another surface. (The Exploratorium science museum in San Francisco suggests a variation on this using binoculars.)

As with the annular eclipse that dazzled millions of people around the world on May 20, the key to viewing the transit of Venus safely is to avoid looking directly at the sun without a protected lens. Sunglasses or ordinary telescope lenses are not enough to protect the eyes.

To be safe, lenses must be treated with something like an aluminized film like Mylar or have a strong filter such as the type found on some welder's glasses. The Royal Astronomical Society of Canada lists a number of safe lens options in a special section of its website devoted to the transit.

Whatever viewing method you choose, the key, says Reid, is to not miss what will be your last chance to see Venus in transit.

"It will not occur again until 2117, so it's worth trying to see it," Reid said.