NASA's Hinode X-ray telescope captured Mercury in transit against the Sun's corona in Nov. 2006. Similar views are possible in H-alpha light. Credit: NASA
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Although the word “transit” can have many meanings, here on Universe Today, we’re talking about astronomical transits. This is where one object in space moves directly in front of another, partly obscuring it from view.
The most famous example of an astronomical transit is a solar eclipse. From our vantage point on Earth, the Moon appears to pass directly in front of the Sun, obscuring it, and darkening the sky. When seen from space, the Moon casts a shadow on the surface of the Earth; only people within that shadowed area actually see the transit.
In order to have a transit, you need to have a closer object, a more distant object, and then an observer. When all three objects are lined up in a straight line, you’ll get a transit. There can be transits of Mercury and Venus across the surface of the Sun, or a transit of Earth across the Sun, seen from Jupiter. We can also see the transit of moons across the surface of their planets. Jupiter often has moons transiting in front of it.
Astronomers use the transit technique to discover extrasolar planets orbiting other stars. When a planet passes in front of a star, it dims the light from the star slightly. And then the star brightens again as the planet moves away. By carefully measuring the brightness of the star, astronomers are able to detect if they have planets orbiting them.
Transits are also helpful for studying the atmospheres of objects in the Solar System. Astronomers discovered that Pluto has a tenuous atmosphere by studying how it dimmed the light from a more distant star. As Pluto began transiting in front of the star, its atmosphere partly obscured the star, changing the amount of light observed. Astronomers were then able to work out the chemicals in Pluto’s atmosphere.
The next transit of Mercury will occur in 2016, and the next transit of Venus is scheduled to occur in 2012.
We have written many articles about astronomical transit for Universe Today. Here’s an article about the transit of Mercury, and here’s an article about the transit of Venus.
A group of astronomers are now on the mysterious Easter Island, one of the few solid places to stand on Earth where a total solar eclipse will be visible on July 11, 2010. The majority of the eclipse’s path is over the ocean, so this will be one of the least observed eclipses ever. “This is one of the most interesting things that is possible for anyone on Earth to see in one of the most interesting places on the Earth that people can go,” said Jay Pasachoff from Williams College, who is the Chair of the International Astronomical Union’s Working Group on Eclipses. This will be his 51st eclipse.
Williams astronomer Jay Pasachoff during the partial phase of the March 29, 2006 solar eclipse. Photo by Anna Tsykalova.
While the eclipse is thousands of miles long in its is path of totality on Earth, it is just a few hundred miles wide. It will pass through French Polynesia on the Cook Islands, but, Pasachoff said, it doesn’t go through any of the main islands. “It misses Tahiti, but there are some atolls off the side of the path, and some eclipse scientists and ecotourists will be on cruise ships that are going into the path of totality. There will also be a group on an airplane observing the eclipse and we hope to compare all the other observations with the ones we get from Easter Island,” he said.
Easter Island is 4023 km (2,500 miles) west of Chile, and is famous for the Moai, giant statutes that were left by a Polynesian culture that mysteriously disappeared. But while the statues’ constant gaze look outward, all human eyes will be on the skies on July 11.
“The actual four minutes and forty-five seconds of totality that we are scheduled to have at Easter Island will be very exciting, as the last sliver of the sun is covered we can then take off the protective filters we’ve been using,” Pasachoff said, “and look at the next few minutes without any protection because the solar corona is about the same brightness as the full moon and is equally safe to look at. In fact we’ve been having a debate recently about whether we can get a very brief warning on the so-called eclipse glasses that many people use because those glasses are only for when any of the everyday sun is visible. They are so dense that they block the solar corona entirely, and the few people who don’t understand what is going on enough to take those glasses off during totality miss the whole event. There are people who have missed past eclipses by not knowing they had to take their glasses off.”
Pasachoff is joined by Professor Marek Demianski and two students. They will be carrying out high-resolution imaging to look for motions in the corona and to follow the varying magnetic-field configuration in the solar-corona as a function of the solar-activity cycle. Though the sunspot cycle remains in an extreme low, some other indications of solar activity have been increasing and we are eager to see the condition of the low and middle corona. They expect to see motions at least in polar plumes.
Also, they will be using the images to fill in gaps between the observations of the corona on the solar disk taken with NASA’s new Solar Dynamics Observatory and the observations of the outer corona taken with the Naval Research Laboratory’s coronagraph on the Solar and Heliospheric Observatory. Pasachoff and his team have contributed to similar images for the past several eclipses but now will have the improved SDO images as part of their montage. Several of the cameras will be computer controlled using software called Solar Eclipse Maestro written by Xavier Jubier of France.
Universe Today hopes to talk with Pasachoff after the eclipse to hear about his experiences.
The Williams College team is accompanied by a documentary crew filming for National Geographic Channel, and their activities will be covered in a special program entitled Easter Island Eclipse partly pre-recorded and partly expected to have new eclipse footage that will air on the National Geographic Channel on the evening of July 11th, at 11 pm.
Here are some resource Pasachoff provided for the eclipse:
On Sunday, July 11th: total solar eclipse on Easter Island (same time zone as Mountain Time in the US)
Partial eclipse begins 12:40:36 Altitude of Sun: 40°
Totality begins 14:08:30 Altitude of Sun: 40°
Totality ends 14:13:10 Altitude of Sun: 39°
Partial eclipse ends 15:34:16 Altitude of Sun: 32°
Duration of totality: 4 minutes and 40 seconds
Times in UT:
18:40 UT 1st contact
20:08 UT 2nd contact: total eclipse begins
20:13 UT 3rd contact: total eclipse ends
Update: I just received a note from Robin Zimmerman at DISH Network, and for those of you who have the DISH Network, there is a special channel, DishEARTH, Ch. 287, that features live 24/7 views of the earth from space and this Sunday the eclipse be shown. Robin said their network will allow people in the US to see the phenomena, live.
