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article imageMercury crossing in front of the Sun is a celestial event

By Karen Graham     May 8, 2016 in Science
On Monday, starting at 7:00 a.m. ET, astronomy buffs and stargazers will get a chance to observe (carefully, I might add), a rare celestial event when Mercury passes in front of our Sun.
The last time we Earthlings were given an opportunity to observe Mercury's transit of the Sun was on November 8, 2006. Mercury's transit of our star occur about 13 or 14 times each century and are much more frequent than the transits ofVenus.
The transit of Mercury, as it is called, is quite similar to a lunar eclipse, when the moon passes in front of the Sun. But where the moon will block the Sun, Mercury will appear as a tiny black dot as it crosses, and it will take seven-and-a-half hours. Remember that Mercury is our smallest planet and takes only 88 days to orbit the sun, compared to our 365 days.
This diagram shows the approximate relative sizes of the terrestrial planets  from left to right: Me...
This diagram shows the approximate relative sizes of the terrestrial planets, from left to right: Mercury, Venus, Earth and Mars. Distances are not to scale.
NASA Mercury image: JHUAPL Venus image: JPL Mars image: HST
Actually, the entire path across the sun will be visible across the Eastern United States but unless you have eye protection and a magnified telescope, it's highly unlikely you will see anything.
The safe way to observe the transit
The best way to observe the event is to go to Slooh, the European Space Agency site, or the NASA site.
“The transit of Mercury reminds us that all of the planets, including Earth, are in rapid and perpetual motion,” said Paul Cox, Slooh’s host, in a press release, according to Time.
For those who may have the necessary equipment to view the transit, NASA stresses that "viewing this event safely requires a telescope or high-powered binoculars fitted with solar filters made of specially-coated glass or Mylar." And if you want to see the tiny dot of Mercury on its celestial crawl, you will need magnification.
The November 15  1999 Mercury Transit as seen by the Transition Region and Coronal Explorer (TRACE) ...
The November 15, 1999 Mercury Transit as seen by the Transition Region and Coronal Explorer (TRACE) spacecraft.
NASA
More than a celestial event
For a transit event to occur, the sun, Mercury, and Earth all have to be aligned with each other. Obviously, this can't be a random thing, but is carefully calculated taking into account the difference in Mercury's inclination as compared to that of the Earth, or in this case, a 7-degree incline.
Knowing this, scientists discovered there were only two spots where the planets could line up with the sun. These were the places where Mercury crosses Earth's orbital plane. Now stay with me, here. The Earth only lines up with these two intersection spots around May 8 and November 10 each year, plus or minus a few days.
So, besides the fact that transits are not random events, but precisely calculated, they are also an opportune time for scientists to study and learn. The first observed transit of Mercury was observed in 1631 and since that time, NASA says transits have been used to measure the distance between the Earth and the sun.
Now we use these celestial events to measure the planets' exospheres, the thin layer of gases that make up the atmosphere. "When Mercury is in front of the sun, we can study the exosphere close to the planet," NASA scientist Rosemary Killen said in a release from the Jet Propulsion Laboratory. "Sodium in the exosphere absorbs and re-emits a yellow-orange color from sunlight, and by measuring that absorption, we can learn about the density of gas there."
A transiting planet causes a drop in the sun s brightness. The subtle change on the light curve can ...
A transiting planet causes a drop in the sun's brightness. The subtle change on the light curve can calculated.
NASA
Scientists have also discovered that a transiting planet causes a slight drop in the sun's brightness. Space scientists use this information from transits in not only helping to find new planets but in determining their size. "This phenomenon is the main way we find planets outside the solar system," NASA says.
If you are wondering what that black dot is going to look like up close, check out the following pictures taken by the MESSENGER spacecraft, the first to orbit Mercury.
These images of Mercury were taken onboard the MESSENGER spacecraft  the first ever to orbit the inn...
These images of Mercury were taken onboard the MESSENGER spacecraft, the first ever to orbit the innermost planet.
NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington
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