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Sun-Earth Day 2007 presents: Living in the Atmosphere of the Sun

Sun-Earth Day 2007 presents: Living in the Atmosphere of the Sun

Mercury Transit


The first telescopic observation of Mercury was in 1610, by Galileo Galilee. Through his crude refractor, he could not resolve any detail in Mercury's disk nor Mercury's phases. It was not until 1639 that Giovanni Zupus observed the phases of Mercury. Like Galileo's observations of the phases of Venus, this proved that Mercury orbits the sun, not the Earth. Studying the ephemeris of Mercury published by Johannes Kepler in 1629, Pierre Gassendi saw that Kepler had predicted a transit of Mercury on November 7, 1631. Because of bad weather, only Gassendi and two other observers are reported to have seen the transit.By recording his observation of Mercury against the sun, Gassendi was able to estimate an angular size for Mercury of 20 arc seconds. The mean value for Mercury's angular diameter at inferior conjunction is about 11 arc seconds.

It should be noted that the transit of Venus, first viewed by Jeremiah Horrocks in 1639 was used to derive the distance to Venus and the sun and so the scale of the solar system. Though the transit of Mercury was viewed first and occurs more frequently, it is too far away and too close to the sun to display an easily observable parallax and so could not be used for this purpose.


Coupling between the orbital periods of Mercury, 88 days, and the Earth, 365.25 days, takes Mercury between the Sun and the Earth, an event called inferior conjunction, every 166 days. But because of the large inclination of Mercury's orbit (7 degrees) to the ecliptic, Mercury most often passes above or below the disk of the sun. Only when inferior conjunction occurs at the ascending or descending nodes of Mercury's orbit - the places where its orbit crosses the plane of the ecliptic - will a transit occur at some point on Earth. These transits then happen only in May or November.


On November 8th 2006, a rare crossing of the planet Mercury across the face of the sun will take place for observers in North and South America, Australia, and parts of Asia. In the US, the "transit" will be visible in its entirety from western states and visible until sunset in the mid west and eastern US. The entire transit will take approximately five hours and is the first such transit since the Mercury transit of May 7, 2003 which was well underway at sunrise for only the most eastern parts of the US but most of the rest of the world. Though not as rare as transits of Venus, there are only about 13 Mercury transits each century. The next transit will not be until May 9, 2016 so you don't want to miss this one!


During the transit, Mercury will be only 10 arc seconds in diameter, roughly 1/195th of the sun's 0.54 degree (1937 arc seconds) angular diameter so you will not be able to see it with the naked eye. Solar projection devices may be able to capture an image of the transit depending on weather and observing conditions. To get a really good look, however, will probably require a solar telescope. Best bets here are probably a refractor with at focal length of at least 500mm and preferably 800-1000mm with a white light or hydrogen-alpha filter.

For more information on the transit, go to:

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