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The black-drop effect occurs when two limbs, such as the edge of
Mercury and the edge of the Sun, are very close together. It is caused by
atmospheric scintillation, or "seeing", the same effect as objects in the
distance shimmering when viewed through heat waves rising off the hot
pavement in the summer. The black-drop effect during a transit of Mercury
was first observed in 1677.
A transit occurs when a planet passes directly between the viewer on
Earth and the Sun. The only two planets in our solar system that can
transit the sun are Mercury and Venus because their orbits around the Sun
are inside of the Earth's orbit.
In its orbit around the sun, Mercury passes the Earth every 116 days.
But only about one out of every 23 of Mercury's passages between the Earth
and the sun result in a transit because Mercury's orbit is tilted seven
degrees in respect to the Earth's orbit. Only 13 other transits of Mercury
have taken place this century.
This transit was unusual however because it combined a total transit
with a partial transit. As viewed from some locations on Earth, Mercury
was not completely in front of the Sun's disk, but skimming just along the
inside edge of the sun. This type of transit is called a partial or
grazing transit. For other locations on Earth, Mercury was completely in
front of the Sun's disk, which is the traditional type of total transit.
This rare combination of total and grazing has not occurred since the
invention of the telescope and will not occur again until the year 2391.
At the time of the transit, Mercury was only 9.9 arc seconds across,
dwarfed by the Sun, which had an apparent size of 1940.47 arc seconds,
some 196 times larger.
Historically, transits of Mercury have been used to determine the
planet's orbits and size more precisely. Today, transits are used to
measure the exact diameter of the Sun, which may vary with the solar cycle
and affect weather on Earth.
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