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 (p.2) |
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| Observation |
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| The
transit starts at about 5:13
Universal Time (UT) and ends at about 11:16
UT. If you’re watching the transit from the northern hemisphere,
Venus will move from left to right on the lower half of the Sun, at
a slight downward angle from start to finish. If you’re in the
southern hemisphere, Venus will move from right to left at a slight
upward angle on the top half of the Sun. (People in the southern hemisphere
are "upside down" relative to people in the northern hemisphere,
and vice versa.) |
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| The
transit starts with the first contact, when Venus’s outside
edge first appears to touch the outside edge of the solar disk, or
"solar limb." It will then take about 19 minutes for Venus
to arrive at the second contact, when Venus touches the inside of
the solar limb. The time between the first and second contacts is
known as the "ingress." A little over six hours later, the
third and fourth contacts will occur, incorporating the "egress."
(The egress repeats the process of the ingress, only in reverse.) |
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View
of the transit path from the northern hemisphere. Depending
on what your viewing location is, the path’s position
will vary slightly. |
A
simulation of the black-drop effect,
projected onto a real image of the Sun. |
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If
you’re able to observe either the beginning or end of the
transit with a telescope (equipped with
a special filter!), you’ll be able to observe something
called the "black-drop effect": Venus seems to develop
a smeared appendage as it enters and leaves the solar disk. The
black-drop effect impeded astronomers from accurately timing Venus’s
ingress and egress; without exact times, their calculation of the
astronomical unit (AU), the distance
from Earth to the Sun, was inexact.
For centuries, no one knew what caused the black-drop effect, although
suggestions included the idea that Venus’s substantial atmosphere
was responsible. In 1999, however, NASA’s Transition Region
and Coronal Explorer (TRACE) observed the black-drop effect during
a transit of Mercury. This was significant because TRACE was beyond
Earth’s atmosphere, and Mercury has no atmosphere to speak
of. It now appears that the black-drop effect may be due to limitations
in telescopes, combined with the fact that the disk of the Sun appears
darker at its edge.
(For more information, go to http://nicmosis.as.arizona.edu:8000/POSTERS/TOM1999.jpg.)
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| Another
thing to watch for as you observe the transit is a "halo"
around Venus. Mikhail Lomonosov, a Russian scientist observing from
St. Petersburg, noticed this halo during the
1761 transit. He proposed that the halo occurred because Venus
had an atmosphere; he was later proven correct by the astronomer William
Herschel. |
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| Complication |
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| Even
the most sophisticated telescope won’t be of any use if Mother
Nature decides to send in the clouds. If clouds are covering the Sun
in your neck of the woods (or at the Exploratorium’s
Webcast venue in Greece), the transit won’t be visible.
The map
on the previous page shows what kind of cloud cover might occur in
various locations. |
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