On November
8, Mercury
will transit the Sun. From Earth, we'll see the silhouette of Mercury
travel across the disk of the Sun. Transits are uncommon but predictable
events. The geometry of Mercury (or Venus), the Earth and the Sun has to be
aligned precisely for a transit to occur. Most of the time, these inner planets
cross above or below the disk of the Sun.
Historically,
transits of Mercury and Venus promised a method to better measure the distance
to the Sun. Johannes Kepler predicted and then endeavored to see a transit of
Mercury in 1607. Likely, he observed a sunspot with his camera obscura--two
years before Galileo discovered sunspots with his primitive telescope in 1609.
Later, telescopes aided transit observers in estimating the scale of the solar
system. This is a difficult method, fraught with problems of timing, optical
effects caused by Earth's atmosphere, poor weather and other complications.
Observing transits gave us the first reasonable estimates for the distance
scale of the solar system, but other methods have superseded transit
observations. Although solar transits are now interesting rather than
compelling for scientists, transit observations are far from dead.
NASA's
Kepler Mission will be the first space mission to search for Earth-size and
smaller planets around other stars in our neighborhood of the galaxy. It's
named for Johannes Kepler, the first astronomer to predict a transit. The
Kepler spacecraft is a special-purpose telescope that will precisely measure
the light variations from distant stars, looking for planetary transits.
Searching for transits of distant "Earths" is like looking for a drop in
brightness when a moth flies across a searchlight. Measuring repeated transits,
all with a regular period, duration and change in brightness, provides a
rigorous method for discovering and confirming planets and their orbits--planets
the size of Earth and smaller in the habitable zones around other stars. NASA's
Kepler Mission launches in 2008, and will search more than 100,000 stars for
evidence of other Earths in transit across their stars. We'll learn whether
Earth-size planets are common or rare.
Here at
home, the transit of Mercury is a good reason for a daylight astronomy event.
The transit will last for almost 5 hours, and observers in the Americas,
Australia, eastern Asia and the Pacific Ocean will be able to see all or part
of Mercury's excursion across the disk of the Sun. (This transit will not be
visible from Europe, Africa and western Asia--it will be nighttime there when
the transit occurs.) If you will be on the sunny side of planet Earth, now's
the time to plan an event with your local astronomy club. [See map of transit visibility here.]
More than
200 astronomy clubs across the US are part of
the Night Sky Network (NSN), a network of amateur astronomers who are
interested in sharing astronomy with the public. Participating NSN clubs
receive kits of materials that support star parties, mini-lessons on astronomy,
demonstrations of astronomical concepts. For NSN members, teleconferences with
scientists offer further information on current astronomical events, like the
transit of Mercury. In early September, a new kit was distributed to NSN clubs:
"Shadows and Silhouettes." This new kit was developed by the Astronomical
Society of the Pacific in collaboration with the Kepler Mission Education and
Public Outreach program, which is conducted by the SETI Institute and Lawrence
Hall of Science at University of California Berkeley in collaboration with NASA. So far, 160 clubs have
received the new materials, with more applying for their kits each day.
"Shadows
and Silhouettes" provides the tools for demonstrations and short lessons on
shadows and silhouettes in the astronomical context: how shadows help us
observe craters on the Moon, phases of the Moon, eclipses, and--of
course--transits. Amateur astronomers across the nation will be reaching out to
the public to demonstrate and explain transits and their importance for NASA's
Kepler Mission. On November 8, some will be observing the transit of Mercury.
You may be able to observe this transit, almost four centuries after Johannes
Kepler first predicted that Mercury would transit the Sun in 1607.
For more
information, please visit these websites:
-
Night Sky Network. Identify a NSN club
in your region, or, learn how your astronomy club can become a member.
-
ASP's
"Universe in the Classroom:" Learn how to observe the transit safely.
-
NASA's Kepler
Mission: Learn about this mission to find other Earths; build a transit
model.
-
SETI Institute: Learn about searching for life
beyond Earth.
-
Lawrence Hall of Science:
Explore science and math education resources.
More information on Planetary Transits:
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