"There will be no other till the twenty-first century of our era has
dawned upon the Earth and the June flowers are blooming in 2004. What will
be the state of science ? God only knows." - William Harkness, U.S. Naval
Image left: Venus Transit seen by NASA's Sun-observing TRACE spacecraft. More images and movies. Credit: NASA/LMSAL
NASA joined the world in viewing a rare celestial event, one not seen
by any person now alive. The planet Venus appeared to cross in front of the
Sun as seen from Earth. The last "Venus transit" occurred more than a century
ago, in 1882, and was used to compute the distance from the Earth to the
Sun. Scientists with NASA's Kepler mission hope to discover Earth-like planets
outside our solar system by searching for transits of other stars by planets
that might be orbiting them.
The Venus transit was visible over about 75 percent of the Earth, and
ended at sunrise over central and eastern North America. The event was over
by the time the Sun rises over the West Coast of North America (but viewers
in Alaska could see the beginning of the transit and, for Northern Alaskans,
the entire transit, because the Sun does not go below the horizon).
"People using a filter approved for safe solar viewing can expect
to see a small black dot, about 1/30 the size of the solar disk, very slowly
moving across the Sun," said Fred Espenak, an eclipse expert at NASA's Goddard
Space Flight Center, Greenbelt, Md.
If people missed the June 8 Venus transit, they will have another chance
in 2012 (June 6). After that, there will not be another Venus transit until
2117 (December 11).
Right: The path of Venus is shown over a
nine-hour period with the Solar X-ray Imager instrument on NOAA's GOES-12
spacecraft. SXI takes a full-disk image of the Sun?s atmosphere once every
minute. The images are used by NOAA and the U.S. Air Force to monitor and
forecast space weather which affects everything from satellites to astronauts
to airplanes. More information. Credit: NOAA
During the 19th century, Venus transits were essential for astronomers
to fathom the scale of the heavens, because they were used to give a relatively
accurate distance from the Earth to the Sun. Once that distance was known
accurately, astronomers could determine the size of our solar system, and
calculate the distances to nearby stars by measuring how much they appeared
to shift against remote background stars as the Earth progressed in its orbit
around the Sun.
Russian and American missions have been
sent to Venus since 1962. The planet is similar to Earth in size, mass, and
composition, but its carbon dioxide atmosphere makes it hot enough to melt
lead. More on Venus and Magellan. Credit: NASA
So critical was this measurement that, beginning in 1761, leading nations
sent expeditions to remote corners of the globe to time exactly when Venus
appeared to begin its transit of the Sun. The precise timing of the transit
depended on location because different places on the globe saw the event
from different angles. The times were compared and the distance to the Sun
calculated using the known distances between expedition locations on the
Earth and trigonometry. Educators and students may do the calculations by
following an activity on the Web site or on the half-hour NASA Connect TV
The transit phenomenon has relevance to the future of astronomy as well.
There is evidence for more than 100 extrasolar planets (planets outside our
solar system) around other nearby stars. However, current techniques can
only detect large planets, gas giants like Jupiter. But a star might have
a planet that appears to pass in front of it by chance alignment with the
Earth, and planets similar in size to the Earth could be detected if they
transit their parent star.
mission, scheduled for launch in October 2007, will allow astronomers to
find smaller, presumably terrestrial extrasolar planets by looking for tiny
dips in the brightness of a star when a planet crosses in front of it. Periodic
brightness dips will signal the presence of a planet in orbit around the
star, even if the planet itself is not directly visible. Kepler will observe
about 100,000 stars in a patch of sky in the direction of the constellation
Cygnus for four years, making brightness measurements every 15 minutes, in
hopes of catching elusive transits. The Kepler mission is expected to detect
50 to 60 extrasolar planets with a similar distance from their parent stars
as the Earth is from the Sun.
Stellar Views from NASA's TRACE Spacecraft
NOAA's GOES Spacecraft Goes Beyond the Weather!
Swedish Solar Telescope High-Res Views
High Resolution Images
Sun-Earth Connection Venus Transit
Exploratorium Transit Page
Transit Predictions & Maps
European Southern Observatory
NASA Goddard Space Flight Center