Where do astronomers work? The answer seems easy. Astronomers
work in observatories, right? They arrive at sundown. They open the
big revolving dome, point the telescope at the heavens, and spend
the night looking at the stars. As the sun begins to rise, they
close up the dome and head home, satisfied with a good night's work.
Not exactly. Only a few astronomers work with telescopes at
night. Most work during the day. They sit at desks, think about the
stars, and try to understand how the universe works. They try to
solve puzzles. Only when they think they have solved one do they run
to their telescopes to see if their solution is correct.
Here's an astronomical puzzle for you: We know that the sun is at
the center of our solar system. Around the sun circle the planets.
Mercury is closest to the sun. Venus is next, followed by Earth and
Mars. The big planets are beyond Mars, after the asteroid belt:
Jupiter, Saturn, Uranus, and Neptune. Pluto is usually farthest from
Most drawings of the solar system show the planets orbiting in
the same plane, as if they were sitting atop the same table. Here is
the puzzle: Is that picture of the solar system correct? And if it
is, how do you know for sure?
Think about it. Picture the planets rolling around the sun on the
same tabletop. If that model is correct, then every now and then
Mercury and Venus would pass between Earth and the sun. This is what
happens a few times every year with our moon. When the moon passes
between us and the sun, we call it a solar eclipse, but you could
also say it was a "transit of the moon." Neither Mercury nor Venus
could blot out the sun partially or fully, the way the moon can.
They are too far away. They would just look like little black dots
on the sun.
But have you ever heard about Venus going across the sun? If our
tabletop solar system is correct, then a little mathematics would
show that Venus should pass between Earth and the sun every 586 days
or so. Yet for the entire 20th century, this did not happen. Now we
know part of the answer to our puzzle: The "tabletop" solar system
is not quite right.
So how do Earth and Venus keep missing each other? Earth must be
a little higher than Venus, or perhaps Venus is a little higher than
Earth. If this is so, then the orbit of Earth and the orbit of Venus
must be tilted a little. Even though the orbits are tilted, the
planets must line up now and then so that Venus is directly between
the earth and the sun. Again, using some mathematics, we can show
that a "transit of Venus," as it's called, will happen only twice
every 120 years or so.
This puzzle was first solved in the 17th century by German
astronomer Johannes Kepler. Kepler, working at his desk, reasoned
his way through the problem just as we have done (though with more
mathematics). He computed that a transit of Venus would occur on
Dec. 6, 1631. Kepler died in 1630, and the transit occurred when it
was night in Europe. No European astronomer observed it.
The next transit, on Dec. 4, 1639, was seen by English astronomer
William Crabtree. Other transits occurred in 1761 and '69, and again
in 1874. The most recent transit occurred on Dec. 6, 1882. The next
transit of Venus is a week from Tuesday, on June 8. Venus will start
passing across the face of the sun just about the time the sun rises
in the Middle East. It will take about six hours to cross in front
of the sun. If you live in the eastern half of the United States you
might be able to see some of the transit from sunrise until about
7:25 a.m. ET.
Transits of Venus occur in pairs, eight years apart. If you miss
this one, the next one is June 6, 2012. But after that, you'll have
to wait until 2117.
How the transit unlocked solar system secrets
When all is said and done, next week's transit of Venus will have
little impact on our daily lives. …