Posted
on December 14, 2014
Today
we celebrate one of the biggest of Europe's early astronomers: Tycho
Brahe.
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| This drawing shows Ptolemy's theory of a geocentric (Earth-centered) universe. |
A
long time ago some very intelligent, learned, and wise men in Ancient
Greece thought long and hard about a lot of things, and they decided
that the heavens – the stuff we see in the sky – was perfect and
unchanging. Everything in the skies seemed to move in regular
rhythms, and they shone with perfect, unblemished light, and things
stayed the same year after year, century after century. The perfect,
unchanging heavens became a part of the Greeks' philosophy about the
universe.
Another
thing that was self-evident as these learned men looked upwards:
everything circled us! We Earthlings were the center of it
all! The Moon and Sun clearly circled the Earth, as did the stars.
The planets had more complicated motions – with a little bit of
backwards action interrupting their progression across our skies –
but they also basically traveled around our world.
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| This is a modern diagram of Ptolemy's system. Note that the Sun and Moon and the sphere of stars simply orbit around the Earth...but those pesky planets with their back-and-forward motion must be doing cycles in their cycle, Ptolemy thought. |
But
the Scientific Revolution upset all of these notions:
In
the 1540s, Nicolaus Copernicus published his theory that the Earth
was NOT the center of the universe. His theory was that the Earth and
other planets traveled around the Sun – although he kept those
perfect circular orbits thought up by the Greeks.
This
was SO not cool with people – after all, we liked being the center
of things!
In
1609, Johann Kepler published his theory that the Earth and other
planets actually traveled around the Sun in non-perfect oval orbits.
Non-perfect
orbits?!? What's the universe coming to?
The
next year, Galileo Galilei published his findings from studying the
universe with his newly built telescope. He had discovered four tiny
lights that clearly revolved around the planet Jupiter. This was
proof that Copernicus was right, at least in the idea that not
EVERYTHING in the universe revolved around us! And Galileo argued
that Copernicus was right about the big idea of the Earth circling
the Sun, rather than vice versa, as well.
Still
not cool with most people! Galileo was put under house arrest.
Sandwiched
between Copernicus, on the one hand, and Kepler and Galileo, on the
other, was today's birthday, Tycho Brahe. His big contribution to
tearing down the Greeks' perfect heavens was proving that the heavens
were not unchanging.
.jpg) |
| One of the supernovas that Tycho observed was one that appeared in 1572. |
Brahe
(who was born on this date in 1546) carefully observed a number of
“new stars,” called stellae novae – now called novas and
supernovas. Because of that “the heavens can't change” rule,
people thought that new stars in the sky were local phenomena, found
in Earth's atmosphere, or at least closer to Earth than the Moon. But
Brahe knew that, if the new stars were that close, they should appear
to be in slightly different spots in the sky, compared to the distant
“fixed stars,” when seen from different observers positioned many
miles away from one another. This parallax didn't occur, however,
thus proving that the stars were, in fact, much farther away from the
Earth than the Moon – that they were, in fact, part of the heavens.
And,
voila, Tycho had proved that the heavens could change!
By
the way, Tycho did not think that Copernicus was right about the
Earth traveling around the Sun. He didn't go along with the Ancient
Greek idea of everything circling the Earth, either – instead, he
came up with his own system:- The other planets circled the Sun.
- The Sun and the Moon circled the Earth.
- The “fixed stars” circled everything.
You
may be wondering why Tycho didn't agree with Copernicus. He had two important arguments:
1) the Earth was too sluggish and heavy to be continuously in
motion, and
2) the Earth's orbit around the Sun, if it
existed, would cause stellar parallax.
(Actually, there IS stellar parallax – the Sun, planets, and nearby stars can be seen to be in slightly different positions, compared to farther away stars, when the Earth is on opposite sides of its orbit. But the
angles are very small and difficult to measure. Tycho knew that the apparent lack of parallax wouldn't be a problem if there was an enormous gap between
the outermost planet – which at the time was Saturn – and the stars. But such an
enormous gap seemed ridiculous to Tycho and other scientists.)
About that enormous gap...
Tycho Brahe thought an enormous gap between Saturn and the rest of the farther-off universe (stars and such) was ridiculous. But that enormous gap is true!
Saturn
is only hundreds of millions of miles away from Earth, but even the
nearest stars are TRILLIONS of miles away!
And
just one trillion is a million millions! So think about how far many
trillions of miles are!
To
give you just a bit more perspective on how much a trillion is, think
of it this way:
Count
one second. It's about as long as it takes you to say “A thousand
one.”
Now
consider this: The United States has not yet existed for a trillion
seconds! As a matter of fact, almost nothing in your history books
had happened yet, a trillion seconds ago! Not Tycho Brahe's birth,
not the Ancient Greeks, not even the Ancient Egyptians!
As
a matter of fact, a trillion seconds ago is about 31 THOUSAND years
ago, and humans were just figuring out how to fire pottery and
inventing the bow and arrow!
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on this date:
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