Download Rotating Sky Have you ever laid outdoors on a starry night, gazing

Rotating Sky
Have you ever laid outdoors on a starry night, gazing up at the stars? As you watch, the stars seem to
move across the sky. The sky seems to be rotating right over your head. In fact, from the Northern
Hemisphere, the sky appears to rotate completely around Polaris, the North Star, once every 24 hours.
Now think about what you see every day. During the day, the sun
appears to move across the sky. From here on Earth, it seems as if Earth
is stationary and that the sun, moon, and stars are all moving around
Earth. But is the sky really moving above you? A century ago, before
there were space shuttles or even telescopes, there was no easy way to
find out.
The Rotating Sky This photo was made by exposing the camera film
for several hours. Each star appears as part of a circle, and all the stars
seem to revolve around a single point.
Wandering Stars
When the ancient Greeks watched the stars move across the sky, they noticed that the patterns of most of
the stars didn’t change. Although the stars seemed to move, they stayed in the same position relative to
each other. For example, the constellations kept the same shapes from night to night and from year to
year.
As they observed the sky more carefully, the Greeks noticed something surprising. Five points of light
seemed to wander among the stars. The Greeks called these objects planets, from the Greek word
meaning “wandering star.” The Greeks made very careful observations of the motions of the five planets
they could see. You know these planets by the names the ancient Romans later gave them: Mercury,
Venus, Mars, Jupiter, and Saturn.
Greek Ideas: Earth at the Center
When you look up at the sky, you can almost imagine that you are under a rotating dome with the stars
pasted on it. The Greeks thought that they were inside a rotating dome they called the celestial sphere.
Most Greek astronomers believed that the universe is perfect and unchangeable and that Earth is
stationary in the center of the celestial sphere. Since geo is the Greek
word for Earth, an Earth-centered explanation is known as a geocentric
system. In a geocentric system, Earth is at the center of the universe.
Meaning all other object move around Earth.
In 140 ad, the Greek astronomer Ptolemy explained the motion of the
planets in another way. Like the earlier Greeks, Ptolemy thought that
Earth is at the center of the system of planets. Ptolemy also thought that
the moon, Mercury, Venus, the sun, Mars, Jupiter, and Saturn revolve
around Earth.
In Ptolemy’s explanation, however, the planets move on little circles that
move on bigger circles. Ptolemy thought that this explained why the
planets seem to move at different speeds, and even backwards, among the stars. For the next 1,400 years,
people believed that Ptolemy’s ideas were correct.
In the 1500s, an astronomy book published this illustration to the right, of Ptolemy’s system.
Copernicus’s Idea: Sun at the Center
In the early 1500s, the Polish astronomer Nicolaus
Copernicus developed another explanation for the motions
of the planets. Copernicus thought that the sun is at the
center of the system of planets. His sun-centered system is
called a heliocentric system. Helios is Greek for “Sun.” In a
heliocentric system, Earth and the other planets revolve
around the Sun. Copernicus’s explanation included the six
planets he knew about: Mercury, Venus, Earth, Mars,
Jupiter, and Saturn.
Galileo’s Observations
In the 1500s and 1600s, most people still believed Ptolemy’s geocentric explanation. However, the Italian
astronomer Galileo Galilei, who lived nearly 100 years after Copernicus, thought that the heliocentric
explanation was correct.
Galileo was the first scientist to use a telescope to look at objects in the sky. With his telescope, Galileo
made two discoveries that supported the heliocentric model. First, Galileo saw four moons revolving
around Jupiter. Galileo’s observations of Jupiter’s moons showed that not everything in the sky revolves
around Earth.
Galileo’s observations of Venus also supported Copernicus’s heliocentric system. Galileo discovered that
Venus goes through phases similar to those of Earth’s moon. Galileo reasoned that the phases of Venus
could not be explained if Earth were at the center of the system of planets. So Ptolemy’s geocentric
system could not be correct. Galileo’s evidence gradually convinced others that Copernicus’s explanation
was correct. Today, people talk about the “solar system” rather than the “Earth system.” This shows that
people accept Copernicus’s idea that the Sun is at the center.
Brahe and Kepler
Copernicus and Galileo had correctly identified the Sun as the center of the system of planets. But
Copernicus, like Ptolemy, assumed that the orbits of the planets are circles.
Copernicus’s ideas were based on observations made by the ancient Greeks. In the late 1500s, Tycho
Brahe, a Danish astronomer, made much more accurate observations. Brahe carefully observed the
positions of the planets for almost 20 years.
In 1600, a German mathematician, Johannes Kepler, went to work analyzing Brahe’s data. Kepler tried to
figure out the shape of the planets’ orbits. At first, he assumed that the orbits are circles. When Kepler
tried to figure out the exact orbit of Mars, however, no circle fit the observations.
Kepler had discovered that the orbit of each planet is an ellipse. An ellipse is an elongated circle, or oval
shape. Kepler found that if he assumed that Mars’s orbit is an ellipse, his calculations fit Brahe’s
observations better.
Inertia and Gravity
Kepler had discovered the correct shape of the planets’ orbits. But he could not explain why the planets
stay in orbit. The work of the English scientist Isaac Newton provided the answer to that puzzle. Newton
concluded that two factors—inertia and gravity—combine to keep the planets in orbit.
Galileo had discovered that a moving object will continue to move until some force acts to stop its
motion. This tendency of a moving object to continue in a straight line or a stationary object to remain in
place is the object’s inertia. The more mass an object has, the more inertia it has. An object with greater
inertia is more difficult to start or stop.
Isaac Newton picked up where Galileo had left off. Late in his life, Newton told the story of how
watching an apple fall from a tree in 1665 had made him think about motion. He hypothesized that the
same force that pulls the apple to the ground also pulls the moon toward Earth. This force, called gravity,
attracts all objects toward each other. The strength of gravity depends on the masses of the objects and the
distance between them.
Newton figured out that Earth keeps pulling the moon toward it
with gravity. At the same time, the moon keeps moving ahead
because of its inertia. Earth curves away as the moon falls toward
it, so the moon winds up in orbit around Earth.
In the same way, the planets are in orbit around the sun because
the sun’s gravity pulls on them while their inertia keeps them
moving ahead. Therefore, the planets keep moving around the
sun and end up in orbit.
Inertia and Gravity If there were no force of gravity, inertia
would make a planet travel in a straight line. But because gravity
pulls the planet toward the sun, the planet actually travels in an
elliptical orbit around the sun.
More to Discover
Since Newton’s time, our knowledge about the solar system has increased dramatically. Newton knew of
the same six planets the ancient Greeks had known—Mercury, Venus, Earth, Mars, Jupiter, and Saturn.
Now astronomers know of two more planets—Uranus and Neptune. Astronomers have also identified
many other objects in the solar system, such as comets and asteroids, and ice worlds (Pluto).
Galileo and Newton used telescopes on Earth to observe the solar system. Astronomers still use
telescopes on Earth, but they have also made close-up observations of the planets from space probes sent
far into the solar system. Our understanding of the solar system continues to change every day. Who
knows what new discoveries will be made in your lifetime!