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“What’s Up”
Planetary Orbits and Gravity
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When Copernicus began studying
astronomy, the tables of planetary data
based on Ptolemy’s model were found to
be quite inaccurate. Copernicus thought a
simpler model with the Sun as the center would produce better results.
This view had been presented by Aristarchus 400 years before Ptolemy,
but Copernicus’ work, published just before his death, led to estimates
of planet-to-sun distances and to mathematical relationships that
produced better tables of planetary motion. His model failed to explain
some anomalies and did not receive widespread acceptance for more
than 50 years.
Kepler (1571 - 1630)
It was Johannes Kepler who provided the first real understanding of
the planets’ orbits. One of the reasons for the shortcomings of
Copernicus’ model was his belief that planetary orbits were circular.
Kepler was able to show that they were, instead, elliptical. With this
understanding he developed his now-famous laws of planetary motion
that gave not only a description of the orbital paths but the
mathematical relationship between a planet’s speed and its distance
from the Sun. (What are Kepler’s three Laws of Planetary Motion?)
Galileo (1564 - 1642)
While Kepler’s laws seemed to provide the support needed for
Copernicus’ model, it was still not widely embraced. Galileo, who was
a contemporary of Kepler, provided the answers to many of the
objections. With his telescope he determined qualities of the Moon
and Sun which banished two thousand year old beliefs. His
discoveries of moons orbiting the planet Jupiter, and of the phases of
Venus provided evidence for the sun-centered model. He also found
that there were far more stars than was previously believed.
Copernicus’ model gained acceptance, and with it came the
awareness that, while the Sun may be the center of our Solar
System, it was not the center of the universe. (Where is our
Solar System situated in the universe?)
Newton (1642 - 1727)
With Kepler’s laws providing a description of planetary
motion, it was time to look at why planets move as they do.
The answer to that would come from Sir Isaac Newton.
Newton was born on Christmas Day, 1642, the same year
Looking a little deeper into this concept, we can understand,
somewhat, the relationship between a planet’s speed and its distance
from the Sun. The gravitational force between two objects decreases
as the distance between them increases. Obviously Earth’s speed is
just right for its distance from the Sun. (What is Earth’s orbital speed?)
If it were farther from the Sun with less gravitational force on it, it
would need to move slower to stay in orbit. If it were closer, it would
need to move faster. Therefore, planets closer to the Sun have higher
speeds along with smaller orbits so they have shorter years. Planets
farther from the Sun have lower speeds and larger orbits so they have
longer years. Recall Kepler’s prediction that the orbits are elliptical
rather than circular. Therefore, the speeds will vary somewhat during a
year. However, the planets’ orbits are close to circular, and the
variations are small. (High school physics students can easily
determine an equation for orbital speed versus orbital radius using
circular paths.)
The concepts apply to any object orbiting the Sun. They also apply
to objects orbiting the planets, such as moons, man-made satellites, and
humans. Astronauts in the International Space Station are not actually
weightless; because they are in orbit about Earth, they are always
falling and, therefore, seem weightless. (How far above Earth’s surface
does the ISS orbit? How long does it take to orbit Earth once?)
For more information about the RASC or the International Year of
Astronomy please go to www.rasc.ca/stjohns. Answers to the questions
in parentheses can be found by clicking on 'Galileo's Universe' or the
NIE icon.
Gary Case RASC, St. John’s Centre
ACTIVITIES
1. The above article contains information developed by historical
figures who have changed the way we look at our world. Looking
through The Telegram find present-day figures who have had a similar
effect.
2. Create a list gathered from The Telegram of objects, both naturally
formed and man-made, affected by the gravitational pull of the earth.
Feb.
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Mar. 2
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Between each planet and the Sun
there is a gravitational force.
Without this force, Newton
determined that a planet
would keep moving in a
straight path… as the
planet tends to move in
its straight path, gravity
draws it to the Sun. In
effect, the planet is falling
toward the Sun as it moves
along. Think of a ball
thrown horizontally; it will, of
Illustration by: Shawn Martin
course, fall to the ground. The
faster it is thrown, the farther it goes. If it could be thrown
fast enough, it could travel all the way around the world… it
would be in orbit.
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ta
Go outside after
dark and point
this astronomical
chart toward North.
on
ati
str
Illu
Planets
by
:
Viewable in a pair of Binoculars or small telescope
Mercury - (magnitude 0) is low in the dawn. Using binoculars, look for it just above the
east-southeast horizon about an hour before sunup (aprox. 6:00 a.m.).
Venus - (magnitude –4.7) The dazzling "Evening Star" in the southwest.
Mars - Visible in the morning sky, along with Jupiter and Mercury within a 5º circle, look
to the East for this planetary conjunction.
Jupiter - Visible in the morning sky, along with Mars and Mercury within a 5º circle,
look to the East for this planetary conjunction.
Saturn - (magnitude +0.8) Viewable in the late evening well after midnight to the East of
the constellation Leo. Saturn is at opposition on March 8.
Uranus - (magnitude 5.9, in Aquarius) is below Venus at nightfall.
Neptune - is lost in the sunset.
Pluto (dwarf planet) - is low in the southeast before dawn.
Ceres (dwarf planet) - will be at opposition on 25 February 2009 when it will be at its
brightest, magnitude 6.9. Ceres was the first Minor Planet or Asteroid to be discovered. It
was discovered by G. Piazzi on 1 January 1801. It resides between Mars and Jupiter, has
a diameter of 932.6 km, and has an orbital period of 4.60 years. Ceres is now in the
constellation Leo and is a very changeling object to observe.
Moon
Waxing
First Quarter
Full
Last Quarter
Waning
New
Mar. 2
Mar. 4
Mar. 11
Feb. 16
Mar. 18
Feb. 18
Mar. 20
Feb. 24
Mar. 26
Libration - An effect caused by the slight wobble of the Moon as it orbits the Earth. The
Moon always keeps the same side toward the Earth, but due to libration, 59% of the
Moon's surface can be seen over a period of time.
Comets
Viewable in a pair of Binoculars or small telescope
C/2007 N3 ( Lulin )
Visible in the morning sky at a magnitude of 6.8, comet Lulin is quickly moving through
the constellation Virgo and into Leo. It is expected to reach magnitude 5, easily seen in
binoculars or a small telescope during February. It’s closest approach to Earth, 0.41 a.u.
(61 million km), occurs on February 24th. As most comets are unpredictable, Lulin’s
maximum brightness may vary. It may be possible to see the comet with the unaided eye
under a dark moonless sky. Comet Lulin won’t return again to the inner solar system for
more than a thousand years.
Use the star chart above to find the positions and dates to view this comet.
Definitions
For more activities go to
www.thetelegram.com and click on
Brought to you by
Magnitude - The units used to describe brightness of astronomical objects. The brighter
the object appears, the lower the value of its magnitude.
Opposition - When a celestial body is opposite the Sun in the sky as seen from Earth.
Its closest approach to the Earth and is best suitable for observing.
WARNING!“ When using a telescope or binoculars, always be sure NEVER TO LOOK
AT THE SUN! This can cause serious and permanent eye damage. To be safe, always make
sure the Sun is fully set belo w the horizon bef ore going outside with your telescope or
binoculars.”
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Newspaper In Education
THE TELEGRAM
0-3471272
Illustra
Copernicus (1473 - 1543)
Newton had a true desire to explain the world around him. Among
the many questions he strived to answer was the one that led to our
present understanding of gravity. (The story of the apple falling from
the tree seems to be true, and the apple tree at his English home is still
there. (Where was Newton’s home in England?)) By solving the Law
of Gravitation along with other laws of motion that he developed,
Newton was able to explain why Kepler’s laws of planetary motion
were true. Gravity is a force that exists between every two objects, and
not just the force that keeps each of us on the ground. For small
objects the force is insignificant…yet it is the force that holds our Solar
System together.
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Ptolemy (85 - 165)
The idea that the sun is at the center of
the Solar System was not
always the accepted
view. In the second century A.D.,
Claudius Ptolemy developed an
Earth-centered universe model. In
this model, Earth, not the Sun,
was at the center of the
universe, and everything else,
Moon, Sun, planets, stars,
revolved around it. In order
to account for certain
observed idiosyncrasies,
Ptolemy added circular
paths within the planets’
orbital paths. This view of
the universe persisted for
1400 years. Our present
understanding of planetary
motion began in the 16th
century with Nicholas
Copernicus.
Galileo died. Throughout his life, Newton received every scientific
honour that could be bestowed. Looking back, we still acknowledge
him as one of the world’s greatest scientists and mathematicians.
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As we know, the planets and
various other objects orbit the sun
in our Solar System. What is not
as well known is that gravity, the
same force that keeps each of us on
Earth’s surface, is responsible for
keeping the planets in their orbits.
February 13 - Mid March
Shawn Martin Observing Director RASC, St. John’s Centre