Orbit Shapes Interactive Animation
This interactive animation illustrates two of the most important parameters
associated with orbits - shape and size. Orbits are ellipses. Mathematicians
and astronomers use the term "eccentricity" to describe how elongated (oval-shaped,
as opposed to circular) an ellipse is. A high eccentricity means an ellipse
is long and skinny; a low value means the ellipse is nearly circular. For an
ellipse, eccentricity can range from zero to one.
Astronomers use the term "semi-major axis" length to describe the size of
an ellipse. The major axis of an ellipse runs lengthwise along the long axis
of the ellipse. The semi-major axis length is the distance from the center
of the ellipse to one end along this major axis. The semi-major axis of an
ellipse is analogous to the radius of a circle.
Use the sliders in the interactive animation (below) to alter the shape and
size of the orbit of "your planet". Earth's orbit is shown for comparison.
(Note: If you cannot see the animation below, or it is not working properly,
you may need to download the latest Flash
player.)
An astronomical unit (AU) is the length of the semi-major axis of Earth's
orbit. It is commonly used to refer to distances
within our Solar System.
Notice how a planet with an elliptical orbit moves closer to and further away
from the Sun. The point of closest approach to the Sun is called perihelion.
The furthest point is called aphelion.
Johannes Kepler, a German astronomer
who lived in the early 17th century, discovered three
important laws about
planetary orbits.
Activities
Here are a few activities you may want to try, using the orbit shapes interactive,
to learn more about orbits:
- A "Hohman transfer ellipse" is a trajectory commonly used to send a spacecraft
between two planets that have circular orbits. The orbits of Earth and Jupiter are very nearly circular. Pretend "your planet" in the
interactive is a spacecraft being sent from Earth to Jupiter. Adjust its
eccentricity and semi-major axis until its perihelion point is just touching
Earth's orbit and its aphelion point is just reaching out to Jupiter's orbit.
About how long does it take a spacecraft on this trajectory to travel to
Jupiter? Note the position of Earth when the "spacecraft" reaches perihelion;
then count how many times Earth circles the Sun while the "spacecraft" travels
to Jupiter. Remember, each time the Earth goes around is one year!
- As you probably know, Pluto orbits further
from the Sun than any of the
other eight planets. You may not know that Pluto's
orbit is also more eccentric
than the orbits of the
other
eight planets. The orbit of Pluto has an eccentricity around 0.25 and a semi-major
axis length of about 39.5 AU. Set the values for "your planet" in the interactive
animation to those of Pluto. How does the orbit of Pluto compare to the orbit
of Neptune, the 8th planet in order from the Sun? Is Pluto always the most
distant planet from the Sun? If not, how
much of the time is it closer to
the Sun than another planet? How does the relatively high eccentricity of
Pluto's orbit influence this situation? If Pluto's orbit was less eccentric,
would it ever be the 8th planet?
- Most comets have very eccentric orbits. The most famous comet, Halley's
Comet, has an orbital eccentricity of 0.967 and a semi-major axis length
of almost 18 AU. This interactive animation only allows eccentricity settings
up to 0.9; check out the comet
orbit interactive to see what Halley's orbit
looks like. Some comets, called short period comets because they orbit the
Sun in less than 200 years, come from a region beyond Neptune's orbit called
the Kuiper Belt. A typical short period
comet might have an eccentricity around 0.9 and a semi-major axis length
of about 25 AU. Try out those values in the interactive animation on this
page. Where does a short-period comet spend most of its time? A comet only
forms its tails and becomes
visible to us when it is close to the Sun, well
inside the orbit of Jupiter. Notice how a comet is only visible for a few
months out of an orbit lasting decades to centuries (Halley's Comet takes
76 years to orbit the Sun). Some comets have much shorter orbital periods
of just a few years. Many of these are "Jupiter-family comets" that were
tossed into the inner Solar
System by the gravity of that giant planet during
a close encounter with Jupiter. Comet
Wild 2, the target of the Stardust
space mission, is one such Jupiter-family comet. Wild 2 has an eccentricity
of 0.54, a semi-major axis length of 3.44 AU, and an orbital period of 6.39
years. Set up Wild 2's orbit in the interactive animation to see a typical
Jupiter-family comet. Finally, some comets come from the far distant reaches
of the Oort
Cloud, the outermost
fringe of our Solar System. One such "long-period comet", Comet
Hale-Bopp, has an eccentricity of 0.995 and a semi-major axis length
of about 186 AU! Hale-Bopp put on a spectacular show in 1997, but won't be
back this way again for more than 2,500 years!
Orbit Shapes Interactive Animation
This interactive animation shows two important features
of orbits - shape and size.
Orbits are ellipses.
An ellipse can be like a circle, or it can be long and skinny. Mathematicians
and astronomers use the term "eccentricity" to
describe the shape of an orbit. An orbit shaped almost like a circle has a low
eccentricity close to zero. A long, skinny orbit has a high eccentricity, close
to one.
If you want to tell someone how big a circle is, you tell her or him the length
of the radius (or the diameter) of the circle. The "semi-major axis" of an
ellipse is like the radius of a circle. Imagine a long, skinny ellipse with
a high
eccentricity. Draw a line from one end of the ellipse to the other, through
the middle, along the long direction of the ellipse. The distance from the
center of the ellipse to one end of the line is the semi-major axis.
Use the sliders in the interactive animation (below) to change the shape and
size of the orbit of "your planet". Earth's orbit
is shown for comparison.
(Note: If you cannot see the animation below, or it is not working properly,
you may need to download the latest Flash
player.)
An astronomical unit (AU) is the length
of the semi-major axis of Earth's orbit. It is commonly used to refer to distances
within our Solar System.
Notice how a planet with an elliptical orbit moves closer to and further away
from the Sun. The point of closest approach to
the Sun is called perihelion.
The furthest point is called aphelion.
Johannes Kepler, a German astronomer
who lived in the early 17th century, discovered three
important laws about planetary orbits.
Activities
Here are a few activities you may want to try, using the orbit shapes interactive,
to learn more about orbits:
- A "Hohman transfer ellipse" is a trajectory commonly used to send a spacecraft
between two planets that have circular orbits. The orbits of Earth and Jupiter are very nearly circular. Pretend "your planet" in the
interactive is a spacecraft being sent from Earth to Jupiter. Adjust its
eccentricity and semi-major axis until its perihelion point is just touching
Earth's orbit and its aphelion point is just reaching out to Jupiter's orbit.
About how long does it take a spacecraft on this trajectory to travel to
Jupiter? Note the position of Earth when the "spacecraft" reaches perihelion;
then count how many times Earth circles the Sun while the "spacecraft" travels
to Jupiter. Remember, each time the Earth goes around is one year!
- As you probably know, Pluto orbits further
from the Sun than any of the
other eight planets. You may not know that Pluto's
orbit is also more eccentric
than the orbits of the
other
eight planets. The orbit of Pluto has an eccentricity around 0.25 and a semi-major
axis length of about 39.5 AU. Set the values for "your planet" in the interactive
animation to those of Pluto. How does the orbit of Pluto compare to the orbit
of Neptune, the 8th planet in order from the Sun? Is Pluto always the most
distant planet from the Sun? If not, how
much of the time is it closer to
the Sun than another planet? How does the relatively high eccentricity of
Pluto's orbit influence this situation? If Pluto's orbit was less eccentric,
would it ever be the 8th planet?
- Most comets have very eccentric orbits. The most famous comet, Halley's
Comet, has an orbital eccentricity of 0.967 and a semi-major axis length
of almost 18 AU. This interactive animation only allows eccentricity settings
up to 0.9; check out the comet
orbit interactive to see what Halley's orbit
looks like. Some comets, called short period comets because they orbit the
Sun in less than 200 years, come from a region beyond Neptune's orbit called
the Kuiper Belt. A typical short period
comet might have an eccentricity around 0.9 and a semi-major axis length
of about 25 AU. Try out those values in the interactive animation on this
page. Where does a short-period comet spend most of its time? A comet only
forms its tails and becomes
visible to us when it is close to the Sun, well
inside the orbit of Jupiter. Notice how a comet is only visible for a few
months out of an orbit lasting decades to centuries (Halley's Comet takes
76 years to orbit the Sun). Some comets have much shorter orbital periods
of just a few years. Many of these are "Jupiter-family comets" that were
tossed into the inner Solar
System by the gravity of that giant planet during
a close encounter with Jupiter. Comet
Wild 2, the target of the Stardust
space mission, is one such Jupiter-family comet. Wild 2 has an eccentricity
of 0.54, a semi-major axis length of 3.44 AU, and an orbital period of 6.39
years. Set up Wild 2's orbit in the interactive animation to see a typical
Jupiter-family comet. Finally, some comets come from the far distant reaches
of the Oort
Cloud, the outermost
fringe of our Solar System. One such "long-period comet", Comet
Hale-Bopp, has an eccentricity of 0.995 and a semi-major axis length
of about 186 AU! Hale-Bopp put on a spectacular show in 1997, but won't be
back this way again for more than 2,500 years!
Orbit Shapes Interactive Animation
This animation explains about the shapes and sizes of orbits.
Orbits are ellipses.
An ellipse is an oval, or a "stretched out" circle. An ellipse can be like a
circle, or it can be long and skinny. Astronomers use a special word to describe
the shape of an orbit. That word is "eccentricity".
If an orbit is almost a circle, the eccentricity is small. If an orbit is
a long, skinny ellipse, the eccentricity is bigger. A small eccentricity would
be a number close to zero, like 0.1 or 0.2. A bigger eccentricity would be
a number close to one, like 0.8 or 0.9. Eccentricity describes the shape of
an orbit.
What about the size of an orbit? Think about a circle. The distance from the
center of a circle to the edge is called the radius. The distance from the
center of an ellipse to the edge is called the "semi-major axis". We measure
the semi-major axis of an ellipse across the long direction of the ellipse.
Use the sliders in the animation (below) to change the shape and
size of the orbit of "your planet". You can also see the orbit of Earth
so you can compare your planet's orbit with Earth's orbit.
(Note: If you cannot see the animation below, or it is not working,
you may need to download the latest Flash
player.)
An astronomical unit (AU) is the length
of the semi-major axis of Earth's orbit. AUs are used to measure distances
in our Solar System.
Notice how a planet with an elliptical orbit moves closer to and further away
from the Sun. The point of closest approach to
the Sun is called perihelion.
The furthest point is called aphelion.
An astronomer named Johannes Kepler figured
out three
important laws about the orbits of planets.
Page created December 19, 2005 by Randy Russell.
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