Plotting Orbits of Planets
Pluto was discovered in 1930. It is said to orbit the sun
in 248 years. We have had 78 years to observe Pluto. How do we know
it takes 248 years to orbit the sun? Also how do we know the orbit
varies 3 billion Km in those 248 years?
Once astronomers figure out exactly where the planet is and what speed and
direction it is moving at that time, they can calculate its past and future
orbit from Newton's equations of gravity and Kepler's Laws of Planetary Motion.
Richard Barrans, Ph.D., M.Ed.
Assistant lecturer, Department of Physics and Astronomy
University of Wyoming
We know that Pluto takes 248 years to orbit the Sun because we now
understand its orbit. You answered your question in the
question! With so many observations, we know it well enough to
plot its obit-- 248 year period-- and know its location well enough
in the future to send a spacecraft to it!
For the same reason, we know that Pluto's orbit sits so far from the Sun.
David H. Levy
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Update: June 2012