William Atkins
Friday, 06 February 2009 23:21
Science -
Space
Page 2 of 4
When this happens, in theory,
all the planets should be roughly of the same size and have about the same orbital shape around the Sun. However, this is not the case with Mercury and Venus, which are much smaller than the Earth and Mars.
In addition, the orbits of these two smaller planets are much more elliptical (a conic section shape that deviates from being circular), than the nearly circular orbits of the two larger planets. Eccentricity is the measure of that deviation from being totally circular.
A very small eccentricity value means that a planet’s orbit is nearly circular (an eccentricity of zero means it is circular).
A larger value means the orbit is much more elliptical. An elliptical orbit has an eccentricity of greater than zero to less than one, with a larger value meaning it is a more elliptical orbit.
Mercury is the nearest planet in the Solar System to the Sun. As the smallest of the inner planets, it has an equatorial radius of about 2,440 kilometers (0.4 Earth-radius), and an eccentricity of about 0.2056 (the most elliptical of the four planets).
Venus, the second closest planet to the Sun, has a mean radius of approximately 6,052 kilometers (0.9 Earth-radius), and an eccentricity of about 0.0068 (the least elliptical and more circular of the four orbits).
Page three gives the radius and eccentricity values for Earth and Mars, along with more details of Hansen's study.
