Orbital Variations
Obliquity (change in axial tilt)
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On the Shoulders of Giants
Left: The eccentricity of the Earth's orbit changes slowly over time from nearly zero
to 0.07. As the orbit gets more eccentric (oval) the difference between the distance
from the Sun to the Earth at perihelion (closest approach) and aphelion (furthest
away) becomes greater and greater. Note that the Sun is not at the center of the
Earth's orbital ellipse, rather it is
at one of focal points.
Note: The eccentricty of the orbit shown in the lower image is a highly
exaggerated 0.5. Even the maximum eccentricity of the Earth's orbit0.07it would
be impossible to show at the resolution of a web page. Even so, at the current eccentricity
of .017, the Earth is 5 million kilometers closer to Sun at perihelion than at aphelion. (Images
by Robert Simmon, NASA GSFC) | ||
Precession
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Left: The change in the tilt of the Earth's axis (obliquity) effects the magnitude of seasonal change. At higher tilts the seasons are more extreme, and at lower tilts they are milder. The current axial tilt is 23.5°. Image by Robert Simmon, NASA GSFC) | ||
next: Milankovitch Theory
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Left: Precessionthe change in orientation of the Earth's rotational axis [this can be seen more clearly in an animation (small (290 kB QuickTime) or large (1.2 MB QuickTime))]alters the orientation of the Earth with respect to perihelion and aphelion. If a hemisphere is pointed towards the sun at perihelion, that hemisphere will be pointing away at aphelion, and the difference in seasons will be more extreme. This seasonal effect is reversed for the opposite hemisphere. Currently, northern summer occurs near aphelion. (Image by Robert Simmon, NASA GSFC) |
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