Elements of Modern Astronomy

  • F. G. Major


The observed eccentricity of the earth’s orbit around the sun is small, amounting to about 0.017. This means that the apsides, that is, the maximum and minimum distances from the sun, differ by only about 3.4 %; this would contribute a negligible amount to the seasonal climatic changes on the earth’s surface. Rather the seasons are due to the obliquity of the earth’s rotation axis relative to the orbital plane, the ecliptic. The angle of obliquity is about 23.5°, as shown in Chap. 3 (Fig. 3.9). The direction of the rotation axis remains fixed in space aside from small perturbations that cause a very slow precession, in which the axis sweeps out a cone, like a spinning top. The consequence of the obliquity of the axis is that during different segments of the earth’s orbit, the northern hemisphere, for example, will be inclined towards the sun increasing the duration of daylight, while in the southern hemisphere the opposite is true. It so happens that the orientation of the spin axis is such that the northern hemisphere is inclined away from the sun at the perihelion point. That point on the major axis of the orbit is nearest to the sun, where by Kepler’s second law, the earth has the highest orbital velocity; at the opposite extreme point, the aphelion, the orbital velocity is minimum. The points in the orbit where the direction of the earth’s spin axis is perpendicular to the radius vector, that is, the line joining the sun to the earth, are called the equinoctial points, because there day and night are of equal length.


Orbital Plane Semimajor Axis Astronomical Unit Corner Reflector Oblate Ellipsoid 
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Copyright information

© Springer Science+Business Media New York 2014

Authors and Affiliations

  • F. G. Major
    • 1
  1. 1.Severna ParkUSA

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