Obliquity Earth orbit

These applications show how easy it is to modify a program, once it has been developed, to explore such questions. Another application, not undertaken here, would be to explore Milankovitch perturbations in the Earth s orbital parameters, eccentricity, obliquity, and date of perihelion. These parameters are specified as constants at the beginning of the program, and it would be simple to change their values as predicted by astronomical calculations, in order to see how the seasonal variation of temperature is affected at various latitudes. 

The variations in the Earth s orbit around the Sun can be grouped into three major cycles—eccentricity, obliquity and precession—each associated with one or more characteristic frequencies (Hays et al. 1976 Berger et al. 1984). 

The rotational axis of the Earth gradually precesses like that of a spinning top, and the ellipitical orbit of the Earth also precesses. The periods of these two precessions are 19 and 23 kyr, respectively, yielding an average of c.21 kyr. Precession controls the distribution of insolation over the Earth s surface by varying the timing of the seasons relative to the perihelion, but, like obliquity, does not affect the total insolation. 

Figure 3 The three most important cycles regulating insolation on Earth are obliquity, eccentricity, and precession (a) obliquity, or tilt of the Earth s axis varies with a period of 41000 years (b) eccentricity of the Earth s orbit varies with periods of 100000 years and 400000 years and (c) precession of the equinoxes has a dominant period of 21 000 years and is modulated by eccentricity.

Obliquity of the ecliptic Angle between the earth s orbit and the Equator. 

According to a basic formula of practical astronomy, cos z depends only on time of day, time of year, and latitude. The zenith angle, and hence /, depends on time of year (seasonal variation) because the angle between the earth s Equator and its orbit around the sun, the obliquity of the ecliptic s, is not zero. Its present value is 23.5° (Fig. 1). The larger s is, the larger are the seasonal contrasts. 

Actually, they provide the standard stratigraphic framework into which Pleistocene climatic events can be placed. In this respect, radiometric dating has been invaluable in establishing an absolute stratigraphy for the past 800,000 years. A model was developed in which the isotope is envisaged as a response of a single exponential system induced by variations in obliquity and precession of the orbit of the Earth (Imbrie et al. 1984). 

This takes into account the hypotheses of Milankovitch and J. Croll requiring that the Earth s climate must respond to orbital forcing (Imbrie 1985). Suitable models include spectral and cross-spectral, gain-and-phase, and dynamic, but cannot be discussed here. However, the first involved four of the main Milankovitch frequencies, i.e., at 100,000, 41,000, 23,000, and 19,000 years, the second including two attempts. One aimed at simulating the entire 782,000-year record, again using the Milankovitch four, which are 100,000 = eccentricity, 41,000 = obliquity, 23,000 and 19,000 = processional factors (O Fig. 16.4). 

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