[Page no. 5.3.2]
[Title: The Earth's Orbit]
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[Text:
Calculations show that the Earth's orbit is relatively stable over very long periods. There is little doubt that the Earth formed in about the same distance from the Sun as it is now and that its orbit did not change much in the meantime.
This orbit is shaped as an ellipse that is nearly circular (eccentricity 0.0167), with a semi-major axis of about 150 million km, the mean distance to the Sun. At the present time, the orbit is placed in such a way that the Earth is closest (about 147 milliom km) to the Sun in January and farthest (about 152 milliom km) from the Sun in July. However, the orbit changes slowly, so this will not always be the case.
It has been found that small changes in the Earth's orbit in the past appear to follow (be "correlated" with) the mean temperature as measured on the northern hemisphere. This may have been a contributing effect to the advent of the recent ice ages, but probably not the only cause.
The Earth's nearest neighbour, its Moon, exerts a beneficial effect. This is because the Moon damps down the tendency of the Earth's spin axis to vary in response to gravitational tugs from the Sun and Jupiter. We can see this effect directly as the tidal effects that changes the level of the sea in a daily rhythm.
The Earth spins completely on its axis once every 24 hours (by definition, the 'day') and makes a complete revolution of the Sun every 365.25 days (the 'year'). The axis of the Earth's spin is at an angle of ~ 23° to the plane of the Earth's orbit (its obliquity), an effect which gives us our seasons.
This tilt angle is fairly stable, and has only varied by a few degrees over the last 600 million years or so. This stability is because the Moon reduces the gravitational pull of both the Sun and Jupiter on the Earth's axis. Without the Moon, the axis would be subject to much greater variation, leading to much wilder swings in seasonal climatic change.
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