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An Extraterrestrial Visitor's Analysis of Earth Help (page 2)

By — McGraw-Hill Professional
Updated on Sep 16, 2011

Year, Day, And Seasons

The equatorial plane of Sol 3, which the two-legged inhabitants call Earth , is tilted by approximately 23.5 degrees with respect to the plane of its orbit around the parent star Sol, which they call the Sun . This results in considerable seasonal variations in the weather that become increasingly dramatic as the latitude increases. The hours of daylight and darkness are always equal at the equator, but fluctuations become greater and greater as one goes nearer to the poles. North of 66.5°N and south of 66.5°S, there are periods when the Sun stays above the horizon for days at a stretch. At the poles themselves, the daylight period lasts for fully half the year, and the darkness period lasts for the other half.

There are about 365.25 solar days in each Earth year. It is difficult for me to describe the length of the Earth day except to say that the two-legged creatures divide each day into 24 equal units called hours . Each hour is divided into 60 minutes, and each minute is divided into 60 seconds. Fractions of a second are expressed in decimal form. For some reason, most Earth inhabitants divide the solar day into two 12-hour segments called and Some of their scientists use an undivided hour system that runs from 0000 (zero hours, zero minutes) to 2359 (23 hours and 59 minutes) and then starts over again at 0000 in the middle of the dark period.

The Earth is farthest from the Sun (that is, it is at aphelion) in the month called July and is closest to the Sun (that is, at perihelion) in the month called January . The mean distance of the Earth from the Sun is 149.6 million km (93 million mi). The variation in orbital radius is only about ±1 percent. The Earth’s greater distance from the Sun in the northern-hemispheric summer results in less solar irradiation over the planet’s greatest land masses at that time. However, this effect is balanced by the fact that the season is lengthened; Earth moves more slowly around the Sun at that time (Fig. 8-1). Conversely, the Earth’s lesser distance from the Sun during the northern-hemispheric winter produces more solar irradiation, but the season is shorter because the Earth moves faster around the Sun.

An Extraterrestrial Visitor’s Analysis of Earth Year, Day, And Seasons

Figure 8-1. Earth is at perihelion in January and at aphelion in July. (In this drawing, the eccentricity of Earth’s orbit is exaggerated for clarity.)

The earth’s axis precesses , or wobbles, slowly like the axis of a spinning top. Every 25,800 Earth years, the axis describes a complete circle whose angular radius is 23.5 angular degrees on the celestial sphere. This means that in 12,900 years, Earth will be closest to the Sun in July and farthest from the Sun in January (Fig. 8-2). It is difficult to say what effect this might have on the overall climate of the planet. There is much more land mass in the northern hemisphere than in the southern; land masses heat up and cool off more rapidly than the oceans. This could have a tremendous cumulative effect when the northern-hemispheric summer is shortened and the winter is lengthened. It is known that the repeated cycles of glaciation that take place on our own planet, Epsilon Eridani 2 (the second planet in orbit around the star Earth inhabitants call Epsilon in the constellation Eridanus), also have taken place on Earth; axial precession might be a contributing factor to these so-called ice ages.

An Extraterrestrial Visitor’s Analysis of Earth Year, Day, And Seasons

Figure 8-2. In 12,900 years, Earth will be at perihelion in July and at aphelion in January. (In this drawing, the eccentricity of Earth’s orbit is exaggerated for clarity.)

Practice problems of this concept can be found at: The Planet Earth Practice Problems

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