An Appreciation of the Earth

We take our home for granted most of the time. We complain about the weather, ignore the splendor of our sunsets, the scenery, and the natural beauties of the lands and seas around us, and cease to be impressed by the diversity of living species that the Earth supports. This is natural, of course, since we are all products of the Earth and have evolved in conformity with the existing environment. It is our natural habitat, and all of it seems very commonplace and normal. Yet how different our world would be if some of the astronomical parameters were changed even slightly.

Suppose that, with everything else being the same, the Earth had started out with twice its present mass, giving a surface gravity of 1.38 times Earth normal. Would the progression of animal life from sea to land have been so rapid? While the evolution of marine life would not have been greatly changed, land forms would have to be more sturdily constructed, with a lower center of mass. Trees would tend to be shorter and to have strongly buttressed trunks. Land animals would tend to develop heavier leg bones and heavier musculature. The development of flying forms would certainly have been different, to conform with the denser air (more aerodynamic drag at a given velocity) and the higher gravity (more lifting surface necessary to support a given mass). A number of opposing forces would have changed the face of the land. Mountain-forming activity might be increased, but mountains could not thrust so high and still have the structural strength to support their own weight; raindrop and stream erosion would be magnified, but the steeper density gradient in the atmosphere would change the weather patterns; wave heights in the oceans would be lower, and spray trajectories would be shortened, resulting in less evaporation and a drier atmosphere; and cloud decks would tend to be lower. The land-sea ratio would probably be smaller. The length of the sidereal month would shorten from 27.3 to 19.4 days (if the Moon's distance remained the same). There would be differences in the Earth's magnetic field, the thickness of its crust, the size of its core, the distribution of mineral deposits in the crust, the level of radioactivity in the rocks, and the size of the ice caps on islands in the polar regions. Certainly man's counterpart (assuming that such a species would have evolved in this environment) would be quite different in appearance and have quite different cultural patterns.

Conversely, suppose that the Earth had started out with half its present mass, resulting in a surface gravity of 0.73 times Earth normal. Again the course of evolution and geological history would have changed under the influences of the lower gravity, the thinner atmosphere, the reduced erosion by falling water, and the probably increased level of background radiation due to more crustal radioactivity and solar cosmic particles. Would evolution have proceeded more rapidly? Would the progression from sea to land and the entry of animal forms into the ecological niches open to airborne species have occurred earlier? Undoubtedly animal skeletons would be lighter, and trees would be generally taller and more spindly; and again, man's counterpart, evolved on such a planet, would be different in many ways.

What if the inclination of the Earth's equator initially had been 60 degrees instead of 23.5 degrees? Seasonal weather changes would then be all but intolerable, and the only climatic region suitable for life as we know it would be in a narrow belt within about 5 degrees of the equator. The rest of the planet would be either too hot or too cold during most of the year, and with such a narrow habitable range, it is probable that life would have had difficulty getting started and, once started, would have tended to evolve but slowly.

Starting out with an inclination of 0 degrees would have influenced the course of development of the Earth's life forms in only a minor way. Seasons would be an unknown phenomenon; weather would undoubtedly be far more predictable and constant from day to day. All latitudes would enjoy a constant spring. The region within 12 degrees of the equator would become too hot for habitability but, in partial compensation, some regions closer to the poles would become more habitable than they are now.

Suppose the Earth's mean distance from the Sun were 10 per cent less than it is at present. Less than 20 per cent of the surface area (that between latitudes 45 degrees and 64 degrees) would then be habitable. Thus there would be two narrow land regions favorable to life separated by a wide and intolerably hot barrier. Land life could evolve independently in these two regions. The polar ice would not be present, so the ocean level would be higher than it is now, thus decreasing the land area.

If the Earth were 10 per cent farther away from the Sun than it is, the habitable regions would be those within 47 degrees of the equator. (The present limit of habitability is assumed to be, on an average, within 60 degrees of the equator.)

If the Earth's rotation rate were increased so as to make the day 3 hours long instead of 24 hours, the oblateness would be pronounced, and changes of gravity as a function of latitude would be a common part of a traveler's experience. Day-to-night temperature differences would become small.

On the other hand, if the Earth's rotation rate were slowed to make the day 100 hours in length, day-to-night temperature changes would be extreme; weather cycles would have a more pronounced diurnal pattern. The Sun would seem to crawl across the sky, and few life forms on land could tolerate either the heat of the long day or the cold of the long night.

The effects of reducing the eccentricity of the Earth's orbit to 0 (from its present value of 0.0167) would be scarcely noticeable. If orbital eccentricity were increased to 0.2 without altering the length of the semi-major axis (making perihelion coincide with summer solstice in the Northern Hemisphere to accentuate the effects), the habitability apparently would not be affected in any significant manner.

Increasing the mass of the Sun by 20 per cent (and moving the Earth's orbit out to 1.408 astronomical units to keep the solar constant at its present level) would increase the period of revolution to 1.54 years and decrease the Sun's apparent angular diameter to 26 minutes of arc (from its present 32 minutes of arc). Our primary would then be a class F5 star with a total main-sequence lifetime of about 5.4 billion years. If the age of the solar system were 4.5 billion years, then the Earth, under these conditions, could look forward to another billion years of history. Since neither of these numbers is known to the implied accuracy, however, a 10 per cent error in each in the wrong direction could mean that the end was very near indeed. An F5 star may well be more "active" than our Sun, thus producing a higher exosphere temperature in the planetary atmosphere; but this subject is so little understood at present that no conclusions can be drawn. Presumably, apart from the longer year, the smaller apparent size of the Sun, its more pronounced whiteness, and the "imminence" of doom, life could be much the same.

If the mass of the Sun were reduced by 20 per cent (this time decreasing the Earth's orbital dimensions to compensate), the new orbital distance would be 0.654 astronomical unit. The year's length would then become 0.59 year (215 days), and the Sun's apparent angular diameter, 41 minutes of arc. The primary would be of spectral type G8 (slightly yellower than our Sun is now) with a main-sequence lifetime in excess of 20 billion years. The ocean tides due to the primary would be about equal to those due to the Moon; thus spring tides would be somewhat higher and neap tides lower than they are at present.

What if the Moon had been located much closer to the Earth than it is, say, about 95,000 miles away instead of 239,000 miles? The tidal braking force would probably have been sufficient to halt the rotation of the Earth with respect to the Moon, and the Earth's day would equal its month, now 6.9 days in length (sidereal). Consequently, the Earth would be uninhabitable.

Moving the Moon farther away than it is would have much less profound results: the month would merely be longer and the tides lower. Beyond a radius of about 446,000 miles, the Earth can not hold a satellite on a circular orbit.

Increasing the mass of the Moon by a factor of 10 at its present distance would have an effect similar to that of reducing its distance. However, the Earth's day and month would then be equal to 26 days. Decreasing the Moon's mass would affect only the tides.

What if the properties of some of the other planets of the solar system were changed? Suppose the mass of Jupiter were increased by a factor of 1050, making it essentially a replica of the Sun. The Earth could still occupy its present orbit around the Sun, but our sky would be enriched by the presence of an extremely bright star, or second sun, of magnitude —23.7, which would supply at most only 6 per cent as much heat as the Sun. Mercury and Venus could also keep their present orbits; the remaining planets could not, although those exterior to Saturn could take up new orbits around the new center of mass.

All in all, the Earth is a wonderful planet to live on, just the way it is. Almost any change in its physical properties, position, or .orientation would be for the worse. We are not likely to find a planet that suits us better, although at some future time there may be men who prefer to live on other planets. At the present time, however, the Earth is the only home we have; we would do well to conserve its treasures and to use its resources intelligently.

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