It is sometimes said of our society that we don’t get to know our neighbors and our neighborhoods as well as we used to, or as well as we perhaps should. Stretching this sentiment more than a little bit, I offer the following tidbits about our solar neighborhood.
The earth is about eight thousand miles in diameter, or about four thousand miles in radius. A straight-line trek around the earth would be a trek of about twenty five thousand miles.
Twenty-five thousand miles is a long way, but if you traveled this distance straight toward the moon you would be only one tenth of the way there. The moon is about two hundred fifty thousand miles from earth.
The moon is about two thousand miles in diameter, or only about one thousand miles in radius, so in this dimension is only about one-quarter the size of earth. By volume, the moon is only about two percent the size of earth (because the volume of a sphere is proportional to the cube of its radius). And because the moon is made of material that is on average slightly less dense than earth, the moon’s mass is only about one percent of earth’s mass.
Next Friday the moon is new, which means that it lies about in the same direction as the sun. A two hundred fifty thousand mile straight-line trip to the moon would get you only about one four hundredth of the way to the sun. The sun is about ninety three million miles away
Once at the sun you would be facing a surfaceless sphere of incandescent gas. And you would be hot! The sun’s visible surface is about six thousand degrees Celcius, which is almost eleven thousand degrees Fahrenheit. But know that the sun’s surface is only an optical illusion. The sun has no surface; what we see as the sun’s surface is the layer where the gas density is just high enough that we can see no deeper.
The sun that we see is something more than eight hundred thousand miles in diameter, about one hundred times the size of earth. By volume, the sun is more than a million times larger than earth. But because the sun is made of gas and the earth of rock, the mass of the sun is only about three hundred times the mass of earth.
What else is there nearby in our solar system neighborhood? A few of rocky planets somewhat like earth – Mercury, Venus and Mars – and a few gas planets – Jupiter, Saturn, Uranus and Neptune – which have more in common with the sun than the earth, and some minor planets and debris.
All of these rocky and gassy bits are arranged in a pancake-shaped neighborhood that is approximately ten billion miles in diameter that is otherwise empty space.
In fact the material world, from large to small, is empty space. All matter in our experience is composed of atoms, with nuclei of relatively big and heavy protons and neutrons surrounded by relatively tenuous shells of tiny and light electrons.
Nearly all of the mass of our world is concentrated in atomic nuclei, which are so compact as to be about one hundred thousand times smaller than the electron shells which push gently together when atoms stack together to make material bits we can hold in our hands.
Like the solar system, the atoms which compose everything in it are almost entirely empty space.
