Ever since Galileo peered through his crude telescope at the moons of Jupiter, man has wondered how far away these strange heavenly bodies are. There are a number of ways that these measurements can be made; the distance to the moon has been accurately measured by radar; we can determine the orbits of planets, comets, and asteroids around the sun; we can measure the time it takes for light to reach us from a distant object such as the sun. It takes 8.3 minutes for the sun's light to reach earth, and since light travels at about 186,200 miles per second, the distance has been calculated to be about 93 million miles, or one astronomical unit (AU).
Another way to measure astronomical distances is by means of parallax. Parallax is the apparent change in position of an object from different points of observation. This can be demonstrated by holding a pencil at an arm's length and looking at it with the right eye and then with the left eye. The pencil seems to move, and by knowing the distance between the eyes and the angle of shift, it should be possible to calculate the length of the arm. The farther apart the eyes are the more accurate the measurement. This is the principle behind the design of optical range finders. The amount or degree of angular displacement can be used to determine the distance to the object, the smaller the angle the greater the distance. The apparent displacement of a celestial object against the fixed background of stars can be used to determine its distance from earth.
Many celestial objects are so far away that ordinary range finders are useless; the two eyes must be very far apart. The farthest apart we can get our eyes on this earth is on opposite sides of the earth's orbit around the sun, equal to two AU's or 186,000,000 miles. With this as a base, when an object subtends a parallax angle of one second of arc, its distance is called one parsec. Thus the parsec becomes a unit of distance equal to 201,265 AU's, or 3.26 light years, or about 20 trillion miles.
Many galaxies are so far away that there is no measurable parallax, so their distance must be determined other ways such as spectral analysis or by variable stars. It has been estimated that the most distant observable galaxies are some 13-15 billion light years away. These tremendous distances boggle the mind; how can one visualize 6 trillion times 15 billion? The vastness of space demonstrates that we worship a God that is far above us in intelligence, power, and wisdom, that He is beyond our ability to measure; much like the parallax of distant stars. With God there is no variableness, no shadow of turning. There is no parallax.
