When we look up at the heavens at night we are looking out into the universe. At the same time, we are also looking into the past, for the light we are seeing has traveled toward us at a finite speed. The light entering our eyes was emitted by the object we are viewing, or reflected by it, in the case of a planet or moon, some time in the past.
Galileo, who lived from 1564 to 1642, tried to measure the speed of light in an experiment that involved covering and uncovering lanterns on hills separated by a few miles, but the time interval was so short -- a few millionths of a second -- that the measurement could not be made. Galileo, however, believed that the speed of light was finite, and reasoned that it must simply be so great that his experiment did not have the precision to measure it.
In 1675 the Danish astronomer Ole Roemer noticed that the times of eclipses of Jupiter's moon Io by the planet did not accurately fit the predictions. He used calculations of the varying distance between Jupiter and Earth as they orbited Sun, and the difference between the observed and computed times of the eclipses to become the first person to measure the speed of light. Other techniques using ground based measurements have improved the results since then, and the speed of light is now one of the most accurately known physical constants -- 186,282.397 miles per second. With this speed, the average light time from Moon is 1.26 seconds. Average light time from Sun is 499.004782 seconds or about 8.32 minutes. Light time from the brightest star in the night sky, Sirius, is 8.6 years, and light travel time from the most distant object we can see without the aid of a telescope, the galaxy in Andromeda, is about 2 million years.
This light travel time, in fact, is the source of the popular unit of distance for objects outside our solar system -- the light year. One light year is simply the distance traveled by light in one year. This is a distance of nearly 6 trillion miles, a number too vast even to imagine. A distance in light years, however, tells us how long the light has been traveling since leaving the object we are viewing and hence how far into the past we are seeing. The light emitted by the supernova observed in the Large Magellanic Cloud in 1987, for example, had spent about 170,000 years on its way to Earth to show us its explosive message. The most distant objects viewed with telescopes, some of the quasars, are believed to be 10 billion light years distant. We see light emitted by them about 10 billion years ago, a time when the universe was less than half the age we now believe it to be. Even the nearby stars are several light years away, so we see them as they were at least a few years ago. We truly look out into the past of the universe.
Astronomers are using the Hubble Space Telescope to peer into the distant past so they can compare shapes of galaxies in clusters 9 billion light years away, and hence as they appeared 9 billion years ago, with the shapes of galaxies in nearby clusters. Hubble's high resolution makes it possible for them to study the detailed shape of the distant galaxies, and they find that there were more galaxies with spiral and distorted spiral shapes then than in, for example, the Coma Cluster, only 100 million light years away. There we see galaxies as they appeared a mere 100 million years ago. Evidently there has been considerable evolution that has changed the shape of galaxies in clusters over the interval of nearly 9 billion years. Computer simulations of galaxies in clusters suggest that a few billion years of collisions and near-misses drastically alter the shapes of the galaxies, changing them from what we see in the distant past to what we view in the more recent past.