Star date: 10:26:98
A Sense of Scale
"The universe, so they say, is big." wrote Douglas Adams. The question is how large. Distances on Earth are measured in units of meters or yards for house sized objects, and kilometers and miles for distances across a city, county, state, or country.
When we speak of objects in space, however, if we use these same units o
f measurement, the numbers we need to use even to describe the distance to the Moon become so large they are meaningless. If I tell you that an object is 10 million kilometers from another object, it is difficult at best to picture what kind of distance this represents. For typical distances between galaxies, the numbers used would be far greater still.
Until the 1920's, we were alone in the universe. We believed our solar system to be the only family of planets in the universe. We knew of the Milky Way, after all, we can see the fuzzy band of stars which we call the Milky Way streching across the sky each Summer. Galileo looked at this fuzzy band of stars through his primative telescope, and was the first to observe this band as many thousands of seperate stars. He was, however, not the first to suggest this idea. This idea was first put forth around the year 150 BC by a scientist working at the Great library of Alexandria, and his name was Democritus of Abdera.
Astronomers throughout the ages had seen fuzzy objects in the sky, and called them nebulae. One of the first people to catalog these nebulae was an unsucessful comet hunter by the name of Charles Messier. He recorded 110 objects (later narrowed down to 108) which looked much like comets, but did not seem to move in the sky. Many of these objects later on were proven to be galaxies. His collection of these objects became known as the Messier catalog, and these objects are referred to as M1, M2, etc. This catalog is still widely used by astronomers, both professional and amateur, to this day.
In the 1920's, when many of these objects, along with still other objects, became known as galaxies, our ideas as to the size of the universe became greater and greater. This meant that another unit of measurement, greater than any we were used to, had to be used. But what?
The fastest object known to the human race is light. In fact, the special theory of relativity states that no material object in the universe can travel at a velocity beyond, or even equal to, the velocity of light in a vacuum. The speed of light in a vacuum is called c (as in E=mc squared), and is equal to about 300,000 kilometers per second, or slightly over 186,000 miles per second. Light travels slower through a medium such as glass, air or water than it does in a vacuum, so that this speed only applies in a theoretical perfect vacuum (which never really exists). Light could travel seven and a half times around the Earth in one second.
This distance can thus be referred to as one light second.
The distance from the Earth to the Moon is around 375,000 km, or about 230,000 miles. Light can travel this distance in about 1-1/4 seconds. If you shine a flashlight on the Moon, the light from your flashlight will hit the surface of the Moon (albeit weakly) 1-1/4 seconds later. Thus, we can say the Moon is 1-1/4 light seconds away from the Earth.
The Earth revolves around the Sun at a distance of around 150 million kilometers, or 93 million miles. The light coming from the surface of the Sun takes about 8 minutes to complete a journey to Earth. We can therefore say the Earth is 8 light minutes from the Sun. Pluto, normally the most distant major planet from the Sun which we know of, receives light from the Sun about 5 hours and 20 minutes after it leaves the surface of our parent star. Even at this remarkable speed, light still takes 4 years to cover the distance to the nearest star, 100,000 years to cross across our Milky Way galaxy, and nearly three million years to reach us from the closest other major galaxy, Andromeda. The light which reaches us now from that distant island of stars left Andromeda before the earliest humans walked on the Savanna Plains of Africa. Even more remarkable is the fact that the latest images from the Hubble Space Telescope show objects which are 12 billion light years distant. This is a little over 100,000,000,000,000,000,000,000,000 meters.
Aren't you happy we speak of these distances in light years?
Clear skies, and good viewing.
