The Pages of The Gray Wizard


My Equipment
Solar System
Milky Way
Deep Skies


How Far  is a Light-Year?

A light-year is the distance that light travels in a year.  Light travels at about 186,000 miles per second.  This means that light can reach the Earth from the Moon in about 1.25 seconds.  The closest approach of the closest Planet, Venus is 100 times further away from Earth than is the Moon, so light from Venus takes about 125 seconds, just over 2 minutes, to reach the Earth.  The farthest planet, Pluto, is about 100 times further away from Earth than Venus, so light from Pluto takes about 200 minutes, or over 3 hours, to reach Earth.  So Pluto, which is about 10,000 times further away from Earth than the Moon, is only  about 3 light-hours away. 

A light-year is about 6 trillion (6,000,000,000,000) miles.  The nearest star system is about 4.3 light-years from Earth.  We can see some stars with the naked eye that are 3,000 to 4,000 light-years away.  The center of our galaxy, the Milky Way, is about 26,000 light-years away.  The Andromeda galaxy is almost 3 million light-years away.  Large telescopes reveal galaxies that are over 5 billion light-years away. The farthest reaches of the universe are estimated to be about 15 billion light-years away. 

Think about these vast distances.  The light from Andromeda that we see through our telescopes tonight started its journey to Earth about 3 million years ago.

The Power Myth!
  When visitors see my telescopes they invariably ask the question, "How much does it magnify?" or "What's its power?"  However, unlike fixed magnification binoculars, the power of an astronomical telescope depends on the eyepiece being used.  Power is a function of the fixed focal length (the distance from the objective to the focal point) of the telescope divided by the focal length of the eyepiece. The power of such a telescope can be varied by exchanging the eyepiece.  By using increasingly smaller focal length eyepieces, the power of any telescope can be increased almost indefinitely.  However, "useable" power cannot be increased without limit.  As the size of the image increases with increased magnification, the image becomes fainter and fuzzier.  What is important, therefore, is the resolution of the original unmagnified image.  This resolution is dependent on the light-gathering ability of the the telescope which in turn is a function of its aperture (the diameter of its objective lens or primary mirror). Contrary to the popular misconception therefore, the most critical characteristic of an astronomical telescope is not its power or magnification, but its aperture or light-gathering ability.