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articles
cosmological distances.

So you think you might have a "big" problem in your life? Let this put it in perspective.

by Bruce Jensen

In astronomy and cosmology, distances are huge (no kidding, right?) but few if any people can intuitively fathom the scales involved. There have been several excellent treatises published about astro and cosmo distances, but here are just a few items to help you get a "feel" for these things (if that is the correct word)...all distances are approximate, and obviously subject to some adjustment as accuracy of measurement improves. I make no guarantees for errors, although I welcome corrections!

The earth is 8,000 miles across. The moon is about 238,000 miles away, or 30 times that distance. OK, so far, so good - these arenít too bad. But then, the sun is 93,000,000 miles away, or 1 astronomical unit, 400 times farther than the moon! (does this mean that, because the moon is just able to eclipse the sun, that the sun is 400 times larger than the moon in diameter? Yes, thatís just about right - the moon is 2,000 miles across, and the sun is right around 800,000 miles across!). Within the solar system, an astronomical unit (AU) is a convenient measure of distance.

So, Jupiter is about 5 AU from the sun, and Pluto is about 40 AU from the sun (about 3.5 BILLION with a "B" miles). you think thatís a long way? Comet Hale-Bopp probably comes from almost ten times farther, about 25 billion miles or 280 AU! The Oort Cloud, possibly the parent for Hale-Bopp in the distant past, lies much farther away at perhaps a whole light year, 5,800,000,000,000 miles (5.8 trillion, for those of you who like big numbers), or 62,400 AU. A long haul for a comet!

But the nearest stars are more than 4 times farther away - Proxima Centauri, faint companion to brilliant Alpha Centauri, is 4.3 light years away (some folks use the term "parsecs" in lieu of light years - a parsec is about 3.3 LY, or the distance to a star whose annual parallax, based on the diameter of the earthís orbit around the sun, is equal to one arc-second compared to the distant stars - but I will stick to LY here). No wonder we never run into any other stars! Space is mostly empty, after all!

Our galaxy contains perhaps 150,000,000,000 stars, plus dust and maybe a black hole in the center. Familiar objects in the sky - the star Sirius at 8 LY, the Orion Nebula at 1,500 LY, the Ring Nebula at 3,000 LY, are all within the Milky Way Galaxy. How much diameter or space does that require? Well, the Milky Way is BIG, so big that it takes us 200,000,000 years to make a full circle around itís hub at our distance from the center, which is about 30,000 LY give or take. The whole diameter is about 120,000 LY, and some of its globular clusters extend tens of thousands of light years above and below its plane. Still, on the grand scheme of things, pretty puny. The Andromeda Galaxy, perhaps a little bigger than the Milky Way and only 2.3 million LY away, is our nearest big spiral galaxy, and yet itís expanse is only about the length of your thumb against the sky!

Well, you get the picture. The Virgo Galaxy Cluster, our nearest big conglomeration of galaxies, is 50,000,000 LY away, and all of its members are telescopic. The nearest and brightest quasar, 3C 273 in Virgo, is 50 times farther away at more than 2,000,000,000 LY. The faintest galaxies seen in the Hubble Deep Field photo are 6 times farther away, 12 to 14 billion LY.

Feel small yet? :o) Many thanks to Bruce!

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