Numbers in Astronomy range from astoundingly large to really very small. Ranging from Billions of Light Years to fractions of an arcsecond, it's no wonder people sometimes feel intimidated.
A light year is one example of measurement for distance that uses the speed of light as a ruler. I've often heard that Grace Murray Hopper had a bit of wire cut to length to represent a light nano-second (one of the really, really small numbers). That wire was thirty centimeters long, which is just under a foot long. One thousand of those make a light-microsecond. One thousand times the light-microsecond is the light-millisecond. Finally, one thousand times that is a light-second. That means that little bit of wire would have to be laid out end to end a billion times to create one light second. That's most of the way to the moon!
Another small measurement is the fraction of an arcsecond that telescopes like the Hubble Space Telescope can resolve as detail (really tiny fractions, too). As an example of how small an arcsecond is, picture the full moon in the sky (or hold your hand out at arm's length and observe the width of your thumb). That is about one half of a degree. That one half of a degree is thirty arcminutes and each arcminute is sixty arcseconds. So the width of the moon, or your thumb at arm's length, is eighteen hundred arcseconds across. The Hubble Space Telescope resolves things about 25 times smaller than that. That is very tiny indeed.
Look at it this way, my monitor has twelve hundred eighty pixels side to side and is thirty four centimeters wide. That's thirty eight pixels per centimeter. Your thumb might be about two centimeters wide, or about seventy six pixels across. Our handy thumb at arm's length is roughly a half a degree, so one degree of screen at arm’s length is about one hundred fifty pixels wide. Each pixel then is twenty to twenty five or so arcseconds across, depending on how close you sit to your monitor. One pixel is five hundred times larger than the fraction of an arcsecond resolution we mention before. Another way to say that is to see that fine a detail on your monitor, it would have to be magnified five hundred times, and probably more to really see it well.
That is enough of tiny. Let's scale up a little bit, the distance to the moon already mentioned is around a second and a half. The distance to the Sun is about five hundred seconds. This brings us to the Astronomical Unit: the AU. For most of the Solar System, objects are measured in their distance AU. Each AU, then, is about five hundred light seconds. A light hour is thirty six hundred light seconds and with a little math we see a light hour is a little over seven AU (thirty six hundred divided by five hundred).
I'm working my way up to the light year here, so hang on. Neptune, the furthest known major planet is right around thirty AU and about four and a quarter light hours from the sun. We haven't even made it a light day yet! To make it to a light day, you have to get twice the distance of the minor planet Sedna from the Sun which is about eighty six thousand, four hundred light seconds and so about one hundred seventy three AU.
Another six days out you are at one light week. Somewhere around forty or forty one light weeks out and you are probably getting into to the famous Oort cloud which will then take another forty weeks at the speed of light to get through (wow, huh?!). Somewhere twelve weeks into the Oort cloud we passed the light year! In Astronomical Units one light year is sixty three thousand, two hundred forty one - I think it's time to abandon the AU!
Another way to look at a light year is back to our original meters: ninety four hundred sixty trillion seven hundred thirty billion four hundred seventy two million five hundred eighty thousand eight hundred meters. That number, only one light year out is so large as to be meaningless for most of us. So we use light years and other large units (did someone mention parsecs?).
OK, since we mentioned the parsec, that is the measure of an object's movement due to parallax of one arcsecond (remember those?) by moving at a right angle by one AU. A little bit of trigonometry and you have a bit over three and a quarter light years. Alpha Centauri moves just over one arcsecond against the deep background sky when the earth moves one AU around in its orbit (a bit over eight weeks worth) which means it is a bit over one parsec away from the sun.
A few thoughts, then:
- The moon is about a second and a half away at light speed.
- The earth is by definition one AU from the sun - and that works out to just about five hundred light seconds.
- Neptune at thirty AU is over four light hours.
- Scaling up, the Oort cloud ranges from just under to well over one light year out from the sun and so is over two light years across.
- Our nearest stellar companion is almost four light years away and the center of the Milky Way is twenty five or thirty thousand light years away.
- Our neighbor the Andromeda Galaxy (I wonder what the inhabitants call it) is two million five hundred thousand light years away.
- Looking at these numbers you can see why meters which are so handy around the house are quickly useless in astronomy.
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