Something to Stretch Your Imagination - Working out Astronomical Distances in Outer Space

(I found this from http://danspace77.com/describing-space-distances/)

Often in astronomy it’s easy to be completely befuddled by all of the different measurements of distance and the terms used to describe them; kilometers (km), miles (mi), lunar distances (LD), astronomical units (AU), light years (LY) so on and so forth. So hopefully, well, my intent is to help break some of these units of measure down into an understandable picture. Weather I succeed will be a matter of opinion.

To start, I expect you to know what kilometers and miles are. You shouldn’t need to be educated in those measurements at this point. Furthermore, were discussing space and in space kilometers and miles are useless unless you are travelling to the Moon (240,000 mi.) or some of the planets (Mercury, Venus and Mars) while they are on the same side of the Sun as Earth. Even for those short distances we can use lunar distances (LD) or fractions of an astronomical unit (AU). YES, of course you can use kilometers and miles for everything but I’m telling you that you will waste a massive amount of time calculating those distances. It’s like saying I want to use millimeters when plotting my cross country drive…..It’s simply unrealistic. So what then, do we do; how do we get around?

Let’s start in our backyard….our vast, almost empty backyard. LUNAR DISTANCES (LD) which equate to 238,900mi or 384,403km is our first real unit of measure in space. It is the average distance to the Moon and is primarily used when reading data pertaining to Near Earth Asteroids (NEA’s) or Near Earth Objects (NEO’s). For example; on twitter I follow an asteroid alert notification system that sends data points on asteroids that have passed or will pass that day. We have had some in the last few years that are less than one LD but most are 15-30 LD’s as they pass.
Once you exhaust a few tens of millions of miles or kilometers you run into the major unit of measuring distance in the solar system; the ASTRONOMICAL UNIT or “AU” which is the average distance between the Sun and Earth. Specifically, an AU is about 93 million miles or 150 million kilometers.

Another way of measuring distance around the solar system is LIGHT SECONDS, MINUTES & HOURS though the AU is the preferred method. Here are some short light distances (hour or less).
Light Second = 186,282mi/300,000km or 7 times around the Earth.
Light Minute = about 11 million miles or 18 million kilometers.
1 AU = 8.3 minutes (Time to get to Earth; 93 million miles).
Light Hour = 671 million miles or 1 billion 80 million kilometers.
Let’s check some distances around the solar system in AU’s and light time.

For the planets I list four (4) measurements in relation to the Sun: 3 of which are in astronomical units (AU). They are; perihelion (closest)/semi major axis (average)/aphelion (furthest) points from the Sun. The fourth measurement; light time, is estimated while the planets are at semi major axis (average distance to the Sun). LM = Light Minutes and LH = Light Hour.
Mercury = 0.3 / 0.4 / 0.46 AU or 3.2 LM
Venus = 0.71 / 0.723 / 0.728 AU or 6.0 LM
Earth = 0.9 / 1 / 1.01 AU or 8.3 LM
Mars = 1.3 / 1.5 / 1.6 AU or 12.6 LM
Jupiter = 4.95 / 5.2 / 5.46 AU or 43 LM
Saturn = 9 / 9.6 / 10 AU or 1.3 LH
Uranus = 18.4 / 19.2 / 20.1 AU or 2.6 LH
Neptune = 29.8 / 30.1 / 30.4 AU or 4.1 LH
Pluto = 29 / 39.5 / 49 AU or 5.5 LH (That’s not a typo; at perihelion Pluto is closer to the Sun than Neptune).
Voyager 1 = 126 AU or 17 LH
Voyager 2 = 103 AU or 14 LH
Light Year = 63,239 AU
Proxima Centauri = 250,000 AU = 4.2 LY

LIGHT YEARS (LY)
Let’s stop and have a little fun with numbers for a moment……If a light second is 186,282 miles, a light hour must be 671 million miles. A year has roughly 365 days, which breaks down to 8760 hours, which breaks down to 525,600 minutes and down even further to 31,536,000 seconds per year……If light travels around the Earth 7 times every second it can orbit Earth 220,752,000 times a year WOW!

Once you leave the solar system you have to be traveling at least that fast to get anywhere and as you will soon realize, that’s simply not going to cut it either.

 The closest star that is not the Sun is Proxima Centauri at 4.2 light years away. To keep your perspective well in hand; the New Horizons spacecraft heading to Pluto at 60,000km/h is the fastest man-made object as it will reach Pluto in about 9 years-time. It would STILL take it 78,000 years to reach Proxima Centauri at those speeds.

 For now, light years are nice, user friendly numbers that we can use for measuring distances beyond the solar system. We can travel around our stellar neighborhood easily counting up light years. In all, the closest 20 stars are all within 12 light years of us. Remember, even Star Trek at warp speed only traveled around our local neighborhood of the galaxy.

PARSEC (Pc)
Derived from the terms PARRALLAX (par) & ARCSECOND (sec).
The next bump up in units of measure in space is the parsec (pc) which is equal to 3.26 light years or 206,264 (AU) or 19.2 trillion miles (30.9 trillion km). So 3260 light years can be shortened to 1000 parsecs and you can see already that 1000 is much easier to write out than 19,200,000,000,000. You can break that down even further because 1000 parsecs = 1 kiloparsec, which we talk about next. Primarily, parsecs are used locally to get us around our local neighborhood of a few thousand stars.

KILOPARSEC (Kpc)
The next step up in describing distance in the universe is the kiloparsec. This one’s pretty easy to understand “kilo” = 1000 + “parsec” = 3.26 light years so in total a kiloparsec = 1000 parsecs.
This unit of measurement is used to move around large distances in the galaxy and even distances to the closer neighboring galaxies. For example the center of the Milky Way galaxy is about 8 kiloparsecs. The Canis Major Dwarf galaxy is approximately 13 kpc from the center of the galaxy and the Large Magellanic Cloud (LMC) is approximately 50 kpc distant. That equals 163,000 light years! NOW you can see just how slow light speed actually is. You would have to travel for 163,000 years at a speed of 7 times around the Earth per-second to reach even a satellite galaxy of our own Milky Way. So that being said, once we leave our neighboring galaxies we need an even larger tool to count with……the megaparsec.

MEGAPARSEC (Mpc)
“Mega” = million (Such as megabyte or megapixel).
The megaparsec is equal to 1000 kiloparsecs, 1,000,000 parsecs or 3.26 million light years.
Andromeda galaxy is approximately 0.78 megaparsecs while the Virgo cluster of galaxies is roughly 16.5 megaparsecs from Earth (Though if you measured from the Sun you wouldn’t notice a difference).

GIGAPARSEC (Gpc)
“Giga” = billion (Such as gigabytes).
Next is the mind numbing distance measurement of the gigparsec. The gigaparsec equates to 1000 megaparsecs, 1,000,000 kiloparsecs, 1,000,000,000 parsecs or 3.26 billion light years.
The entirety of the observable universe has a radius of approximately 14 gigaparsecs or 46 billion light years!

TERAPARSEC (Tpc)
“Tera” = trillion (Such as terabyte).
Whoa, I think here is a great place to end the write up because this is where we come close to running out of room to actually use these massive measurements. We have reached the number that denotes one trillion parsecs. That equates to 3.26 trillion light years and we just don’t have anywhere to use that at this time.

Oddly enough these numbers ARE useful right here on Earth when referencing computing data and memory. You could even go further; PETAPARSECS, EXAPARSECS, ZETTAPARSECS, YOTTAPARSECS, XENNAPARSECS, VENDEKAPARSECS, GOOLGOLPARSECS and GOOGOLPLEXPARSECS…………good luck with those.