It’s pretty common in astronomy to be put off or confused by all of the different measurements of distance and the terms used to describe them such as 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. Hopefully.
To start, I expect you to know what kilometers and miles are. You shouldn’t need to be educated in those measurements as they are pretty basic. I’m not saying you should have the conversions memorized, but just a basic understanding of what they are. Furthermore, were discussing space and in space kilometers and miles are pretty much useless unless you are travelling to the Moon (240,000 mi. or 386,242 km.) or some of the closer planets such as 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 distance (LD) which equates to 238,900 mi. or 384,403 km. 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). We’ve had some in the last few years that are less than one LD but most are 15-30 LD’s or more as they pass.
Once you exhaust a few tens of millions of miles or kilometers you run into one of the more widely used units of measuring distance in the solar system; the astronomical unit (AU) which is the average distance between the Sun and Earth. Specifically, an AU is about 93 million mi. or 150 million km.
Another way of measuring distance around the solar system is light seconds, light minutes and light hours, though the AU is the preferred method.
Example: Here are some short light distances (hour or less).
Light Second = 186,282 mi. or 300,000 km. or 7 times around the Earth
Light Minute = about 11 million mi. or 18 million km.
1 AU = 8.3 light minutes (how long it takes light to travel the 93 million mi. from the Sun to Earth)
Light Hour = 671 million mi. or 1.8 billion km.
Let’s check some distances around the solar system. As you probably already know, orbits aren’t perfect circles; they’re slightly elliptical which means in every orbit around the Sun, a solar system body has a closest point to the Sun (perihelion) as well as a furthest point from the Sun (aphelion).
For the planets I list four (4) measurements relative the Sun; three (3) of which are in astronomical units (AU).
Semi major axis (average)
The fourth measurement is light time using either light minutes (LM) or light hours (LH) while the planets are at semi major axis (average distance to the Sun).
Mercury: 0.3 AU (closest), 0.4 AU (average), 0.46 AU (furthest) or 3.2 LM
Venus: 0.71 AU (closest), 0.723 AU (average), 0.728 AU (furthest) or 6.0 LM
Earth: 0.9 AU (closest), 1 AU (average), 1.01 AU (furthest) or 8.3 LM
Mars: 1.3 AU (closest), 1.5 AU (average), 1.6 AU (furthest) or 12.6 LM
Jupiter: 4.95 AU (closest), 5.2 AU (average), 5.46 AU (furthest) or 43 LM
Saturn: 9 AU (closest), 9.6 AU (average), 10 AU (furthest) or 1.3 LH
Uranus: 18.4 AU (closest), 19.2 AU (average), 20.1 AU (furthest) or 2.6 LH
Neptune: 29.8 AU (closest), 30.1 AU (average), 30.4 AU (furthest) or 4.1 LH
Pluto: 29: AU (closest), 39.5 AU (average), 49 AU (furthest) or 5.5 LH (NOTE: that’s not a typo; at perihelion Pluto is closer to the Sun than Neptune).
Voyager 1: 137 AU or 19 LH (as of January 2017)
Voyager 2: 113 AU or 16 LH (as of January 2017)
Light Year: 63,239 AU
Proxima Centauri: 268,000 AU or 4.25 LY
Light Years (LY): Let’s stop and have a little fun with numbers for a moment. If a light second is 186,282 mi. a light hour must be 671 million mi. A year has roughly 365 days, which breaks down to 8,760 hours, which breaks down to 525,600 minutes and down again 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. To get to Proxima Centauri, the next closest star to Earth, you would have to travel that speed for 4.25 years, 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 the speed of light simply isn’t going to cut it either. To keep perspective; the New Horizons spacecraft that recently past Pluto at 60,000km/h is the fastest man-made object as it reached Pluto in about 9 years-time. It would still take it 78,000 years to reach Proxima Centauri at that speed.
For now light years are nice, user friendly units of measurement that we can use for measuring distances beyond the solar system. We can travel around our stellar neighborhood easily just counting up light years. After all the closest 20 stars are all within 12 light years of us. Remember, even Star Trek at warp speed only traveled around in the local neighborhood of the galaxy.
Parsec (Pc): Derived from the terms parallax (par) and arcsecond (sec). The next bump up in units of measure for measuring space distances is the parsec (pc), which is equal to 3.26 light years or 206,264 (AU) or 19.2 trillion mi. (30.9 trillion km.). So 3,260 light years can be shortened to 1,000 parsecs and you can see already that 1,000, is much easier to write out than 19,200,000,000,000. You can break that down even further because 1,000 parsecs = 1 kiloparsec (Kpc), which we talk about next. Primarily we use parsecs 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 around our own Milky Way. So that being said, once we leave our local group of galaxies we need an even larger tool to count with. The megaparsec (Mpc).
Megaparsec (Mpc): “Mega” = million (such as megabyte or megapixel). The megaparsec is equal to 1000 kiloparsecs or 1,000,000 parsecs or 3.26 million light years. Andromeda galaxy is approximately 0.78 megaparsecs away while the Virgo cluster of galaxies is roughly 16.5 megaparsecs away.
Gigaparsec (Gpc): “Giga” = billion (such as gigabytes). If these distances don’t already hurt to comprehend, welcome to the mind numbing distance measurement of the gigparsec. The gigaparsec equates to 1000 megaparsecs or 1,000,000 kiloparsecs or 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 for now because this is where we come close to running out of room to actually apply these massive measurements. We have reached the number that denotes one trillion parsecs which equates to 3.26 trillion light years and we just don’t have anywhere in the universe to use that distance at this time.
Oddly enough these numbers are useful right here on Earth when referencing computing data and memory. You could even go further if you feel up to it. Scale up to petaparsecs, exaparsecs, zettaparsecs, yottaparsecs, xennaparsecs, vendekaparsecs, goolgolparsecs and googolplexparsecs. Good luck with that.
I hope that you found this write up at least interesting and I hope that it gave you food for thought in relation to just how expansive the universe really is and what terms we use to describe those distances. If you have any questions, criticisms or ideas that I can add into this feel free to let me know be leaving a comment.