To be honest, I think gigameters and petameters are rarely used, au, light years and parsecs are much more common. In the media, even in science articles, "large counting words" will be combined with kilometers. The earth-sun distance is commonly described as 150 million kilometers when 150 Gm would be more correct. When a "large counting word" is used to avoid a string of zeroes in a metric quantity, a more suitable metric prefix should be substituted.
For the solar system, including outer planets and the Kuiper belt, both the gigameter and the terameter would be reasonable candidates. The scale ranges from around 50 Gm for Mercury to 8 Tm for the outer Kuiper belt, and it might be best to pick one or the other for all solar system differences to preserve sense of scale. Either 50 to 8000 Gm or 0.05 to 8 Tm is manageable, I think the first might be preferred, but in the end, they are the same thing. As for the diameter of the universe, the SI has larger prefixes, up to yotta (10^24). Exa (10^18) is frequently used in national energy discussions (the US uses a bit over 100 EJ per year), the larger ones see less usage. ________________________________ From: Paul Rittman <[email protected]> To: U.S. Metric Association <[email protected]> Sent: Sat, April 9, 2011 12:10:10 AM Subject: [USMA:50315] re: astronomical measurements John, Thanks for the clarification. Yes I see that using the meter (or its powers to the ten) could preserve scale that the terms astronomical units or parsecs don't. It would be nice to see metric units used more often with inter-stellar systems. I wonder why they aren't used more often. I'm sure there are at least a few astrophysicists and astronomers who are metric-conversant. Anyways I'll keep looking at this--I realize there is a massive scale here, going from our own solar system all the way up to the 92 billion light-year-wide--err... that is, 900 billion petameter-wide universe, so it it very well might be the case that metrically-minded astronomers might want to use two metric prefixes (one for shorter and the other for longer distances). Are gigameters and petameters the two most commonly used ultra-metric measurements? [USMA:50264] re: astronomical measurements John M. Steele Sun, 03 Apr 2011 07:41:49 -0700 No. I find comparing in meters (with a suitable prefix) gives me a better sense of scale than mixing AU, light years, parsecs, and other obscure units with obscure inter-relationships. Determining that a light year is some 63000 AU is not particularly helpful. As a general comparison, it is much easier to learn a few metric prefixes, usable with all units, to provide a short hand alternative to scientific notation for scale than to learn all the absurd interrelationships of Customary units.
