PostscriptSome specific gravities
On noticing the moon overhead one may reflect that, since the specific gravity of the earth's moon is almost the same as that of the mineral jade, one would have the same skimtime (19.2 Planck minutes) on the moon as on a small planet made of jade.
Depending on one's vacation plans it could doubtless of great utility to know that on a planet made of pure gold it would take 8 minutes for a skimming orbiter to travel the length of the planet's radius. The skim-orbit radian arctime, or "skimtime", for gold is 8 minutes. On a cheaper planet, for instance one made of aluminum, the skimtime is 21 minutes. (Planck minutes, about 90 percent of ordinary, are used throughout.)
Conventional specific gravity compares materials with uncomfortably cold water at normal earth-surface pressure. On a planet made of this "normal" water (which would not be nice for swimming) the skimtime would be 35.086 minutes. To discover the skimtime on nicer-to-visit planets one divides 35.086 by the square root of the conventional specific gravity.
I've listed some uncopyrighted specific gravitiesfor imaginative use only. I can't vouch for them and have not checked them in the standard reference works. If you notice any are way off, please let me know.
Discussion copyright 2002 Leonard Cottrell. Data is public
Table of contents for the New Metric Fables
Aluminum 2.702sq rt=1.64421.34 minutes
Gold, pure 19.32sq rt=4.3957.982 minutes
Gold, alloys 15.3-19.3
Iron, pure 7.86
Silver, pure 10.5
Amber 1.25-1.55 sq rt=1.118-1.24528.18-31.38 minutes
Beryl 2.36- 2.91
Jade 3.33sq rt=1.82519.23 minutes
Sand, dry 1.62
Cedar, red 0.38
Ebony 0.96- 1.12
Lignum Vitae 1.28- 1.37 (bowling balls)
Mahogany 0.54- 0.85
Specific gravity data is public domain and I can't certify its reliability.