On the scale of things

When someone tells me that a thing is so many microns or parsecs across, I am sometimes somewhat at a loss to picture the scale they're referring to, or to relate it to the scales of things familiar to me from other discourses. So here's some information on the scale of things, specifically to make it easier to compare and contrast between different fields of experience. I am probably wrong about some of this – please feel free to correct me !

Each page deals with a particular kind of quantity, measured in its unit within the Système Internationale (SI). I group things by SI quantifier on the standard unit, putting any thing into the group for which its value lies between about 0.1 and about 100, with some arbitrariness near the boundary.

• Length
• Time; see also my historical time-line
• Frequency
• Data-rate
• Speed and velocity
• Mass
• Energy
• Force
• Temperature

Sources

Where practical, I've simply linked to the source for data in relevant pages: but books aren't so amenable to HREFs and there are some sites I've plundered so heavily that it makes sense to mention the site itself here, as distinct from the various links into it where referenced.

• Isaac Asimov, The Tragedy of The Moon, ISBN 0 340 19879 6; chapter 10, Down from Amoeba.
• Isaac Asimov, From Earth to Heaven, US Library of Congress 66-20967 [1964–1967].
• Nuffield Advanced Science Book of Data (Chemistry, Physical Science, Physics), ISBN 0 582 82672 1.
• Douglas Scott's web pages on the cosmic microwave background.
• NASA's astronomy picture of the day and its archive.
• Since I began maintaining this page, my colleague Johan Herland has pointed me at Wikipedia's article on the same topic.

Strays

I don't yet have pages for all types of quantity – most of them would be rather short on illuminating examples. So I collect strays here; if I get a lot, it might become another page.

Area density

a human eye's retina has about a fifth of a million photoreceptors per square millimetre; that of a hawk has about a million per square millimetre.

Power

tropical huricanes dissipate energy at of order 50 to 200 terawatts. Humanity does so at closer to ten terawatts.

20115-112-26, LIGO observes grav-waves; two black holes, 14.2 +7.5 solar masses merge to a 20.8 solar mass result in "a couple of seconds"; solar mass per second power conversion, to G-radiant. This is 1e4 times as bright as a GRB. For contrast: our sun has burnt O(1e-6) of its mass in 5e9 years.

Energy price

price of (coal) gas in around 1897: 2 shillings 10 pence per 1,000 cubic feet i.e. (2 + 10/12)/20 quid / (10 ft)³ = 5 milli quid / metre³

See also

For visualising large numbers, you might find the MegaPenny project helpful. Lengths, masses and energies are all included in a spiral scale of which the internet has propagated some rather crude copies.

Randall Monroe (author of xkcd) occasionally makes charts to help visualise the scales of things:

• The angular sizes of various celestial objects, compared to terrestrial features subtending the same angle at Earth's centre.
• radiation dosages
• money
• The surface areas of the substantial bodies with visible solid surfaces in our solar system.
• The Right Way to think about converting to metric is to learn some real-world points of reference and get into the habit of using them – rather than converting via familiar units.
• XKCD also had some worth-reading points about security and what actually matters; and Randall, though he favours plain language, is farily clear about the boundaries of sanity.

Written by Eddy.