Astronomical unit

Astronomical
unit:  mean distance between
the Earth and the Sun.

Catenary

Supported at each
end, a hanging cable forms
a catenary.

Speed of sound

Sound travels faster
in liquids and non-porous
solids than in air.

The speed of sound

The speed of sound in
dry warm air is about one
mile in five seconds.

Light year

A light year is the
distance light travels in a
year in a vacuum.