Absolute. Not relative. Independent of all scale and comparisons. E.g., zero temperature, number, the speed of light.
Acceleration. Rate of change of velocity. Increasing velocity is positive acceleration; decreasing velocity is negative acceleration. The average acceleration of any body falling to Earth in a vacuum is 32 feet per second per second.
Achromatic. Applied to optical apparatus which gives images free from colored fringes. A. lenses have one sense of crown glass and one of flint glass. The flint lens corrects dispersion caused by the crown glass.
Aerolite. (Sometimes called ‘aerolith’). A stony meteorite, as distinct from a metallic one. A meteorite that is a mixture of stone and metal, but preponderantly stone would be called aerolithic.
Albedo. A measure of the brightness of celestial bodies that shine by reflected light. Technically, it is the amount of light a body reflects in proportion to the amount that falls on it. The Moon’s albedo is 7%; that of Venus 65%.
