density
- measured in grams per cubic centimeter (or
kilograms per liter); the density of water is
1.0; iron is 7.9; lead is 11.3.
- disaster
- literally "bad stars"; particularly apt
in reference to a major asteroid impact.
- disk
- the visible surface of the Sun (or any heavenly
body) projected against the sky.
- doppler effect
- the apparent change in wavelength of sound or
light caused by the motion of the source,
observer or both.
- dinosaurs
- large reptiles that lived in the Mesozoic Era
from 230 to 65 million years ago; most probably
wiped out by the impact of a large asteroid or
comet.
- direct
- rotation or orbital motion in a counterclockwise
direction when viewed from the north pole of the
primary (i.e. in the same sense to most
satellites); the opposite of retrograde. The north
pole is the one on the same side of the ecliptic
as the Earth's north pole. (The word
"prograde" is sometimes used to mean
"direct" in this sense.)
- dorsum
- ridge.
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