Glossary D

Dark Matter: Matter that exerts a gravitational influence on its surroundings but does not emit detectable amounts of radiation.

Declination: The angular distance of a celestial body north or south of the celestial equator.

Doppler Effect: The observed change in wavelength or frequency of radiation caused by the motion of its source towards or away from an observer.

Double Star: Two stars that appear close together in the sky because they happen to lie in almost exactly the same direction when viewed from Earth.