D

Dark nebula

A nonluminous cloud of interstellar dust and gas that obscures the light of the stars behind it.

 

Declination

The angular distance north or south of the celestial equator measured along the hour circle that passes through the body.

 

Deferent

The circumference of the large circle in the ptolemaic system on which the center of the epicycle of a planet moves.

 

Descending node

The point in the orbit of celestial body at which it crosses the reference plane-either the ecliptic or the celestial equator-from north to south.

 

Diffuse nebula

A cloud of interstellar dust and gas dense enough to be seen as a dark nebula or close enough to nearby stars to either reflect light (reflecting nebula) or emit light (emission nebula).

 

Distance modulus

The difference between the apparent and absolute magnitudes of a stars. A quantity used to express the star's distance (d) in parsecs, mathematically equal to 5 log d - 5.

 

Diurnal circle

The daily apparent path of a celestial body yhat is parallel to the celestial equator.

 

Diurnal motion

The daily apparent motion of a celestial body caused by the daily rotation of the earth.

 

Doppler effect

The apparent change in the frequency of radiation caused by the relative motion between the radiation source and the observer.

 

Dwarf star

A main sequence stars comparable to the sun in size, mass, and luminosity.