R

Radial velocity

The rate of change of the distance to an object, usually corrected for the Earth’s motion with respect to the solar system barycenter.

 

Refraction

The change in direction of travel (bending) of a light ray as it passes obliquely from a medium of lesser/greater density to a medium of greater/lesser density.

 

Refraction, astronomical

The change in direction of travel (bending) of a light ray as it passes obliquely through the atmosphere. As a result of refraction the observed altitude of a celestial object is greater than its geometric altitude. The amount of refraction depends on the altitude of the object and on atmospheric conditions.

 

Refraction, horizontal

The astronomical refraction at the astronomical horizon; often, an adopted value of 34′ is used in computations for sea level observations.

 

Retrograde motion

For orbital motion in the solar system, motion that is clockwise in the orbit as seen from the north pole of the ecliptic; for an object observed on the celestial sphere, motion that is from east to west, resulting from the relative motion of the object and the Earth. (See direct motion.)

 

Right ascension

Angular distance on the celestial sphere measured eastward along the celestial equator from the equinox to the hour circle passing through the celestial object. Right ascension is usually given in combination with declination.