right-hand screw rule
razan-e pic-e rast-gard
Fr.: règle de la vis droite
A rule that gives the direction of the resultant vector in a → vector product: A x B = C. It is the direction of advance of a right-hand screw whose axis, held perpendicular to the plane of A and B, is rotated in the same sense that rotates the first-named vector (A) into the second-named vector B through the smaller angle.
râstâl, râstdast (#)
Using the right hand with greater skill or ease than the left.
Fr.: brillance du ciel
Atmospheric (→ airglow, → auroral emission, → artificial light) or extraterrestrial (→ scattered → sunlight from Moon, scattered → starlight, → interplanetary dust) foreground light that → interferes with → observations.
deraxandegi-ye ruyé, ~ ruye-yi
Fr.: brillance de surface
The brightness of an extended object, such as a planet, nebula, galaxy, or the sky background, expressed as magnitudes per unit area (usually square arc second). Surface brightness is calculated by dividing the object's magnitude by its size (→ isophotal radius).