An Etymological Dictionary of Astronomy and Astrophysics
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Of or pertaining to geometry or to the principles of geometry.

Adj. of → geometry

A measure of the → reflectivity of a surface, especially of the solar system bodies (planets, satellites or asteroids). It is the ratio of a body's brightness at zero → phase angle to the brightness of a perfectly diffusing disk with the same position and apparent size as the body. Geometric albedo depends on the radiation wavelength. The bolometric geometric albedo refers to reflectivity in all wavelengths. Compare with the → Bond albedo.

→ geometric; → albedo.

One of the two factors contributing to → gravitational lensing time delay that arises from the fact that the bent trajectory is longer than the straight one. The other factor is due to the → Shapiro time delay.

→ geometric; → delay.

Where the apparent → sea horizon would be if there were no → atmospheric refraction.

→ geometric; → horizon.

The middle term in a → geometric progression. Of two terms, the geometric mean is the square root of their product. For example, the geometric mean of 4 and 9 is ± 6. For a series of n terms, it is expressed as: (a₁.a₂. ... .a_n)^1/n.

→ geometric; → mean.

A branch of physics that deals with reflection and refraction of rays of light without reference to the wave or physical nature of light.

→ geometric; → optics.

A → sequence in which the ratio of a term to its predecessor is the same for all terms. In general, the nth term has the form ar^(n-1), where n is a positive integer, and a and r are nonzero constants; r is called the ratio or common ratio. The sum of the first n terms is given by: S_n = a(1 - rⁿ)/(1 - r). Also called → geometric sequence.

→ geometric; → progression.

A type of scattering in which the wavelength (of the light or the sound) is much smaller than the size of object causing the scattering.

→ geometric; → scattering.

→ geometric progression.

→ geometric; → sequence.

Libration resulting from changes in the location of the observer with respect to body. More specifically, a lunar libration motion that results from the Earth based observer seeing the Moon from different directions at different times. There are three types of geometrical libration: → libration in longitude, → libration in latitude, and → diurnal libration. See also → physical libration.

→ geometric; → libration.