An Etymological Dictionary of Astronomy and Astrophysics
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The form of the → Earth obtained by taking average sea level surface and extending it across the continents. In other words, the → equipotential surface ("mean sea level") of the Earth's → gravitational field. The geoid is considered to represent the true physical figure of the Earth, in contrast to the → reference ellipsoid, which is idealized geometrical figure of the Earth.

→ geo-; → -oid.

Of, pertaining to, or based on → geology. Also geological.

From geolog(y), → geology, + → -ic.

The long span of time from the end of the formation of Earth during which our planet underwent its major transformations.

→ geologic; → time.

The scientific study of the composition, structure, and physical history of the Earth.

→ geo- + → -logy.

Of or pertaining to → geomagnetism.

→ geo-; → magnetic.

The natural variations in the → geomagnetic field due to interactions of the Earth's field and → magnetosphere with energetic particles from the Sun.

→ geomagnetic; → activity.

A geophysical event, distinguished from the → magnetic reversal, in which the Earth's magnetic field departs for a relatively short time from its usual near axial configuration, without establishing a reversed direction. During the excursion the intensity and direction of the Earth's magnetic field undergo drastic changes. Palaeomagnetic measurements have revealed that since the last full reversal the Earth's magnetic field has, for brief intervals, deviated from the behavior expected during "normal" secular variation.

→ geomagnetic; → excursion.

The magnetic field that is generated within the Earth and extends out around the Earth. The intensity of the magnetic field at the Earth's surface is about 0.32 → gauss at the equator and 0.62 gauss at the north pole.

→ geomagnetic; → field.

A violent disturbance of the Earth's magnetic field, distinct from regular diurnal variations, following a → solar flare or → coronal mass ejection.

→ geomagnetic; → storm.

A branch of geophysics concerned with the study of the Earth's → geomagnetic field, including its origin, spatial extent, and variations in time.

→ geo-; → magnetism.

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.