An Etymological Dictionary of Astronomy and Astrophysics
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For a body that moves with respect to a rotating → reference frame, the vector sum of the observed acceleration, the → Coriolis acceleration, and the → centrifugal acceleration. See also the → Coriolis theorem.

→ absolute; → acceleration.

The rate at which the velocity of an object changes with time.

Acceleration, from accelerate, from L. accelerare "quicken," from → ad- "to" + celerare "hasten," from celer "swift" (cf. Skt. car, carati "to move, go, drive," Gk. keles "fast horse, horse race," Av. kar- "to walk, move, go around," Mod.Pers. cal, calidan "to move, to go, to walk" (jald? "quick, active, brisk"), Gilaki/Hamadâni jal "quick, fast,"Lori žil "motion, impulse"); PIE *kel- "to drive, set in swift motion."

Šetâb "quickness, haste, speed," Mid.Pers. ôštâp "hurry, haste," ôštâftan "to hurry, hasten," from *abi.stap-, from abi- " to; in addition to; against" + *stap- "to oppress," Arm. (loanword) štap "haste, trouble."

The acceleration that an object experiences because of gravity when it falls freely close to the surface of a massive body, such as a planet. Same as → gravitational acceleration.

→ acceleration; → gravity.

A measure of the departure from a constant rate of the acceleration of the Universe, expressed by: q(t) = R(t)R ^..(t)/R ^.2(t), where R(t) represents the size of the Universe at time t. Traditionally, a negative sign is inserted in the above equation for the → deceleration parameter.

→ acceleration; → parameter.

The rate of change of → angular velocity. It is equal to the → first derivative of the → angular velocity: α = dω/dt =d²θ/dt² = a_t/r, where θ is the angle rotated, a_t is the linear tangential acceleration, and r is the radius of circular path.

→ angular; → acceleration.

Of a body traveling from A to B, the change of → velocity divided by the time interval: ā = (v₂ - v₁) / (t₂ - t₁).

→ average; → acceleration.

Of a point rotating in a circle round a central point, the outward acceleration away from the rotation axis. It corresponds to → centrifugal force. The centrifugal acceleration is given by ω x ω x r, or v²/r, where ω is → angular velocity, r the distance to the rotating axis, and v the → tangential velocity. The centrifugal and → centripetal accelerations are equal and opposite.

→ centrifugal; → acceleration.

The rate of change of the → tangential velocity of a body moving along a circular path. The direction of centripetal acceleration is always inward along the → radius vector of the → circular motion. The magnitude of the centripetal acceleration is related to the → tangential velocity (v) and → angular velocity (ω) as follows: a_c = v²/r = rω². According to → Newton's second law, an object undergoing centripetal acceleration is experiencing a → centripetal force.

→ centripetal; → acceleration.

The apparent acceleration corresponding to the → Coriolis force. It is the acceleration which, when added to the acceleration of an object relative to a rotating → reference frame and to its → centrifugal acceleration, gives the acceleration of the object relative to a fixed reference frame. Coriolis acceleration equals 2ω x v, where ω is the → angular velocity of the rotating reference frame and v is the radial velocity of a particle relative to the center of the rotating reference frame.

→ Coriolis effect; → force.

→ accelerating Universe.

→ cosmic; → acceleration.

The acceleration caused by the force of gravity. At the Earth's surface it is determined by the distance of the object form the center of the Earth: g = GM/R², where G is the → gravitational constant, and M and R are the Earth's mass and radius respectively. It is approximately equal to 9.8 m s^-2. The value varies slightly with latitude and elevation. Also known as the → acceleration of gravity.

→ gravitational; → acceleration.

The → acceleration of a particle at time t defined by a = lim Δv/Δt = dv/dt. It is the limiting value of Δv/Δt at time t as both Δv and Δt approach zero.

→ instantaneous; → acceleration.

The rate of change of the → linear velocity with time. It is defined by the expression Δv/Δt and is equal to the → first derivative of the → linear velocity.

→ linear; → acceleration.

The acceleration exerted on the plasma particles according to the → magnetocentrifugal model.

→ magnetocentrifugal model; → acceleration.

The acceleration imparted to matter by → radiation pressure.

→ radiative; → acceleration.

An acceleration that results from the vector addition of two or more distinct accelerations.

→ resultant; → acceleration.

The apparent gradual increase in the → Moon's motion in its orbit, as measured relative to → mean solar time. Secular acceleration corresponds to an extremely gradual reduction in the speed of the → Earth's rotation. The slow-down of the Earth's spin comes mainly from → tidal frictions from the Moon. Historically, Edmond Halley (1656-1742) was the first to suggest that the Moon's mean rate of motion relative to the stars was gradually increasing. In 1693, Halley compared eclipses of recent, medieval, and classical Babylonian time, and discovered that the Moon's mean motion had been gradually increasing. Using Lunar Laser Ranging measurement, based on laser reflectors left by the Apollo astronauts on the Moon's surface (1969 to 1972), the secular acceleration is derived to be -25".4 ± 0".1 century ² (Xu Huaguan et al., 1996, in Earth, Moon and Planets 73, 101). This corresponds to a linear increase of about 3.5 cm yr^-1 in the mean Earth-Moon distance.

→ secular; → acceleration.