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a term used for phenomena that spread radially, especially of two:
In the narrow sense, some type of electromagnetic wave: radio, microwave, light (infra-red, visible or ultra-violet), x-rays or gamma rays are all types of electromagnetic radiation.
Colloquially, an abbreviation of "ionizing radiation" meaning any spreading emission which can penetrate matter and ionize its atoms. That includes x-rays and gamma rays, but also high-energy ions and electrons emitted by radioactive substances, accelerated by laboratory devices or encountered in space (e.g. the "radiation belt" and "cosmic rays," also known as the "cosmic radiation").
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Instability of some atomic nuclei, causing them to change spontaneously to a lower energy level or to modify the number of protons and neutrons they contain. The 3 "classical" types of radioactive emissions are (1) alpha particles, nuclei of helium (2) beta-rays, fast electrons and (3) gamma-rays, high-energy photons.
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Electromagnetic waves of relatively low frequency.
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The added force implied by the lack of motion (equilibrium) when an applied force exists (e.g. gravity).
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(atmospheric re-entry)-- The return of a spacecraft from orbit to Earth, in which the kinetic energy of the orbital motion is converted into heat. Since that heat is sufficient to melt the spacecraft, if the spacecraft is to land intact, the heat must be safely dissipated. Heat-resistant shields of various types are used, and the reentry is at a shallow angle, to stretch out the process.
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Temporary reversal of the apparent motion of a planet along the ecliptic. Caused because (by Kepler's 3rd law) a planet moves faster the closer it is to the Sun, so that (for instance) Jupiter appears to move backward when the faster-moving Earth overtakes it.
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The angle formed when two straight lines intersect and the 4 angles at their crossing are all equal. When measured in degrees it equals 90o.
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Two angles marking the position of a star on the celestial sphere. Imagine a line from the observer to the star, and draw its projection (like a shadow) onto the celestial equator. Declination d is the angle between the line and its projection (d = 90o - q, where q is the angle to the direction to the celestial pole); it is negative south of the equator. RA is the angle between the projection and the direction to the vernal equinox or first point in Aries.
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A device shooting out a fast jet of gas, in order to produce a force in the opposite direction. See center of gravity, also Newton's laws of motion in Mach's formulation.
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The imaginary line around which the Earth turns. Its inclination of about 23.5o to the ecliptic is the reason for the seasons of the year.
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