The point in an object's orbit when it is the closest to the Earth.

A physical phenomenon that occurs when pairs or groups of particles are generated or interact in ways such that the quantum state of each particle cannot be described independently – instead, a quantum state may be given for the system as a whole.

The range of all possible frequencies of electromagnetic radiation. The electromagnetic spectrum extends from below the low frequencies used for modern radio communication to gamma radiation at the short-wavelength (high-frequency) end, thereby covering wavelengths from thousands of kilometers down to a fraction of the size of an atom. The limit for long wavelengths is the size of the universe itself, while it is thought that the short wavelength limit is in the vicinity of the Planck length, although in principle the spectrum is infinite and continuous.

An image of a small region of space... a composite of data accumulated over a period of time.

In Jungian psychology, the shadow or "shadow aspect" may refer to (1) an unconscious aspect of the personality which the conscious ego does not identify in itself. Because one tends to reject or remain ignorant of the least desirable aspects of one's personality, the shadow is largely negative, or (2) the entirety of the unconscious, i.e., everything of which a person is not fully conscious.

A technical term in thermodynamics which can be used in two senses. One sense is that of thermal equilibrium within a system for itself. The other sense is that of a relation between the respective physical states of two bodies. Thermal equilibrium in a system for itself means that the temperature within the system is spatially and temporally uniform. Thermal equilibrium as a relation between the physical states of two bodies means that there is actual or implied thermal connection between them, through a path that is permeable only to heat, and that no energy is transferred through that path.

The change in length measurements of an object ("linear dimension" as opposed to, e.g., volumetric dimension) due to thermal expansion is related to temperature change by a "linear expansion coefficient". It is the fractional change in length per degree of temperature change.

The tendency of matter to change in volume in response to a change in temperature.