An Etymological Dictionary of Astronomy and Astrophysics
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One of the postulates used in the → Bohr model, whereby the atom emits (absorbs) a quantum of electromagnetic energy (→ photon) when the electron passes from an orbit with a greater (lesser) n value to one with a lesser (greater) value. The energy of the quantum is equal to the difference between the energies of the electron on its orbits before and after the transition or "jump": hν = ε₁ - ε₂, where h is the → Planck's constant and ν the frequency of the transition.

→ Bohr; → third; → postulate.

The ratio between the square of a planet's → orbital period (P) to the cube of the mean distance from the Sun (a) is the same for all planets: P²∝ a³ (year 1618). More accurately, P² = (4π²a³) / [G(M₁ + M₂)], where M₁ and M₂ are the masses of the two orbiting objects in → solar masses and G is the → gravitational constant. In our solar system M₁ = 1. The → semi-major axis size (a is expressed in → astronomical units and the period (P) is measured in years.

→ Kepler; → third; → law.

In a system where no external forces are present, every action force is always opposed by an equal and opposite reaction.

→ Newton; → third; → law; → motion.

Same as → principle of excluded middle.

→ principle; → exclude; → third.

Next after the second; the ordinal number for three. → Newton's third law of motion; → third contact; → third dredge-up; → third law of thermodynamics.

M.E. thirde, O.E. (north) thirda, variant of ridda, from P.Gmc. *thridjas (cf. O.Fris. thredda, O.S. thriddio, M.L.G. drudde, Du. derde, O.H.G. dritto, Ger. dritte, Goth. thridja).

Sevom, ordinal number for sé, → three.

The end of the total phase of a solar eclipse marked by the trailing edge of the Moon first revealing the Sun.

→ third; → contact.

A → dredge-up process that occurs in the stellar interior during He shell burning, as in → asymptotic giant branch (AGB) stars. These stars consist of a degenerate carbon-oxygen core, surrounded by a helium-rich region, above which lies a hydrogen-rich convective envelope. Following thermal pulses of the helium-burning shell, the convective envelope moves inward in mass, penetrating the hydrogen-exhausted regions. This is known as third dredge-up. As convection moves inward, nuclear processed materials are carried to the surface.

→ third; → dredge-up.

The → entropy of an idealized state of maximum order is zero at the temperature of → absolute zero. Another version of this law: As a system approaches absolute zero, all processes cease and the entropy of the system approaches a minimum value.

→ third; → law; → thermodynamics.

The phase of the Moon when half the visible disk of the waning Moon is illuminated. This quarter occurs when the celestial longitude of the Moon is 270° greater than the Sun's.

→ third; → quarter.