1) The fourth letter of the Greek alphabet (δ, Δ).
M.E. deltha, from L. delta, from Gk. delta; from the Phoenician name of the corresponding letter daleth "tent door."
Fr.: Delta Cephée
The prototype of classical → Cepheid variables, which is a pulsating → yellow supergiant. John Goodricke was the first in 1784 to discover its variability. The star shows a quick and sharp rise from minimum to maximum, and slowly declines to its minimum again. The changes in brightness are accompanied by and principally caused by changes in stellar temperature and also by changes in radius. δ Cephei was actually the second Cepheid variable to be discovered. The first one, Eta Aquilae, had been discovered earlier the same year by Edward Pigott. δ Cephei varies with a period of 5.366341 days (or 5 days 8 hours 37.5 minutes) from magnitude 3.48, spectral type F5 Ib in its maximum to magnitude 4.37, spectral type G2 Ib in its minimum. It lies at a distance of 1,340 → light-years.
râne-ye deltâ , ~ vâkil
Fr.: entraînement en déclinaison
Fr.: fonction delta
Same as → Dirac function.
Fr.: offset en déclinaison
Fr.: delta Orionis
Same as → Mintaka.
Delta Scuti variable
Fr.: variable δ Scuti
A member of a class of → pulsating stars with periods less than 0.3 days, → spectral types A or F, and visual light amplitudes in the range from a few thousands of a magnitude to about 0.8 mag. On the → H-R diagram, δ Scuti stars form a group which lies in an → instability strip which includes the classical → Cepheids at its brightest end and the pulsating → white dwarfs at its faintest limit. These stars can show very complex light variations since, while some of them are pulsating in one radial mode only, others may be pulsating simultaneously in several radial and non-radial modes.
Deltâ T (ΔT)
Fr.: Delta T (ΔT)
A measure of the variation in → Earth's rotation, which is the difference between → Terrestrial Time (TT) and → Universal Time (UT). TT is uniform and related to the → International Atomic Time, whereas UT, which is directly tied to the Earth's rotation, is not strictly uniform and shows small erratic fluctuations. Between 1970 and 1990, ΔT changed from +40 to +57 seconds, and was +67 seconds for 2010.
Δ, Gk. letter of alphabet indicating a difference; T for → time.
deltâ-ye Kronecker (#)
Fr.: delta de Kronecker
The function δik of two variables i and j defined by δik = 1 if i = j, and δik = 0 if i ≠ j.
Leopold Kronecker (1823-1891), a German mathematician; delta, Gk. letter of alphabet.