An Etymological Dictionary of Astronomy and Astrophysics
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1) The fourth letter of the Greek alphabet (δ, Δ).
2) Anything triangular, like the Greek capital delta (δ).
3) Math.: An incremental change in a variable.

M.E. deltha, from L. delta, from Gk. delta; from the Phoenician name of the corresponding letter daleth "tent door."

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.

→ delta; Cephei, genitive of → Cepheus.

The → mechanism that imparts or transfers power to a → telescope so that it can move along the → declination direction. See also → tracking.

→ delta; → drive.

Same as → Dirac function.

A short distance from the target, in → declination, where the → telescope is pointed for various purposes.

→ delta; → offset.

Same as → Mintaka.

→ delta; → Orion.

A → binary star in the constellation → Scorpius. Its other designations include BD-22 4068, HD 143275, HR 5953, IRAS 15573-2228, SAO 184014. The → primary star is called → Dschubba. δ Scorpii is one of the brightest stars in the sky. Until 2000, its → visual magnitude was V = 2.32; since then, and due to its transition to a Be phase (→ Be star), it has been even brighter (V = 1.6 mag). It was resolved interferometrically into two components in the 1970s, and the observations indicated a very → eccentric orbit (e ~ 0.94) with a period of ~10.6 years. The → binary system is not → eclipsing, and the → secondary star is 1.78 ± 0.03 mag fainter than the primary one. The → spectral type of the primary is B0.5 V and that of the secondary B2V. The components are therefore of similar size and thus may produce strong interactions between themselves and affect the → circumstellar disk at, or near → periastron. δ Sco was first classified as a Be star when a small amount of Hα emission was observed in its spectrum. Since the reclassification of δ Sco as a Be star, two periastrons have passed, once in 2000, and again in 2011. Spectroscopic observations around the 2000 periastron revealed a large increase in the Hα emission compared to that found previously in 1993, with further noticeable month-to-month variations in its Hα → equivalent width and visual magnitude. It has been suggested that these small variations are due to the disk's inability to grow greater than the → Roche lobe of the primary, which caused a density increase on the side of the disk facing the secondary (See Miroshnichenko et al., 2013, AJ 766, 119 and references therein).

Delta Scorpii is the system's → Bayer designation.

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.

Named after the prototype star δ of constellation → Scutum; → variable.

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.

The function δⁱ_k of two variables i and j defined by δⁱ_k = 1 if i = j, and δⁱ_k = 0 if i ≠ j.

Leopold Kronecker (1823-1891), a German mathematician; delta, Gk. letter of alphabet.