Deneb el Okab

Deneb el Okab joins two Arabic words, dhanab, tail, and uqab, eagle: the tail of the eagle. The name is used in modern star atlases for the pair Delta and Zeta Aquilae, distinguished as Deneb el Okab Australis and Borealis, the southern and northern; the older astrological lists do not always make that distinction and sometimes carry the name as a single entry. This star and the entry immediately following it in the atlas stand at the same recorded ecliptic longitude, which strongly suggests they are two spellings of the same object rather than two distinct stars, a common condition in catalogues assembled by copying from several sources.

Delta and Zeta Aquilae, the two stars usually intended by this name, are themselves unrelated to each other, a hot subgiant and a cooler evolved star at different distances that happen to sit near each other in the sky along the eagle's tail; their pairing under one descriptive phrase is a matter of appearance from Earth rather than any physical association. Both stars are considerably hotter and more distant than the Sun, and neither has any recorded planetary system; their interest to the tradition rests entirely on position within the eagle's outline rather than on any distinguishing physical property.

The atlas records this star at Capricorn 23 degrees 38 minutes, projected from Aquila.

The entry Deneb Okab carries the same sign, the same degree and the same minute, in the same constellation. Two separate stars agreeing to the arcminute does not happen; these are one star recorded twice under two spellings of the same Arabic name, deneb al-uqab, the tail of the eagle. The atlas holds both because the modern fixed-star lists hold both, and those lists were built by compilers copying earlier compilers and treating a variant transliteration as a new object.

A third entry in the same constellation, Dheneb, carries a third form of the same root and is not a duplicate: it is recorded at Capricorn 19 degrees 48 minutes, three degrees and fifty minutes away, so it is a different star that happens to share a name. Distinguishing the two cases is the point. A shared name proves nothing; a shared position to the arcminute proves a great deal.

On precession, a fixed star gains about fifty seconds of arc a year, a degree in roughly seventy two years. Taking the figure for epoch 2000, this star entered Capricorn early in the fourth century and will cross into Aquarius in about four hundred and fifty years, sooner than most of the entries in this group.