Dheneb

Dheneb is one of the many star names built on the Arabic dhanab, a tail, which marks the tail of whatever figure the star belongs to. The same root gives Deneb in Cygnus, Denebola in Leo, Deneb Algedi in Capricornus and Deneb Kaitos in Cetus, and the spelling of each has drifted independently through Latin transliteration, which is why so many similar forms crowd the catalogues. Here it marks the tail of Aquila, the eagle. Because several stars in the tail of the same bird were described in the same way, the Aquila tail-names in particular multiplied, and different compilers assigned them to different stars. No single traditional planetary nature is recorded for this entry.

Denebola in Leo and Deneb Algedi in Capricornus both appear elsewhere in this same set of tables as stars in their own right, and setting all the dhanab-derived names side by side is a genuinely useful way to see the method of the old Arabic sky at work: rather than inventing a fresh proper name for every star, the tradition frequently just stated the star's place on the animal or figure it belonged to, so the same word for tail recurs wherever a constellation happened to have one. No planetary nature is recorded for this entry specifically, which is the ordinary condition for names built purely on anatomical position rather than on a star bright enough to have drawn Ptolemy's individual attention.

The atlas records this star at Capricorn 19 degrees 48 minutes, projected from Aquila.

Three entries in this constellation carry a form of the Arabic dhanab, the tail: Dheneb here, Deneb el Okab and Deneb Okab. It would be reasonable to assume all three are the same star under three spellings, and the assumption would be half right. The other two share a sign, a degree and a minute with each other, Capricorn 23 degrees 38 minutes, and are certainly one object entered twice. This one is three degrees and fifty minutes away from them and is therefore a different star.

That is a small result and it is the kind that only the recorded positions can settle. Names in the fixed-star lists are unreliable as identifiers: the same star acquired several transliterations, and several stars in one figure could all be called its tail. The longitude, whatever else it is, is a number that can be compared.

Precession dates it. At about fifty seconds of arc a year, one degree in roughly seventy two years, and taking the value for epoch 2000, this star entered Capricorn towards the end of the sixth century and will reach Aquarius in about seven hundred and thirty years.

Aquila lies well north of the ecliptic, so as with every star in this group the sign is a projection and not a location.