Thanih Ras al Akab

Thanih Ras al Akab is Arabic for the second in the head of the eagle, from thani, second, ras, head, and uqab, eagle. It belongs to the same descriptive family as the tail-names of Aquila and shows the method of the older Arabic star nomenclature at its clearest: rather than giving each star an arbitrary proper name, the tradition named stars by their place in the figure and their order within that place, so that a catalogue entry is really a short sentence of position. Names of this length were commonly shortened or garbled in Latin transmission, and several familiar modern star names are the worn-down remains of such phrases. No traditional planetary nature is recorded for this star, and the astrological tradition attaches no meaning to it beyond its position in the constellation.

A name built this precisely, second, head, eagle, is functionally a coordinate rather than a proper name in the modern sense, and the habit of naming stars by their exact anatomical place in a figure, rather than giving each an independent word, is one of the clearest structural differences between the older Arabic descriptive layer and the later custom, more Latin and Renaissance in spirit, of giving a star a single fixed proper name regardless of its position. Names of exactly this kind are the staple of Richard Hinckley Allen's 1899 Star Names, the standard English reference for this whole layer of the catalogue's nomenclature.

The atlas records this star at Aquarius 4 degrees 55 minutes, projected from Aquila, whose stars lie well north of the ecliptic and are never seen among the constellations of the zodiac.

Precession dates the figure and, for a star this near the head of a sign, dates it usefully. At about fifty seconds of arc a year, one degree in roughly seventy two years, and taking the recorded value for epoch 2000, the star crossed from Capricorn into Aquarius around the middle of the seventeenth century. Fixed-star tables printed before about 1650 place it in Capricorn, and the two placements are the same claim seen from different centuries.

It is one of only two entries in this group of twelve already in Aquarius, the other having crossed the boundary as recently as about 1969. The rest of the Aquila stars here are still in Capricorn and will cross over the coming centuries, in an order set entirely by how far into the sign each one already is.

The name means the second in the head of the eagle, which is an ordinal within a figure rather than a description of the star, and like the other Arabic names in this constellation it is an identifier of position within Aquila rather than of the object itself. Where an entry carries little else, the longitude is the more reliable of the two.