The instrument is a nocturnal on one side and a Regiomontanus dial on the other side.
The nocturnal has the handle at the 20th of August. On the limb of the nocturnal is a calendar scale divided to 10 days, subdivided to one, numbered by 10 from 10 to 30 or 31 or 28 as appropriate. The names of the months are engraved in Latin on an inner circle and alternate with star symbols. The equinoxes are at 11th March and 15th September. Further to the centre is a zodiac scale divided to 10 and subdivided to one degree, numbered by 10 from 10 to 30. Further to the centre is a ring with the representations of the zodiacal signs and their symbols. The last ring towards the centre is engraved with the names of the zodiacal signs alternating with star symbols. A smaller brass disc fixed on the centre has an index engraved 'INDEX SOLIS' with a sun effigy. Two scales are engraved on the disc, the outermost one numbered 1 to 12 twice and divided to the quarter hour, the inner one with lunar days numbered from 1 to 29 divided to the quarter. A third disc has an index engraved 'INDEX LVNAE' (repaired close to the centre) with a moon effigy, and is pierced to show the moon phase. It is inscribed 'PRIMA LVNAE QVADRA IVVENILIS', 'SECVNDA LVNAE QVADRA VIRILIS', 'TERTIA LVNAE QVADRA SENILIS', 'QVARTA LVNAE QVADRA DECREPITA'. It is also engraved with an aspectarium named 'PLANETARVM ASPECTVS', with the positions named, 'OPPOSITVS', 'TRIGONVS', 'TETRAGONVS', HEXAGONVS'. The index is engraved next to the centre 'STELLA POLARIS' and 'ALRUKABA', on the arm 'INDEX HORA' and 'DVAE PARILES PLAVSTRI POSTREMAE A STELLA POLARI IN RECTAM DVCTAE SOLIS INDICE AD DIEM PRAESENTEM POSITO HORAM OSTENDVNT NOCTV ?'. Next to the end of the arm is 'PLAVST VRSAE' and seven stars in the shape of the Big Bear. The handle is engraved with a coat-of-arms and with the inscription 'HOROMETRI DORSVM NOCTVRNAS HORAS CONIVNCT ? OPPOSITI ASPECTVS Q3 INDICAT'.
The back of the instrument has a Regiomontanus-type dial, inscribed 'HORARIVM VNIVERSALE'. The scale of latitudes is divided to ten and to five, from 0 to 65, numbered from 10 to 60 by 10 and is inscribed 'ZODIACI LATITVDINVM' and 'ZODIACI LATITVDINVM QVAE ET POLI ELEVATIO SIVE BOREI SIVE NOTII'. Common hours are numbered 1 to 12 twice in the opposite directions, and inscribed 'HORAE ANTEMERIDIANE' and 'HORAE POMERIDIANAE'. The thread for the dial arm is missing. On the right hand side of the dial is a latitude scale, marked 'MERIDIANVS ZODIACVS GENERALIS' with the symbols of the zodiacal signs. On the limb of the instrument are two shadow scales the outer one from 0 to 90, divided to 5 and to1, numbered from 10 to 90 by 10, and inscribed 'QVADRANS ASTRONOMICVS'. The inner scale is divided from 0 to 60 to 0 by 5 and by 1, numbered from 5 to 60 to 5 by 5, and named 'QVADRANS GAEOMETRICVS'. On the right hand side of the circle is inscribed 'VMBRA RECTA' and on the left hand side 'VMBRA VERSA', 'MEDIVM NOCTIS SEPTENTRIO', and 'CENTRVM QVADR ?'. On the left hand side is also the signature 'CASPAR VOPELI<VS> FACIEBAT COLONIE AGRIPPI ANNO 1551'. On this side the handle is inscribed 'HOROMETRI VNIVERSALIS FACIES AD OMNEM REGIONVM LATITVDINEM DELINIATA' with three star punches. One of the sights at the top of the instrument is missing, as is probably a central alidade.
A very similar nocturnal by Caspar Vopel was sold at Sotheby's auction, on 4th December, 1961 (lot 106), and another one is in the collection of the Museum of the History of Science in Oxford (see Epact 97152).
The instrument was purchased in 1895 and is described in F. A. B. Ward, A Catalogue of European Scientific Instruments in the Department of Medieval and Later Antiquities of the British Museum (London, 1981), p. 57, no.151.