A very rare armillary sphere by one of London's leading instrument makers. Benjamin Martin was "one of the great popularizers of science in the mid-eighteenth century" (DSB) and his fine instruments can be seen in many museum collections today. After the fire in 1764 he supplied Harvard College with an order of several instruments, and he was held in high regard internationally. Orreries and planetaria by him a rare, and this model of armillary is not mentioned in any of his publications. He did write that "Gentlemen may have the orrery constructed in what manner soever they chuse", so it is likely that this piece will have been a commission.

The armillary sphere is a demonstrational model of the universe. Composed of several rings (Armillae in Latin) and a band for the Zodiac, it represents the apparent movement of the celestial sphere around the Earth and marks the Sun's annual progress around the ecliptic. This example also carries additional arms to mark 60 stars on the celestial sphere. Their use can be traced back to antiquity and the handful of earliest extant examples date from the Middle Ages. But it was in the 16th through 18th centuries that their construction reached a peak and they became such iconic instruments of science. Elaborate and decorative examples were made for princely collections and they became symbolic of astronomy in paintings and engravings of the time.