Astronomical Images : The appearances of Saturn, Mars and Jupiter

Johannes Hevelius

Astronomical Images

The Selenographia, literally meaning 'descriptions of the Moon' was published by Johannes Hevelius (1611-1687) in 1647. In addition to descriptions of the surface of the Moon, the book also contained other telescopic observations by Hevelius, as well as explanations of instruments. The Selenographia is an unusual publication in the extent to which an author controlled the printed presentation of his work. The book contained observations made by Hevelius at his observatory using instruments (several of which he invented or improved upon), drawn and noted down by himself, then engraved by himself ' several images bear the signature: auctor sculpsit (the author engraved) ' and published at his own expense. This avoided complications and errors that might be introduced during the publication process by other artists and printers. The book itself contains a wide range of representations of the Moon, including a series of the surface of the Moon through its phases. This assemblage of figures of Saturn, Jupiter and Mars demonstrates how improvements to the telescope could lead to better understanding, in particular, of the shape of Saturn. Though Saturn was originally believed to be round, following observations with the first telescopes the planet began to appear oval, with arms or with two smaller globes. Hevelius confirmed that he had frequently observed Saturn with such globes, as shown in figure B. Figure A was taken from Matthias Hirzgarter's Detectio dioptrica (1643), which was the first printed image of a 'handled' Saturn observed earlier by Francesco Fontana, the Neapolitan instrument-maker. Hevelius corrected Fontana's depiction of the handles, saying that they were more parabolic, as depicted in figure C. Hevelius calls this figure the 'true' face of Saturn, and the inscribed dates of his observations (September to November 1643) add weight to his claim. Hevelius emphasises his ability to observe accurately by adding the figure of Jupiter and its satellites, which he said he observed repeatedly between 1642 and 1645. The figure at D, showing the phase of Mars, also serves to show the power of Hevelius' telescope.