Editing Robert Hooke (section)

=== Astronomy ===
[[file:Saturn Robert Hooke 1666.jpg|thumb|Hooke noted the shadows (a and b) cast by both the globe and the rings on each other in this drawing of Saturn. |alt=Hooke's drawing of the planet Saturn]]
[[file:Moon Micrographia Hooke.png|thumb| Drawings of the Moon and the Pleiades from Hooke's ''Micrographia'']]

In May 1664, using a {{cvt|12|ft}} [[refracting telescope]], Hooke observed the [[Great Red Spot]] of [[Jupiter]] for two hours as it moved across the planet's face. In March 1665, he published his findings and from them, the Italian astronomer [[Giovanni Cassini]] calculated the [[rotation period]] of Jupiter to be nine hours and fifty-five minutes.{{sfnp|Inwood|2003|p=51}} 

One of the most-challenging problems Hooke investigated was the measurement of the distance from Earth to a star other than the Sun. Hooke selected the star [[Gamma Draconis]] and chose the method of [[parallax]] determination. In 1669, after several months of observing, Hooke believed the desired result had been achieved. It is now known his equipment was far too imprecise to obtain an accurate measurement.{{sfnp |Hirshfeld |2001 |pages=[https://archive.org/details/parallax00alan/page/144 144–149] }}

Hooke's ''Micrographia'' contains illustrations of the [[Pleiades]] star cluster and [[lunar craters]]. He conducted experiments to investigate the formation of these craters and concluded their existence meant the Moon must have its own gravity, a radical departure from the contemporaneous [[Gravity#Ancient world|Aristotelian celestial model]].{{sfnp|Gribbin|Gribbin|2017|p=57}} He also was an early observer of the [[rings of Saturn]],{{sfnp|Alexander |1962 |pages=108–109}} and discovered one of the first-observed [[double-star]] systems [[Gamma Arietis]] in 1664.{{sfnp|Aitken|1935| page = [https://archive.org/details/dli.ernet.15387/page/1/mode/1up 1]}}

To achieve these discoveries, Hooke needed better instruments than those that were available at the time. Accordingly, he invented three new mechanisms: the [[Hooke joint]], a sophisticated [[universal joint]] that allowed his instruments to smoothly follow the apparent motion of the observed body; the first [[clockwork]] drive to automate the process; and a [[micrometer screw]] that allowed him to achieve a precision of ten [[second (arc)|seconds of arc]].{{sfnp|Jardine|2003|pp=44,45}}{{sfnp|Inwood|2003|p=82}} Hooke was dissatisfied with [[refracting telescope]]s so he built the first practical [[Gregorian telescope]] that used a silvered glass mirror.{{sfnp|Atkin|n.d.}}{{sfnp|Gribbin|Gribbin|2017|p=98}}{{efn|Between Gregory's initial effort and Hooke's improvement, Isaac Newton had built a reflecting telescope{{snd}}but because its mirror was made from polished steel, it tarnished and rapidly became useless.{{sfnp|Gribbin|Gribbin|2017|page=96}}}}