γ Arietis

Gamma Arietis is a binary star system in the constellation of Aries. The double star nature of this system was discovered by Robert Hooke in 1664. It has the traditional name Mesarthim. The combined apparent visual magnitude of the two stars is 3.86. The distance to Gamma Arietis is approximately 164 light-years (50 parsecs).

The two components have an angular separation of 7.606 arcseconds, which can be resolved with a small telescope. The orbital period of the pair is greater than 5000 years. The brighter component is a magnitude 4.58 B-type main sequence star. The secondary is a Lambda Boötis (chemically peculiar) star with a magnitude of 4.64. It is classified as an α2 CVn type variable star and its brightness varies by 0.04 magnitudes with a period of 2.61 days.

Some sources claim it has been called "the First Star in Aries" because it was once the nearest visible star to the equinoctial point (cf. First Point of Aries).


Micrographia had diagrams of Hipparchus, Pleiades, Orion Nebula


Comet C/1664 W1 appeared in December 1664 which he investigated along with Wren. In fact the fact that the comet of 1664 was followed by one in 1665, and these comets were then followed by the Great Plague and Great Fire in London obviously gave rise to alleged connections between these phenomena

He investigated the comet of 1677, which led to his book of the folllowing year, Cometa. This book included a a statement of the Law of Inverse Squares and the effect of the Sun on comet tails.

The book can be viewed here : Cometa


He attempted to detect the parallax of a star for which purpose he had a telescope installed in Gresham College - from summer 1666. On 22nd. October 1668 he reported to the Royal Society his project. He chose to observe γ Draconis viewed in the zenith from London. Four measurements from July 1669 to October 1670 produced a parallax of 30 seconds of arc which appears at the time to be taken as proof of stellar parallax but as Hooke admitted (eventually) was far too big.

In 1674 this was related in 'Attempt to Prove the Motion of the Earth' based on his 1674 Cutlerian Lecture.

The book can be viewed here : Attempt to Prove the Motion of the Earth

System of the World

Accuracy of Telescopic Observations

After Hooke had received a copy of Hevelius’ Cometographia in 1668, both were in contact with each other. Hooke sent a description of his telescope. He tried to convince Hevelius, who observed exclusively with the naked eye, that the accuracy of telescopic observations are higher by several magnitudes. In 1673, Hevelius published his work Machina Celestis, in which he describes his observing instruments and which contains a part of his decades-long observational results. His work was taken on by many members of the Royal Society, including Edmond Halley, with wonderment.

Hooke on the other hand used his Cutler Lecture Animadversions on the First Part of the Machina Coelestis of the following year to present heavy criticism of Hevelius’ work. On 17th. January 1674 he demonstrated to members of the Royal Society, that the human eye can only differentiate distances of about one minute of arc. On the other hand Hevelius maintained that naked-eye observing could differentiate one second or arc.

This lecture was written up in a book in 1679 - you can read the book here : Animadversions.

In autumn 1685, an anonymous critique of Hevelius's latest work Annus Climactericus appeared in the Philosophical Transactions , which was probably written by John Wallis. The critique contained extracts from various letters of Hevelius, which openly query Hooke's competence as an astronomer. The personal attacks on Hooke and their publication in the Philosophical Transactions led finally to a scandal. Francis Aston and Tancred Robinson who were responsible for the Philosophical Transactions resigned and in the aftermath one of the consequences was that Edmond Halley was appointed to the paid position of Clerk of the Royal Society and entrusted with the publication of the Philosophical Transactions.

In 1679, Helvelius published his Machine Coelestis (part 2). In September, his observatory suffered major damage in a fire.

Greenwich Observatory

On 23 Sept. 1674 he travelled with Jonas Moore to Chelsea College, with a view to using it as an observatory.

Plans were put in place and in March 1675 Flamsteed was appointed as Astronomer Royal.

However in June 1675 it was decided to switch to Greenwich, with Wren as the architect of a new observatory there.

The site of Chelsea College was used soon after for the Chelsea Hospital.


In 1672, Newton presented a reflecting telescope to the Royal Society

In 1672 presented a paper on light and colors

Hooke attempted to build a reflector not subject to tarnishing.

Christopher Cook was Reeve's successor as London's foremeost lens maker

Hooke was the first person to build a practical Gregorian reflecting telescope. 1674


improved quadrant, sextant and backstaff

1667 filar micrometer

Discourse of a new Instrument to make more accurate Observations in Astronomy, than ever were yet made 1661