Page 26. " celatone "
by hector

Galileo's celatone was named after a sallet, or light helmet with a curving rear.  

The embedded video can also be found here.


Page 27. " revised map "
by hector

Louis XIV
Public DomainLouis XIV
The astronomers placed France’s western coast one degree further east than previously believed.

Page 27. " Giovanni Domenico Cassini "
by hector

Jean Dominique Cassini
Public DomainJean Dominique Cassini
Jean Dominique Cassini (1625-1712) was an Italian-French astronomer.  He discovered the great red spot on Jupiter.  

Page 28. " Uraniborg "
by hector
Public DomainUraniborg

When the great Danish astronomer Tycho Brahe (1546-1601) was granted the funding and land to build a research institute, he named it after Urania, the muse of Astronomy.   Constructed between 1576 and 1580 on the island of Hven near Copenhagen (now part of Sweden), the elaborate building and gardens cost 1% of the Danish state’s total budget – the largest state subsidy ever made to a scientific project.  It was however found to be unsuitable, and was abandoned just 17 years after construction, and destroyed four years later.


Page 28. " Christiaan Huygens "
by hector

Christiaan Huygens
Public DomainChristiaan Huygens
 Christiaan Huygens (1629-1695) was a Dutch astronomer and watchmaker who invented the pendulum clock and advanced the fields of calculus and probability theory.   He also designed a pocket watch and an internal combustion engine powered by gunpowder.

Page 28. " astronomical observatory "
by hector

The Observatoire de Paris
Public DomainThe Observatoire de Paris - Credit: Emmanuel Cordisco
The Observatoire de Paris was designed by Claude Perrault and built between 1667 and 1671.   It continues to serve as France’s leading astronomical centre.   The International Time Bureau is located here, and the Paris meridian runs through it.

Page 29. " velocity of light "
by hector

Jupiter Eclipsing Io
Public DomainJupiter Eclipsing Io - Credit: Ole Roemer
Ole Roemer did not actually calculate the speed of light, but he laid the groundwork for others, including Christiaan Huygens, to do so.

Page 31. " John Flamsteed "
by hector

John Flamsteed
Public DomainJohn Flamsteed - Credit: Klaus-Dieter Keller
John Flamsteed (1646 – 1719) was a precocious astronomer, correctly predicting a solar eclipse at the age of 20, and going on to make first sighting of the planet Uranus.   Originally ordained as a deacon, he became the first Astronomer Royal in 1675, and later President of the Royal Society.  

Page 32. " Royal Observatory "
by hector
Flamsteed House
Public DomainFlamsteed House

Designed by Sir Christopher Wren, the original Observatory at Greenwich was built at a cost of £520 on the site of the old Duke Humphrey’s Tower.   It was later renamed Flamsteed House.  Although Greenwich remains a focal point of astronomy, time-keeping and navigation, the Royal Observatory was moved in 1948 to Herstmonceux Castle in East Sussex to escape the light pollution in London.

Page 32. " star catalog "
by hector

Detail from Atlas Coelestis
Public DomainDetail from Atlas Coelestis
Flamsteed’s Atlas Coelestis can be viewed online

Page 35. " Gemma Frisius "
by hector

Gemma Frisius
Public DomainGemma Frisius
A mathematician and instrument maker, Gemma Frisius (1508-1555) was the first person to describe how longitude could be established using clocks.   He also invented the surveying technique of triangulation.

Page 37. " really a ring "
by hector

Public DomainSaturn - Credit: NASA
Saturn’s ring is composed of ice particles and fragments of rock.   But Saturn does also have 61 moons including Titan, the second largest in the solar system.   Saturn itself is composed mostly of gaseous hydrogen.  

Page 38. " first pendulum-regulated clock "
by hector

The Huygens Clock
Public DomainThe Huygens Clock
Huygens did not build the clock, but provided the design for the Dutch clockmaker, Salomon Coster.    Huygens also discovered a fascinating property of pendulums: two attached to the same beam will swing in perfectly opposite directions.   He called this property “odd sympathy”.

Page 38. " spiral balance spring "
by hector

Drawing of a Balance Spring by Huygens
Public DomainDrawing of a Balance Spring by Huygens
The timekeeping device that replaces the pendulum is the balance wheel.   The wheel is weighted, and spins first one way then the other.   It is the balance spring that reverses the motion of the wheel.

Page 39. " Hooke-Huygens conflict "
by hector

Hooke had form in these conflicts.   He clashed with Newton, claiming credit for work on gravitation, and with others over similar disputes.   Hooke had a tendency to exaggerate his already considerable achievements.   It is quite possible that both Hooke and Huygens came up with the coiled balance spring independently.   Both men were in the habit of communicating their breakthroughs as anagrams, which protected their intellectual property but could also muddy the waters.   Hooke gave a lecture in 1664 discussing the use of a spring to regulate a watch, and so clearly was working on the idea long before Huygens’ 1675 patent.   Nevertheless, Hooke had not, by that stage, built a functioning spring-operated watch.

Page 39. " Robert Hooke "
by hector

The Hooke Microscope
Public DomainThe Hooke Microscope
Robert Hooke (1635-1703) was a man of extraordinary and diverse talents.   His interest in microscopy led him to coin the fundamental biological term “cell”, while his mechanical studies led to his law of elasticity.   As an architect, he was chief assistant to Christopher Wren, and worked with him rebuilding London after the great fire.   Despite his achievements, Hooke was roundly considered to be a deeply dislikeable and intellectually jealous man.   

Page 43. " put out his own eye "
by hector

Public DomainBackstaff
The backstaff was a great improvement on the cross-staff because it allowed the navigator to work with the sun behind him, sparing his retinas.   He would measure the position of the shadow cast by an upper vane above on the horizon vane in front of him.

Page 43. " gimbals "
by hector

Compass on Gimbals
Public DomainCompass on Gimbals
Gimbals are pivoted rings that allow a compass or other object to remain stationary while the supporting base moves in any dimension.   Supposedly the first use of gimbals was in a Greek curiosity: an octahedral inkpot with a hole in each of its eight sides.   The inkwell was held upright inside on gimbals, whichever way up the inkpot was turned.

Page 43. " binnacle "
by hector

Creative Commons AttributionBinnacle - Credit: Andyrob, Flickr
A binnacle is simply a robust, built-in case to hold and protect navigational instruments on deck.   Over time, engineers have learnt to avoid the use of iron nails in their construction, as the ferromagnetic material distorts the compass.   Later developments on iron ships include Lord Kelvin’s invention of a binnacle incorporating two magnets to counter the effects of the ship’s plates.

Page 44. " two north poles "
by hector

 The magnetic pole is the point at which the Earth’s magnetic field points directly downwards.   It lies to the north of Canada, but its position shifts with time. The embedded NOAA video can also be found here



Page 44. " North Star "
by hector

The North Star is not a specific star.   It is the title bestowed upon whichever star happens to be closest to the north pole.   In our age, that happens to be Polaris, but the title will transfer to Gamma Cephei around 3000AD.

Page 45. " terrestrial magnetism "
by hector

The Earth’s core is thought to be composed mostly of iron (around 80%) and nickel.   This forms a planetary magnetic field which varies in strength and polarity.   From studies of prehistoric lava flows, it is believed that the polarity of the magnetic field reverses periodically, on average every 250,000 years.

Measurements suggest the strength of the magnetic field has declined by around 5% over the last 150 years.

Page 46. " Noah’s flood "
by hector

William Whiston
Public DomainWilliam Whiston
William Whiston (1667-1752) was a mathematician and historian who held strong but unorthodox Christian beliefs.   He sought to reconcile his scientific and religious beliefs, and postulated that the Great Flood was caused by a comet skimming past the Earth and deluging it with forty days and nights of water from its tail.   The comet had also stretched and split the Earth’s crust, causing fountains of water to burst out of the core, adding to the flood.  


The Lucasian Chair of Mathematics is required by charter not to be active in the Church, hence Whiston’s removal.   His adherence to the Arian creed also set him at odds with his theologian colleagues.   After being expelled from Cambridge, he went on to spend forty years working on the Longitude problem, and even managed to cause a major public panic in 1736 by predicting the world’s imminent destruction due to – what else? – collision with a comet.

Page 48. " Thanksgiving Day "
by hector

Fireworks in 18th Century London
Public DomainFireworks in 18th Century London
The Peace in question is the Peace of Utrecht, which signalled the end of the War of Spanish Succession, by then in its twelfth year.   By the Treaty of Utrecht, Philip V was removed from the French line of succession, avoiding a union of France and Spain under his rule.   The war finally ended the following year.  

Page 48. " anchor chains "
by hector

A fathom is six feet, or 1.8 metres.   The average depth of the Atlantic is 12,880ft.   As sailors like to have a chain five times as long as the depth of the anchorage, the signal boats would have needed to carry over 60,000 feet of heavy-duty chain just to anchor at average depth.   Someone else can work out how much that would weigh…