Amateur Astronomers Association of Princeton

by Rex Parker, PhD director@princetonastronomy.org

Summertime in Jersey.  I hope you are beating the heat and that your personal telescope isn’t sitting idle too long despite the smoke from the Canada wildfires this summer.  Regarding plans for the club’s anticipated new Celestron C14 Edge telescope, we prudently await renewal of the State Park lease for the observatory grounds before purchasing the scope. I hope to see you out there on a clear Friday night at AAAP’s Washington Crossing observatory in August.  AAAP will hold our next monthly meeting on campus Sept 8.

On Light Speed – What I Did on Summer Vacation!  Quick now – off the top of your head, what is the speed of light?  Why would light even have a velocity, and when did it become accepted among scientists and philosophers that it does?  Does light slow down or speed up in different regions of the galaxy or universe? 

Attempts to answer these questions have a remarkable history. Two centuries before American Nobel laureate Albert Michelson’s rotating mirror apparatus gave an accurate light speed in 1879 (see Sidereal Times Mar 2025), the Danish astronomer Ole Romer (1644-1710) addressed the fundamental question of whether light even had a finite speed.  He used the precise timing of eclipses of Jupiter’s moons as convincing evidence to overturn the prevailing wisdom that light was instantaneous, infinite in speed. 

My recent trip to Copenhagen led me to the Round Tower (Rundetaarn) built in 1642 in the heart of old Copenhagen (picture below, left panel).  At the top was an astronomical observatory established by Ole Romer.  The Tower has an unusual helical equestrian staircase on which a horse could be ridden to the top 100 ft above the city!  Unfortunately, virtually all of Romer’s observations and instruments were lost in the great fire in Copenhagen in 1728, and much of his accumulated knowledge has been lost.  But not all.  This is a tale of brilliant original thinking going up against the dogma of established authorities, no less than Descartes himself.  Following in Galileo’s footsteps, Romer used the recent invention of the refracting telescope with instruments he invented and beautifully crafted of brass and jewels (picture below, right panel), in Copenhagen and in the Paris Observatory where he worked for a decade in the 1670’s. 

Devices of the famed astronomer and engineer, Ole Romer.  (Left) The 100 ft high Rundetaarn (Round Tower) in Copenhagen.  (Upper Right) Romer observing through a refracting telescope.  (Lower panel) Romer’s mechanical model of the Jupiter-moon system made of brass and pearls could predict timing of the moons’ eclipses. The drawing at lower right

In order for us to realize how great Romer’s and colleagues’ discovery was, we have to put ourselves in the mind-set of three and a half centuries ago.  It was difficult to use the small single-lens refractor telescopes which were hand-made in the mid-1600’s.  The teachings of Rene Descartes and the older writings of Aristotle still dominated thinking about math, physics, and light.  Descartes held that light propagates instantaneously, and does not present a measurable finite velocity at all.  Galileo in the early 1600’s was skeptical of this but his lantern flashing experiments were inadequate, and he concluded that light travelled extraordinarily rapidly if not instantaneously.  Yet Galileo’s discovery of Jupiter’s moons using telescopes promoted new thinking among natural philosophers (the term “scientist” wasn’t used yet) and inspired improvements in optical instruments. 

By the 1670’s the Paris Observatory emerged as Europe’s leading center for astronomy.  Its patron was none other than the Sun King himself, Louis XIV, who funded it for power, prestige, and advancing France’s naval interests. The crown recruited the leading astronomers of Europe, now famous names in history.  These included Italy’s Giovanni Cassini, Holland’s Christiaan Huygens, and France’s Jean Picard who recruited Ole Romer.  Romer had impressed the crown by successfully designing the challenging water fountains of the Versailles palace then being built. Romer must have needed little sleep because it was during this period that he made the detailed observations of Jupiter’s moons that led to his key insight about light speed. 

Galileo had discovered the 4 main moons of Jupiter in 1610, and proposed that their periodic eclipses into the shadow of Jupiter could be a natural clock better than any human-made clock of the day.  By 1670 the Paris Observatory with Romer at the telescope became the leading center studying the Jovian moon eclipses.  Romer focused on Io, the innermost and fastest moon with an orbital period ~42 hours. Over the years at the Paris Observatory, Romer had timed dozens of these eclipses more accurately than anyone else. He realized from his data that the precise schedule of Io’s eclipses in Jupiter’s shadow shifted unexpectedly by 10 minutes or more as the distance between Earth and Jupiter changed during the course of a year in earth’s orbit (Figure below).  When earth was farther away from Jupiter the intervals between eclipses of Io grew longer;  when earth was closer the intervals were shorter.  Romer deduced that Io’s actual orbit wasn’t changing at all — the delay was the time it took light to travel across the extra distance of Earth’s orbit!  This was revolutionary.  He concluded that because light required more time to cover additional distance, its speed could not be infinite.  In Nov 1676, Romer presented his findings to the French Royal Academy of Sciences, correctly predicting that Io’s Nov 9 eclipse would occur 10 minutes late.  He calculated that light would take ~22 minutes to cross the full diameter of Earth’s orbit.  He published his findings in Dec 1676, stating that light has a “successive motion” and takes time to travel. 

Romer’s drawing of the eclipses of Io from 1672.  The drawing shows how the timing of eclipses of Io at different dates of the year on earth would lead to the conclusion that light has a finite measurable speed.From the library of the Paris Observatory.

History shows that Cassini, then Director of the Paris Observatory, also came up with the conclusion that light must have a finite speed.  But Cassini soon retracted, in part because of inconsistencies in the pattern of the other 3 moons of Jupiter, while Romer focused his studies only on Io.  Cassini may also have been inhibited going against Descartes, who had said that a finite speed would refute his entire theory of light, and disregarding Descartes could be a serious academic risk.  But Romer stood by his conclusions, though he apparently didn’t actually calculate or at least did not report the velocity (keeping in mind the Copenhagen fire of 1728 destroyed most of Romer’s writings). 

After all the painstaking eclipse timing measurements, it was actually a simple calculation.  Dividing the diameter of earth’s orbit by the Io eclipse time delay at two times of the year yields the velocity of light.  The size of earth’s orbit had only recently been determined for the first time by Cassini and colleagues. They observed Mars during its opposition from two continents apart, and used parallax against background stars to get the Mars-Earth distance by trigonometry.  That along with Kepler’s 3^rd Law gave them the distance of the earth to sun, with a value of about 87 million miles, only 7% short of the modern value. It was up to their colleague Christiaan Huygens to take Romer’s Io timing data along with Cassini’s distance to do the calculation.  Huygens obtained 136,000 miles per second for the velocity of light (73% of the modern value) and presented this in 1678 to the French Royal Academy of Sciences, and published it in his renowned Treatise on Light in 1690.  Romer along with Cassini and Huygens had once and for all shattered the long-held belief that light travels instantaneously, and reset the stage for the great advances in physics and astronomy and technology that we see today. Ole Romer, observer extraordinaire, hero of science, icon of Copenhagen!