Jupiter’s Discoverers – the Astronomers

Jupiter was never discovered, that is, no individual can be credited with its discovery. It has always shone brightly in our night sky and must have been known to the first humans who gazed skywards. The ancient civilisations were well aware that Jupiter moved steadily through the heavens and was one of five wandering planets known in those times. The word ‘planet’ is derived from the Greek planaein, meaning to wander.

Distant past

In 360 BC Chinese astrologer/astronomer Gan De observed a ‘star’ next to Jupiter which is now thought to have been Ganymede. Back then he had the distinct advantage of no light or air pollution but it might be worthwhile trying to see Ganymede with the unaided eye during NAW. Gan De is reputed to have occluded Jupiter’s light by lining it up with a nearby tree branch, thus allowing Ganymede to be seen.

Copernicus - Portrait, 1580
Copernicus – Portrait, 1580
Toruń Old Town City Hall

In 1530, a Polish cleric turned astronomer named Nicolaus Copernicus wrote ’De Revolutionibus’, a magnum opus treatise about the universe which led to the birth of modern astronomy and revolutionised our perception of mankind’s place in the universe. Copernicus’s remarkable proposal, based on his own observations, questioned the classical and theological belief of a geocentric universe where the Earth was at the centre of everything. His observations led him to the startling conclusion that the Earth and planets revolved around the Sun as a heliocentric system. This was controversial stuff, especially for medieval times. Essentially, it meant that mankind was not at the centre of the universe and we were but a tiny, insignificant part of the whole. In the event, Copernicus was reluctant to publish his findings and died before proof of them was established.

The age of the telescope

Portrait of Galileo
Portrait of Galileo Galilei, 1636 by Giusto Sustermans

When the telescope was invented in the early 17th century, Man’s perception of the universe took a quantum leap forward. Astronomy emerged as a science and the leading exponents were quick to make startling new discoveries. Copernicus’s controversial heliocentric theory was proven by Italian astronomer Galileo Galilei in January 1610, when he used one of the newly invented telescopes to observe Jupiter and the four large moons which today bear his name.

To quote Galileo, “…our sense of sight presents to us four satellites circling about Jupiter, like the Moon about the Earth, while the whole system travels over a mighty orbit about the Sun in the space of 12 years.”

His observations confirmed that not everything in the universe revolves around the Earth, something which sent shockwaves through the establishment and led to repercussions which he would suffer for the rest of his days.

Early measurements of the speed of light

Observations of the Jovian system had thus far led to proof of the heliocentric nature of the solar system and the first inklings that it was but a tiny part of an immense universe. In 1676, a Danish mathematician and astronomer named Ole Romer used observations of the Jovian system to measure the speed of light.

It was well known that sound travelled at finite speed, as witnessed by the muzzle flash of a distant cannon followed by the bang after a pause dependent upon the distance from the observer. As the muzzle flash always preceded the bang the assumption had been that light was instantaneous, that is, the flash was seen at the instant it happened, no matter how far away the observer.

Romer realised that discrepancies in timing the eclipses of Io, when it passed behind Jupiter, were due to the distance between Earth and Jupiter. When Earth was closer to Jupiter the eclipses occurred earlier than expected, when Earth was further away they occurred later. Romer concluded that the speed of light was finite rather than instantaneous although he did not publish a specific figure. Subsequent and more precise observations proved him correct.

NAW 2014 is running the “Speed of Light Challenge” where we invite amateur astronomers and groups to repeat Romer’s experiment. Participants are invited to make observations at any time in the period around NAW, and submit them to us.  The results will be published on this website. Good luck and clear skies!

Rotation period

Jupiter rotates in just under 10 hours, a fact first measured by British scientist Robert Hooke in 1664,  350 years ago. Hooke constructed a telescope and worked out the rotation period of Jupiter from the movement of a large spot across the face of the planet. This is sometimes credited as the first recorded observation of the Great Red Spot, although this is unlikely, as discussed at https://en.wikipedia.org/wiki/Great_Red_Spot#Great_Red_Spot_studies

This might be another opportunity for amateur astronomers who could repeat Hooke’s measurements.

Early observations by Thomas Harriot

Although Galileo gets most of the credit for early telescopic observations, it was a British astronomer called Thomas Harriot who was the first person, in 1609, to use a telescope for astronomical purposes when he drew an image of the Moon some months before Galileo.

Harriot carried out most of his observations from Syon House, to the west of London and in 1610 he began to concentrate his observations on the planet Jupiter.  On October 17th 1610, Harriot recorded in his notes that he had made “My first observations…… Of the new found planets about Jupiter”.  He meant of course the moons of Jupiter.

Harriot’s first observation of Jupiter’s moons. Copyright image

On December 14th 1610 his notes tell us  “The first time I saw 4 all clere”. So he had identified Io, Europa, Ganymede and Callisto.

Harriot went on to consistently observe Jupiter and to record the movement of the moons around the planet in meticulous detail.  By 1611 he had calculated the orbital period of each of the four moons.

Thomas Harriot was a brilliant scientist and astronomer but received little of the recognition that he deserved.