Astronomy: The moons of Jupiter


It’s hard to miss Jupiter these days. The largest of all the planets sits high in the southeastern sky just after dark. Soon thereafter, Saturn rises in the east.

The presence of the two planets can’t help but remind me of the great achievements of past astronomers as they probed the mysteries of those enormous “gas-giant” planets.

They also remind me that among the ranks of famous, one can find a few grade-A, natural-born sleazeballs.

Among the great accomplishments of Giovanni Cassini was the discovery of a large break in the rings of Saturn, a gap we still call Cassini’s Division. However, Cassini was prone to raging fits of jealousy that ruined more than one promising scientific career.

In 1669 he was invited by the King of France to be the first director of the Paris Observatory. As a boss, Cassini left a lot to be desired. Living accommodations at the observatory were spartan. One of Cassini’s assistants spent his nights sleeping on a windowsill.

Another assistant, Jean Richter, helped Cassini to measure accurately for the first time the distance between the Earth and the sun. His work gave Richter a certain measure of fame. Ever the narcissist Cassini sent Richter to a remote province to design military fortifications. So much for Richter’s career in astronomy.

Cassini’s greatest anger was reserved for Danish assistant Ole Roemer. Over the years, Cassini had plotted the orbits of the four main moons of Jupiter. Periodically, the moons pass behind Jupiter and are eclipsed by it.

Once he knew the time each moon took to travel around Jupiter, Cassini should have been able to predict when each moon should pass behind the planet. But mysteriously, the intervals between eclipses weren’t always the same.

Roemer studied Cassini’s data and noticed that when the Earth and Jupiter were far apart, the times between eclipses were longer. Six months later, when the Earth was on the other side of the sun and closer to Jupiter, the intervals got shorter.

Roemer realized that the light from Jupiter’s moons simply had to travel farther when the Earth and Jupiter were farther apart. That meant that the light took time to get where it was going!

That contradicted what most scientists believed about light. They assumed that light got where it was going instantaneously. Galileo had tried to measure the speed of light with signal lanterns set at varying distances from each other. He had failed because the lanterns were much too close to measure differences. (Light can travel several times around the earth in one second.)

Roemer noticed the difference because his “lanterns” were hundreds of millions of miles apart. From the data, Roemer was able to calculate the speed of light with amazing accuracy, his lasting contribution to science.

Cassini, however, was incensed and refused to accept the results. By 1681, Roemer was so disgusted that he returned to Denmark.

Cassini made many important astronomical discoveries before his death in 1712, and despite his obnoxious personality, he certainly deserves the praise that history gives him. He certainly deserved to have a groundbreaking Saturn probe named after him.

However, Roemer deserves to be more than a footnote in astronomy books. His calculation of the speed of light was the first in a long series of discoveries that led almost 250 years later to Einstein’s theory of relativity, which has at its linchpin the constancy of the speed of light.

Relativity revolutionized our view of the size and structure of the universe, and we owe that understanding in no small measure to Ole Roemer and the moons of Jupiter.

Poetry at Perkins

On June 3 at 8 p.m., Perkins Observatory will hold its annual “Poetry at Perkins” celebration. The event, cosponsored by Full Crescent Press and the Ohio Poetry Association, will feature Central-Ohio poets reading their favorite astronomical poetry.

After the poetry, we’ll be observing the night sky with telescopes, including the observatory’s own Schottland Reflecting Telescope, weather permitting. Tours of the observatory, including a ghost story, will also be available if the sky fails us. Tickets are $10 in advance and can be obtained by calling 740-363-1257. Seating is limited, so advance tickets are recommended.

Tom Burns


Tom Burns is director of the Perkins Observatory in Delaware.

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