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The View from Mars Hill: Jupiter and its moons

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Le Scienze Astronomia

A headgear known as a celatone worn by Galileo, with a small telescope attached to one eyepiece.

Next week, on March 8, the planet Jupiter reaches opposition. Opposition refers to the time when a planet is opposite the Sun in the sky, rising at about the same time the Sun is setting and reaching its highest point in the sky at around midnight.

As one of the brightest objects in the current evening sky, Jupiter is easy to distinguish from its celestial neighbors. It will look like a bright star (though, of course, it’s a planet) rising in the east just after sunset.

Jupiter is the largest planet in the solar system (about 1,300 Earths could fit inside it), containing 71 percent of the planetary material in our solar system. Like Saturn, Uranus, and Neptune, it is a gas giant planet with rings, though its rings are much less prominent than Saturn’s. Jupiter takes nearly 12 Earth years to orbit the Sun once, but a day on Jupiter (the time it takes to rotate on its axis once) lasts only about 10 hours.

Jupiter has more than 60 known moons, though if current trends of discovery continue, this number will rise. The four largest moons are called the Galilean satellites, named after the Italian astronomer Galileo Galilei, who in 1610 was the first person to point a telescope at the heavens and describe his observations.

Galileo’s interest in Jupiter’s moons went well beyond their mere discovery. In fact, he saw in these satellites a utilitarian purpose. Galileo lived in the days before an accurate method of calculating longitude was known. After a year of careful study, Galileo determined the orbits of these satellites and predicted the time at which the moons would be eclipsed by Jupiter. His predictions were accurate enough that they could be used to set a watch, the key to determining longitude. However, his process proved to be quite impractical.

To view the satellites, Galileo designed a special helmet which he called a celatone. This headgear had two eyeholes. One was an empty hole through which the observer located Jupiter, and once this was accomplished, the observer would then view the satellites through a telescope that covered the other eyehole. This was difficult, especially when attempted on a rocking boat at sea. Moreover, this method could be used only at night, and even then only on clear evenings during the seasons in which Jupiter and its moons were visible. Despite these obstacles, this method of determining longitude was used for years, albeit primarily on land.

The Galilean satellites also were used to help determine the speed of light in the mid-1600s. Galileo had attempted to measure the speed of a light signal sent from one hill to another, but no matter how far he removed himself from the signal, he detected no difference in speed. Years later, in 1676, the Danish astronomer Ole Roemer observed that the expected time of Jupiter’s satellite eclipses changed, depending on Earth’s proximity to Jupiter and its family of satellites. When Earth was at its closest point to Jupiter, the eclipses occurred ahead of schedule; when Earth was at its most distant point, the eclipses occurred behind schedule.

As scientists now know, the speed of light is much too high to measure across short distances with the technology that was available to Galileo. However, with much larger changes in distance, as Roemer had used, the speed is measurable.

Roemer measured the speed of light for the first time, giving a slightly low but still fair approximation of the modern accepted value of about 186,000 miles per second (roughly seven million times faster than new Arizona Diamondbacks ace Zach Greinke can throw a fastball).

Jupiter and its moons will be in the evening sky for the next several months. With a good set of binoculars and a steady hand, you can see Jupiter’s disk, along with the Galilean satellites that were so useful in advancing our knowledge of the cosmos.

Kevin Schindler is the Lowell Observatory historian.


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