THE SOUTHWORTH PLANETARIUM
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Founded January 1970
Julian Date: 2459331.18
2020-2021: CXIX
THE DAILY ASTRONOMER
Tuesday, April 27, 2021
Exploratorium XLVIII: Galileo's Worlds
LOCATION
Jupiter
TIME
Early 17th century
The Geocentric model, the one that placed Earth in the solar system's center, was taught as fact for centuries. However, in the early 17th century, the Florentine astronomer Galileo Galilei used the newly invented telescope* to observe what he described as "worlds" around Jupiter. As he wrote in his 1610 publication Sidereus Nuncius ("Starry Messenger"):
"On the 7th day of January in the present year, 1610, in the first hour of the following night, when I was viewing the constellations of the heavens through a telescope, the planet Jupiter presented itself to my view, and as I had prepared for myself a very excellent instrument, I noticed a circumstance which I had never been able to notice before, namely that three little stars, small but very bright, were near the planet; and although I believed them to belong to a number of the fixed stars, yet they made me somewhat wonder, because they seemed to be arranged exactly in a straight line, parallel to the ecliptic, and to be brighter than the rest of the stars, equal to them in magnitude . . .When on January 8th, led by some fatality, I turned again to look at the same part of the heavens, I found a very different state of things, for there were three little stars all west of Jupiter, and nearer together than on the previous night."
He concluded that these worlds were moons revolving around Jupiter, just as the planets are the moons of the Sun. With the publication of this discovery, the beloved Geocentric Model sustained the death blow. For the first time, an astronomer directly observed objects in orbit around a body other than Earth. Even the highly ingenious Ptolemaic contrivances devised to reconcile certain planetary motions with the geocentric model,**
As Galileo saw -and as you would, too, were you to observe Jupiter through a telescope- the worlds in orbit around Jupiter appear as bright dots that quickly change position relative to the giant planet. Although Galileo only saw three dots on the first observation night (January 7, 1610), he eventually discovered another and established that four moons revolved about the fifth planet.
The image above shows Galileo's observations of Jupiter and its attendant moons over the course of many nights. From our perspective, these moons look like little dots on either side of the planet. These four moons, now known as the Galilean moons, are Io, Europa, Ganymede and Callisto.*** Were one to see them up close, their distinctive features would become readily apparent.
- Ganymede, the solar system's largest moon, is larger than Mercury or Pluto, though it is less dense owing to its icy composition
- Europa, coated entirely by ice, might harbor life within a deep subterranean ocean. Tidal forces exerted between Europa, Io and Jupiter impart sufficient heat energy to maintain a liquid ocean beneath its surface.
- Io is known for its erupting volcanoes. The same tidal stresses that produce Europa's ocean are also responsible for Io's volcanic activity.
- Callisto is the "battered" moon. Craters cover more than 90% of its surface. While all bodies have withstood repeated asteroid impacts, Callisto still shows its scars all over its body.
More than four centuries ago, Galileo saw the first moons beyond the Earth system. Although this discovery finally shifted Earth away from the Universal center, it did help us to understand the solar system and to appreciate the beauty of its many attendant worlds.
*Galileo Galilei did NOT invent the telescope. Dutch lens maker Hans Lipperhey (1570-1619) deserves the credit for this invention. However, Galileo was the first to use the instrument for astronomical observations. Lipperhey intended it for maritime observations.
**The geocentric model placed Earth in the solar system's center. It also posited that all planets moved along circular orbits. In the Ptolemaic system, named for 1st century astronomer Claudius Ptolemy, each planet moved along a circular epicycle that traveled along a circular orbit. This system "explained" retrograde motion, the tendency of planets to periodically stop and reverse course relative to the background stars.
In this system, the planet is in orbit along an epicycle that is in orbit around Earth. All orbits were Earth-centered. The observation of Jupiter's moons lent credence to the Copernican notion that the Sun, not Earth, occupied the solar system's center.
***German astronomer Simon Marius (1573-1625) gave each of these moons their individual names. He claimed to have discovered these four moons in 1609. However, he lacked documentation substantiating these observations and so credit was ultimately conferred onto Galileo.
Simon Marius (who bears an uncanny resemblance to Johannes Kepler)
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