What Galileo Saw

September 29, 2009

Recently, we passed the anniversary of Galileo‘s trial before the Congregation for the Doctrine of the Faith (Inquisition) for teaching that the Earth orbits the Sun. As the current International Year of Astronomy honors Galileo’s observations and how they transformed astronomy, now is a good time to consider just what he saw through his telescope and why it was so revolutionary.

Loch Duich from Eilean Donan
Creative Commons License photo credit: atomicjeep

From November 30 to December 17, 1609, Galileo observed the Moon.  For a long time, medieval scholars had accepted the view of Aristotle, who  had taught that the heavens were perfect, unblemished, and unchanging.  This belief also dovetailed with the religious view of the heavens as the eternal abode of God.   Galileo’s telescope, however, revealed the mountains and valleys on the Moon’s surface.  Galileo could even see the shadows cast by lunar mountains.

In January 1610, Galileo turned his attention to Jupiter, the brightest object in the evening sky at the time, aside from the Moon.  Also, Jupiter was just past opposition and therefore high in the sky for much of the night.  On January 7, Galileo observed three ‘stars’ in a straight line with Jupiter, two to the left and one to the right like so:

**                            O                                                       *

Galileo knew that Jupiter was just past opposition and was therefore in retrograde motion.  (Earth had just passed between the Sun and Jupiter, and Earth’s faster orbit was making Jupiter seem to drift backwards against the background stars).  Thus, Galileo expected Jupiter to have shifted to the west, or to the right in his telescopic view, by the following night.  Instead, on January 8, Galileo saw this in his telescope:

O           *             *              *

Jupiter seemed to have gone the wrong way! Thus intrigued, Galileo continued observing Jupiter for the following week.  He saw that the ‘stars’ always appeared in line with Jupiter and to its left or right, but not in exactly the same place night to night. Although Jupiter was changing position against the background stars, it never left these companions behind.

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Galileo also began to notice  fourth ‘star’, which had been too far from Jupiter and thus out of the field of view on January 7 and 8.  These ‘stars’, Galileo realized, were in fact satellites of Jupiter.   He published his findings in his book Sidereus Nuncius (“Starry Messenger”)in March 1610.  Galileo called the moons the ‘Medicean Stars’ in honor of his patron Cosimo II of Medici and numbered them  1 to 4 in his observing notebooks.  It wasn’t until the 19th century that astronomers, following a suggestion made by Johannes Kepler to Simon Marius, began using names from Greek myth.  Thus, today we know the Galilean moons of Jupiter as Io, Europa, Ganymede, and Callisto.

That there were moons orbiting Jupiter did not disprove the idea that the Sun, Moon and all planets orbit Earth.  However, this observation answered one of the main objections to accepting the Sun as the center of the solar system.  When Nikolai Copernicus proposed (correctly) that the Moon orbits Earth while Earth and the other planets orbit the Sun, philosophers objected that there could not possibly be two centers of motion in the solar system.  Galileo’s observation that Jupiter is a center of motion with moons orbiting it made this objection moot.

Two antique goddesses
Creative Commons License photo credit: fdecomite

Towards the end of 1610, Venus reappeared in the evening sky.  Turning his telescope on it, Galileo observed that Venus, when magnified, can show phases like the Moon.   Observing the moons of Jupiter convinced Galileo that not everything orbits the Earth, but it was these observations which convinced him that planets orbit the Sun.

The dominant view of the solar system at the time, based on Claudius Ptolemy’s views, placed the Earth at the center of the system with ‘planets’ orbiting it in this order:


The order is based on how quickly the planets change position against the background stars.  (The Moon and the Sun were ‘planets’ because we see them change position against the background stars).  Based on this model, Venus should have to be virtually opposite the Sun in our sky in order for its full day side to face us.  Given that Venus never appears more than 47 degrees from the Sun, a ‘full’ phase should be impossible.  Galileo observed a full set of phases, including a full phase (the whole day side facing us) and a crescent phase (most of the night side facing us), all with Venus roughly in the Sun’s direction.  This was impossible, according to the prevailing model of his day.

In his telescope, Galileo also observed that Venus’ disk was much bigger when in crescent phase than in full phase.  Thus, he surmised that Venus was orbiting the Sun, not Earth.  When Venus enters our evening sky, we’re seeing it emerge from behind the Sun.  Venus is then smaller in our telescopes, because it is farther from Earth.  During  its evening apparition, Venus is coming around to our side of the Sun.  It therefore looms a bit larger in our telescope each day.

Also, we begin seeing Venus more from the ‘side’, with the day/night terminator in view–Venus goes from ‘full’ phase to ‘gibbous’ phase to ‘quarter’ phase. Venus appears largest when it is about to pass between the Sun and the Earth.  At that time it shows a crescent phase, as most of the sunlit side faces away from the Earth.  We can’t observe Venus when it is directly in line with the Sun (unless it also transits the Sun), but it soon reappears in the morning sky, again as a large crescent.  As the ‘morning star’, Venus goes from crescent to full and gets smaller in our telescopes as it recedes to the far side of the Sun.   In fall 2009, Venus is nearing the end of an appearance as the morning star.  It therefore shows a small, nearly full disk in telescopes now.  It will pass behind the Sun in January 2010.

And if you want to observe Jupiter tonight, look southeast at dusk for the brightest thing there.  Towards the end of the year, Jupiter will have shifted to the southwest.  With Venus in the morning sky, only the Moon can outshine Jupiter on an evening this fall.

Any observing equipment you have today is better than what Galileo was using in 1610, so even the smallest telescopes today will show you the Galilean moons of Jupiter.  If you can’t see all four, keep in mind that sometimes moons are behind Jupiter, in Jupiter’s shadow, or passing in front of Jupiter (and thus lost in its glare).  The outermost of the four moons, Callisto, is often much farther from the planet than the others–this is why Galileo couldn’t see it on January 7-8, 1610.  As you watch Jupiter’s moons orbit, you’ll be repeating one of the observations that changed astronomy. 

Authored By James Wooten

James is the Planetarium Astronomer at the Houston Museum of Natural Science. He teaches students every school morning in the planetarium, and also answers astronomy questions from the public.

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