What Galileo Saw

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.

Creative Commons License photo credit: ComputerHotline

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
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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. 

The gift that keeps on giving: Darwin and the Origin of Species

In conjunction with Darwin2009 Houston, a year-long celebration of Darwin’s 200th birthday and 150th anniversary of the publication of “On the Origin of Species,” HMNS will host a series of events exploring the contributions of this famous scientist.

Today’s guest blogger is Francisco J. Ayala, who shares some his findings here prior to his Feb. 24 lecture at the Museum, on “Darwin’s Gift to Science and Religion,” a part of HMNS’ Distinguished Lecture series.

The Origin of Species #1
Creative Commons License photo credit: gds

Darwin occupies an exalted place in the history of Western thought, deservedly receiving credit for the theory of evolution. In The Origin of Species, he laid out the evidence demonstrating the evolution of organisms.  However, Darwin accomplished something much more important than demonstrating evolution. Indeed, accumulating evidence for common descent with diversification may very well have been a subsidiary objective of Darwin’s masterpiece.  Darwin’s Origin of Species is, first and foremost, a sustained argument to solve the problem of how to account scientifically for the design of organisms. Darwin seeks to explain the design of organisms, their complexity, diversity, and marvelous contrivances as the result of natural processes. Darwin brings about the evidence for evolution because evolution is a necessary consequence of his theory of design.

The advances of physical science brought about by the Copernican Revolution had driven mankind’s conception of the universe to a split-personality state of affairs, which persisted well into the mid-nineteenth century.  Scientific explanations, derived from natural laws, dominated the world of nonliving matter, on the Earth as well as in the heavens.  Supernatural explanations, which depended on the unfathomable deeds of the Creator, were accepted as explanations of the origin and configuration of living creatures. Authors, such as William Paley in his Natural Theology of 1802, had developed the “argument from design,” the notion that the complex design of organisms could not have come about by chance, or by the mechanical laws of physics, chemistry, and astronomy, but was rather accomplished by an Omnipotent Deity, just as the complexity of a watch, designed to tell time, was accomplished by an intelligent watchmaker.

It was Darwin’s genius to resolve this conceptual schizophrenia.  Darwin completed the Copernican Revolution by drawing out for biology the notion of nature as a lawful system of matter in motion that human reason can explain without recourse to supernatural agencies. Darwin’s greatest accomplishment was to show that the complex organization and functionality of living beings can be explained as the result of a natural process—natural selection—without any need to resort to a Creator or other external agent.  The origin and adaptations of organisms in their profusion and wondrous variations were thus brought into the realm of science.

crab on the rocks
Creative Commons License photo credit: angela7dreams

Evolution can be seen as a two-step process. First, hereditary variation arises by mutation; second, selection occurs by which useful variations increase in frequency and those that are less useful or injurious are eliminated over the generations. “Useful” and “injurious” are terms used by Darwin in his definition of natural selection. The significant point is that individuals having useful variations “would have the best chance of surviving and procreating their kind.” As a consequence, useful variations increase in frequency over the generations, at the expense of those that are less useful or injurious.

Natural selection is much more than a “purifying” process, for it is able to generate novelty by increasing the probability of otherwise extremely improbable genetic combinations.  Natural selection in combination with mutation becomes, in this respect, a creative process.  Moreover, it is a process that has been occurring for many millions of years, in many different evolutionary lineages and a multitude of species, each consisting of a large number of individuals. Evolution by mutation and natural selection has produced the enormous diversity of the living world with its wondrous adaptations.

Francisco J. Ayala is a noted biologist and philosopher at the University of California at Irvine’s Department of Ecology and Evolutionary Biology. Don’t miss his lecture on Feb. 24 - or any of the other Darwin2009 events planned at HMNS this year.

Eight is Enough?

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Two years ago this month, the International Astronomical Union adopted a new definition of ‘planet’ which excludes Pluto. Not only do I, as Planetarium Astronomer, continue to get questions about Pluto’s ‘demotion’, but scientists themselves continue to debate it. Right now (August 14-16, 2008), a conference called “The Great Planet Debate:Science as Process” is underway at the John’s Hopkins University Applied Physics Laboratory in Laurel, Maryland. The saga of Pluto and of the definition of ‘planet’ offers some insight into our solar system and into how science works.

northern tier sky
Creative Commons License photo credit: truello

The definition of ‘planet’ has changed before. Ancients looked at the sky and saw that certain ‘stars’ in the sky changed position, while most stars seemed to form the same patterns all of the time. The Ancient Greeks called the moving stars ‘planetes‘, or wanderers–this is the origin of the word. The Moon, too, appears near different stars each night. The Sun’s apparent motion is less obvious, since we don’t see the Sun and stars at the same time. Careful observers, however, can see that different stars rise and set with the Sun at different times of year. The full list of ‘planetes’, then, included the Sun, the Moon, Mercury, Venus, Mars, Jupiter, and Saturn. (Astrologers still use this archaic definition of planet).

Thanks to Copernicus and Galileo, people began to realize that the Sun, not the Earth, was the center of the solar system. The definition of ‘planet’ changed from ‘object which moves against the background stars’ to ‘object in orbit around the Sun’. The Sun and Moon, which had been planets, no longer were.

The position of Uranus, discovered in 1781, seemed to fit a pattern described by astronomers Johann Titius and Johann Bode. That same ‘Titius-Bode rule’ also predicted a planet between Mars and Jupiter, so when Giuseppe Piazza discovered Ceres at just the right distance in 1801, it was considered a planet. By 1807, four new ‘planets’ had been found between Mars and Jupiter (Ceres, Pallas, Juno, and Vesta). By the middle of that century, however, dozens of these new objects were being discovered; up to 100 had been found by 1868. It thus became clear that astronomers had in fact found a new category of solar system object. Astronomers adopted the term ‘asteroid‘, which William Herschel had recommended in 1802; ‘planet’ was redefined to exclude very small objects that occur in bunches. This is how science works; we must constantly revise even long standing definitions as we learn more about the universe around us.

In the late 19th century, astronomers noticed that Uranus and Neptune seemed to deviate ever so slightly from their predicted positions, suggesting that another planet was perturbing them. in 1906, Percival Lowell started a project to find the culprit, which he called ‘Planet X’. In 1930, Clyde W. Tombaugh located Pluto in sky photographs he took at Lowell Observatory in Arizona. It soon became apparent, however, that Pluto was not massive enough to influence the orbits of Uranus or Neptune. Throughout the mid 20th century, astronomers continued to revise Pluto’s estimated size downwards. From 1985 to 1990, Pluto’s equator was edge on to us, such that we saw its moon Charon pass directly in front of and behind Pluto’s disk. This allowed scientists to measure Pluto’s diameter more precisely, proving that it had not been the Planet X that Percival Lowell sought. Pluto’s diameter is just under 2400 km, a little less than the distance from the Rio Grande to the US/Canadian border. Pluto’s discovery, it turns out, was an accident.

In addition to small size, Pluto has an unusual orbit. Planetary orbits are ellipses rather than perfect circles. The eccentricity of an ellipse indicates how ‘out-of-round’ it is on a scale from 0 (perfect circle) to 1 (parabola–far end at infinity). Pluto’s orbit has an eccentricity of about 0.25, much greater than that of planets such as Earth (0.01) or Venus (0.007). The planets orbit nearly (but not exactly) in the same plane; Mercury‘s orbit, inclined by 7 degrees, is the most ‘out of line’. Pluto’s orbit, however, is inclined by 17 degrees.

Released to Public: Solar System Montage (NASA)

Behold: a pluto-less solar system.
Creative Commons License photo credit: pingnews.com

We divide the planets of our solar system into two categories: the inner planets (Mercury, Venus, Earth, and Mars) which are made mostly of rock, and the outer planets (Jupiter, Saturn, Uranus, and Neptune) which are gas giants with no solid surface. Pluto, however, fits in neither of these categories, as it is made of ice and rock (by some estimates, it’s 70% rock and 30% ice; by others, it’s about 50/50).

With its small size and abnormal orbit and composition, Pluto was always a misfit. Textbooks noted how Pluto fit in with neither the rocky inner planets nor the gas giants in the outer solar system. Still, Pluto remained a ‘planet’ because we knew of nothing else like it. There was simply no good term for what Pluto is.

That began to change in 1992, when astronomers began finding Kuiper Belt objects. The Kuiper Belt is a group of small bodies similar to the asteroid belt. Kuiper Belt objects (KBOs), however, orbit beyond Neptune’s orbit. Also, the Kuiper Belt occupies more space and contains more mass than does the asteroid belt. Finally, while asteroids are made mostly of rock, KBOs are largely composed of ice, including frozen ammonia and methane as well as water–just like Pluto. In addition to the Kuiper Belt proper, there is a scattered disc of objects thought to have been perturbed by Neptune and placed in highly eccentric orbits. Objects in the Kuiper Belt, scattered disc, and the much more distant Oort Cloud are together called Trans-Neptunian Objects (TNOs)

With the discovery of more and more KBOs, astronomers began to wonder if Pluto might fit better in this new category. Not only was the composition similar, but there is even a group of KBOs called plutinos, with orbits similar to Pluto’s. In the Kuiper Belt and the scattered disc, astronomers began to find objects approaching Pluto’s size, including Makemake, Quaoar, and Sedna.

Pluto can't get no respect
Pluto takes advantage of the wildly (?)
popular LOLcats to plead its case
with mankind.
Creative Commons License photo credit: the mad LOLscientist

To call Pluto a planet, but not these others, seemed arbitrary.

Finally, in 2005, a team of astronomers located Eris, which is slightly bigger than Pluto. Clearly, Eris and Pluto are the same kind of thing; either both are planets or both are not. If they both are planets, however, then should we include Quaoar et al., above? We have only just begun to explore and understand the Kuiper Belt and the scattered disc. Might we eventually find dozens of new ‘planets’ like Eris? Hundreds? Thousands?

This is what led the International Astronomical Union to reconsider the definition of ‘planet’ two Augusts ago. The IAU decided it was simpler to limit the number of planets to eight (Mercury through Neptune) and classify Pluto (and Eris, Quaoar, et al.) among the Trans-Neptunian objects. A new term, “dwarf planet,” includes the biggest asteroids and TNOs–those big enough to have assumed a spheroid shape. Still, other astronomers remain dissatisfied, hence the discussion going on in Maryland now.

There are two things we must keep in mind if we’re wondering when the Pluto question will be ‘resolved.’ First, decisions and conclusions of scientists are not holy edicts to be obeyed and never questioned. Quite the contrary, all such conclusions are provisional, pending new discoveries and better information. Any new decision reached this weekend is likely to be revised when the IAU meets again in 2009, and again in 2015 when the New Horizons mission arrives at Pluto. If it were any other way, science could not function.

Secondly, all categories which help us organize and understand things in our minds (including ‘planet’) are pure human inventions that only roughly correspond to nature. Although we need to categorize the things we see, nature does not; no matter how we classify objects, nature presents us with borderline cases that challenge us. Pluto is the same thing today as it was in 2005 or even before it was discovered in 1930. We need to distinguish our need for neat categories from our need to explore and describe nature.

Proud to be a space cadet? Learn more about astronomy:
Dust off your telescope – or visit the George Observatory – to see what’s in the night sky this month.
Ten billion trillion trillion carats – the universe has great taste in diamonds
If it blew a hole in your roof, you’re on the right track – how do you tell a rock might be a meteorite?