2009: International Year of Astronomy

Look
Creative Commons License photo credit: judepics

We could say that modern astronomy began in 1609.  That was the year when the telescope, invented by the Dutch in 1608, was first used to observe and describe celestial objects.  Until telescopes were used, astronomy was primarily about measuring the positions of the Sun, Moon, and planets in the sky.  This helped early astronomers make calendars and to plan their harvests, but people were unable to study the celestial bodies and learn their characteristics.  A recently discovered lunar map indicates that Thomas Harriot of England was the first to observe and draw a magnified image of the Moon in July 1609. 

Galileo Galilei, of course, is most well-known for building and using early telescopes.  He did his lunar observations in December 1609 while observing from Padua, Italy.   The prevailing idea at the time was that everything in the heavens had to be perfect and unblemished.  Drawings of mountains, valleys, and craters on the Moon contradicted this idea, showing the Moon to be an ‘imperfect’ world like Earth.  As Galileo published his drawings and Harriot did not, Galileo gets the credit for changing our concept of the universe, helping us realize that celestial bodies are worlds and not just sources of light.

On January 9, 1610, Galileo saw three ‘fixed stars’ next to Jupiter.  Four days later he discovered a fourth and realized that these ‘stars’ orbited Jupiter.  Today, those four moons– Io, Europa, Ganymede, and Callisto, are called the Galilean moons.  The direct observation of moons orbiting Jupiter disproved Claudius Ptolemy‘s model of the universe, already centuries old at the time, which held that all bodies in the universe orbited the Earth.

Moon n Venus played hide-and-seek
Creative Commons License photo credit: voobie

In December 1610, Galileo observed Venus and saw that Venus showed phases like the Moon’s when magnified in his telescope.  This meant that sometimes the sunlit side of Venus faces Earth, while at other times we see the night side, although Venus is never opposite the Sun in the sky.  This could happen only if Venus orbits the Sun rather than Earth.

By the way, Galileo did far more than just astronomy.  Rice University’s Galileo Project has more on his extraordinary life, including a timeline.

It was also in 1609 that Johannes Kepler published his New Astronomy, containing his first two laws.  The first law states that each planet’s orbit is an ellipse rather than a perfect circle.  The second law states that a planet sweeps out equal areas in equal times.  Kepler published his third law, which relates the square of a planets period (time for one orbit) to the cube of its average distance, in 1619. 

This makes 2009 the 400th year of modern astronomy.  Appropriately, the United Nations declared this year to be the International Year of Astronomy.  At that link, you can learn about events taking place all over the world promoted by the International Astronomical Union (IAU) and the United Nations Educational, Scientific, and Cultural Organization (UNESCO).  Their goal is for people all over the world to discover the wonders of the sky and to appreciate our place in the universe.

Star Cloud Over Saskatchewan.jpg
Creative Commons License photo credit: Space Ritual

You can participate in the International Year of Astronomy right here in Houston.  Several of the Fun Hundred events we’ve set up to celebrate our 100th anniversary are astronomy-related.  They include Sun-Earth Day at the vernal equinox, our annual viewing of the Perseid meteor shower in mid-August, members nights at the George Observatory, and a winter solstice event on our sundial. 

Also, you can observe the phases of Venus in the first three months of this year, just as Galileo did through his telescope.  Keep in mind that Galileo’s telescope looked like this; anyone with a good pair of binoculars has better observing equipment.  Go outside at dusk and look west southwest for the brightest point of light in the sky.  That is Venus.  Through a telescope, you’ll notice that Venus appears half-lit in mid-January 2009.  As you keep observing through March, you’ll see Venus become a more and more pronounced crescent.  This is because Venus is coming around to our side of the Sun and thus turning more and more of its night side to Earth.  The very skinny crescent of mid-March is so pronounced that it is noticeable in binoculars.

Remember, the great discoveries, or aha moments, as my co-blogger described, are not limited to great, historic scientists.  The beauty of science is that anyone who takes the time to observe can share in the act of discovery.

Looking back…

In case you were wondering about notable science events that occured the week following August 22…

On August 24, 79 A.D. Mount Vesuvius erupted, covering the cities of Pompeii (hopefully you had the chance to see the exhibit here in Houston at the Museum of Fine Arts), Herculaneum, and Stabiae under volcanic ash. The city was lost for 1,700 years – until it was accidentily rediscovered in 1748. The excavation of the city has given valuable insight into the city during the height of Roman Empire, acting as a time capsule, allowing scientists to study the buildings, food, and even people that were buried that fateful day.
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This is Mount Etna erupting in 2006 (there is no footage of the 79 explosion of Mount Vesuvius for obvios reasons.)

Also on August 24, in 2006, the International Astronomical Union (IAU) redefined the term “planet,” and Pluto was sent on its cosmic way (read the post about the controversy that ensued, by our astronomer James.) Pluto was “demoted” to the status of Dwarf Planet. There are currently eight planets and four dwarf planets in our solar system. The new definition of a planet is a celectial body that meets the following criteria:
    (a) is in orbit around the Sun, 
    (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and
    (c) has cleared the neighbourhood around its orbit.

Karoli looking foward
Creative Commons License photo credit: ckaroli

On August 25, 1609, Galileo Galilei demonstrated his first telescope to Venetian lawmakers. He was one of the first men to build a telescope, and did so without actually ever seeing one of the few that existed. He was the first to discover any of Jupiter’s moons (he found 4), now known as the Galilean satellites.

On August 27, 2003, Mars made its closest approach to Earth in nearly 60,000 years. The last time Mars was that close to Earth, man had just began to migrate out of Africa. Man wouldn’t start settling down, farming, and beginning to live in cities for another 48,000 years. Mars passed approximately 34,646,416 miles (55,758,006 kilometers) from Earth.

Eight is Enough?

Creative Commons License photo credit: CommandZed

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?