Join Us for Astronomy Day! This Saturday, at the George Observatory

Girl ScoutsOur Fun Hundred events continue this Saturday, October 24, as our annual Astronomy Day takes place at the George Observatory in Brazos Bend State Park. Join us from 3 p.m. to 10:30 p.m. for a wide variety of activities.  Of course we’ll begin stargazing at dusk (about 7:30 p.m.) if the weather is clear.  Our main telescopes (36”, 18” and 14”) will be open, and our observing deck will be full of telescopes of all shapes and sizes showing a wide variety of objects.  However, we also have many fun activities in the afternoon, including solar observing on the observation deck and simulated missions to the Moon in the Observatory’s Expedition Center.   Special indoor and outdoor presentations begin at 4 p.m.  Outdoor presentations occur every half hour until dusk; indoor presentations occur every hour, with the last one starting at 9 p.m.  You can even win a telescope!  All events at the Observatory on Astronomy Day are free of charge; you pay only to enter the state park itself.   Go to for a full description of everything going on.

The theme for this year is the International Year of Astronomy, as 2009 marks the 400th anniversary of the first observations of the sky through a telescope.  Our event even coincides with the Galilean Nights (October 22-24), a Cornerstone Project of International Year of Astronomy 2009 .  Accordingly, many of the indoor and outdoor presentations will focus on telescopes, the history of telescopes, and Galileo’s observations.  You can personally repeat one of Galileo’s historic observations by observing Jupiter’s moons through one of the many telescopes on our deck.  If you observe between 8:19 and 8:24, you can see one of the moons, Io, occult (partially block) Europa.

The Houston Astronomical Society (HAS) sponsored Astronomy Day events as early as 1982.  Many gatherings in the ’80s took place at Rice University.  In fall 1985, Comet Halley returned to our region of the solar system for the first time in 76 years. When HMNS and the newly formed Fort Bend Astronomy Club (FBAC) arranged for a Saturday night viewing of Halley that fall, over 10,000 visitors came to Brazos Bend State Park to get a glimpse of the comet.  Such an expression of local interest in observing celestial events led to the creation of the George Observatory in 1989.

On August 12, 1994, in conjunction with the annual Perseid Meteor Shower, HAS sponsored the first Astronomy Day to be held at the George Observatory.  As the event grew in popularity, organizers shifted the event to October, a month with (on average) more comfortable temperatures and clearer skies in the Houston area.  Also, more local area clubs became involved, including the Johnson Space Center Astronomical Society (JSCAS), the North Houston Astronomy Club (NHAC) and the Astronomical Society of Southeast Texas (ASSET), based in Beaumont.   Joining us as sponsors in 2009 are the Huntsville Amateur Astronomical Society (HAAS)  and the Community of Humble, Administaff Observatory Society.  Introducing local astronomy clubs to the public, and vice-versa, has become an important part of Astronomy Day.  If you are interested in any of the astronomy clubs in the immediate vicinity of Houston; you will be able to learn about all of them at Astronomy Day.

The involvement of more and more clubs and volunteers has gone hand in hand with much greater attendance in recent years.  As you can see in the accompanying chart, not even Hurricane Ike’s aftermath could depress our attendance numbers back to what they were just four years ago.

Year Attendance
1999 1900
2000 500 (rain)
2001 1500
2002 1300
2003 1800
2004 1200
2005 1586
2006 2028
2007 3997
2008 2400 (Ike)

Also, we are not alone in holding a huge star party on October 24.  The Astronomical League sponsors Astronomy Day events worldwide.  In 2009, most of these were in the spring, back on May 2.  But there are at least four other events this Saturday, including one at the University of Texas at Arlington.  In 2007, the Astronomical League recognized our event as the best run Astronomy Day of the year.

Creative Commons License photo credit: fdecomite

We hold Astronomy Day every year in mid-to-late October.  If possible, we select a Saturday with a first quarter Moon.  This puts the Moon, a popular viewing target for the public, high in the sky right at dusk yet not so bright as to overwhelm everything else in the sky.   This year, Astronomy Day is one day before First Quarter, so a big crescent Moon will be in the south-southwest at dusk.  Dominating the southern sky all evening, outshining everything else but the Moon, is the planet JupiterUranus and Neptune, though not visible with the naked eye, will also be in the south.  Other objects visible every October include the Andromeda Galaxy (the nearest galaxy to or own, not counting the Milky Way’s companions) and the Ring Nebula in Lyra (the remains of a star similar to our Sun).

As I write this, the weather forecast for Saturday is looking good.  A cold front should have cleared the area by then, leaving us with clear skies and perfect temperatures.  Therefore, we invite everyone to join us this Saturday for a wonderful afternoon and evening under the stars.  See you Saturday!

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
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. 

2009: International Year of Astronomy

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.