If you caught the eclipse this week, you probably took care to use eclipse glasses or a pinhole camera. But you may have caught a glance at the Sun with the unaided eye. Perhaps while looking at the Sun during totality you saw a bit of the Sun come back before looking away. If you did, you wouldn’t be alone. Lots of people have looked at the Sun before, including Galileo. 

Galileo is most famous for his telescopic observations. His discovery of the the moons of Jupiter and the phases of Venus changed our understanding of the solar system. But Galileo also made naked-eye observations of the heavens. As part of his work he looked directly at the Sun many times. This was not one of Galileo’s best ideas, but he did try to be careful about it. He never looked directly at the Sun when it was high in the sky. He looked when the Sun was low on the horizon at dawn or sunset, often when somewhat obscured by fogs or clouds. When the Sun is low in the sky its light must travel through much more atmosphere to reach us. This not only makes the Sun less intense, it gives the Sun its reddish glow. Galileo found the addition of fog particularly useful, because it allowed Galileo to observe sunspots on the Sun.

Although we don’t know how often Galileo looked directly at the Sun, we know from his letters that he made several observations. He may even have looked at the Sun through a low power telescope. Later, Galileo’s student Benedetto Castelli discovered the projection method for looking at the Sun (using a pinhole camera), which Galileo felt was a far superior method of solar observation. Toward the end of his life, Galileo did go blind, but this was more than two decades after his direct solar observations.

Looking directly at the Sun is always risky. This is particularly true during a total eclipse when the Sun is mostly covered. If you take an extended look at the Sun during a partial eclipse you can damage your eyes permanently. There are several recorded cases of this. But a quick glance isn’t likely to be harmful. If you caught a quick glance during the eclipse, you shouldn’t worry.

And if you still have your solar glasses, you can hang on to them until the next great American eclipse in 2024, or you can donate them to Astronomers Without Borders for others to use.

The post Did You Look At The Sun? So Did Galileo appeared first on One Universe at a Time.

On January 7, 1610 Galileo Galilei pointed a small twenty-power telescope at Jupiter. What he observed changed the way we understood the universe.Galileo noticed what appeared to be three small stars near Jupiter. The next evening he again observed three faint stars, but they now appeared on the other side of the planet. Over the next several weeks he watched up to four faint stars weave back and forth near Jupiter. Galileo named them the “Medicean Stars” in honor of his patron Cosimo II de’ Medici, but we now know them as the Galilean moons of Jupiter. 

At first it wasn’t clear to Galileo what these “stars” were, or why they were always found near the king of planets. But Galileo was a patient and accurate observer, and over time it became clear that the motion of these objects followed Kepler’s laws. The same laws that described the motion of the planets around the Sun. Clearly they orbited Jupiter in much the same way as our Moon orbits the Earth. And if moons could orbit a planet, then perhaps it was true that the Earth orbited the Sun after all.

A comparison of Galileo’s observations of the moons with their actual positions as determined from modern measurements. Galileo’s observations are astoundingly accurate. Credit: Ernie Wright

With this discovery and his observations of the phases of Venus later that same year, Galileo gave us proof of a heliocentric universe. Earth was not fixed at the center of the cosmos, but rather moved around the Sun just as other planets did. But Galileo’s discovery not only changed our view of the heavens, it also changed the Earth. Quite literally.

One of the great challenges of cartography has been determining just where on Earth you are. Determining your latitude can be done by observing the position of the stars. For example, the angle of the “north star” Polaris above the horizon is a good basic indication of your latitude. Determining longitude, however, is a very different matter. The Sun, planets and stars travel east to west across the sky, and so there is no clear point of reference for measuring longitude. To make an accurate longitude measurement, you need an accurate clock you can use to measure when particular stars pass overhead, for example. Since the Earth rotates at a steady rate, a time measurement can be used to determine your position east or west of a reference location.

Left: Willem Bleu’s 1650 map of Europe. Right: Robert Janvier’s 1764 map of Europe.

Galileo realized that since the moons of Jupiter obeyed Kepler’s laws, they could serve as a kind of heavenly clock. A clock more precise than any human-made clock of the time. So he began to compile a table of eclipses of the Galilean moons. That is, when a particular moon would pass into Jupiter’s shadow or reappear from behind Jupiter. In 1668, Giovanni Domenico Cassini improved upon these tables, creating a time table accurate enough for cartography. For the first time cartographers could make truly accurate longitude measurements. Many of the accepted distance between cities (used since the Roman Empire) were found to be off by hundreds of miles. The affect of Galileo’s moons can be seen in the difference of world maps made before and after Cassini’s tables.

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There’s a new video from Human Universe where Brian Cox shows how, in a vacuum, a bowling ball and feathers fall at the same rate. The idea that all objects fall at the same rate regardless of their mass is often attributed to Galileo. It’s commonly said that Galileo proved this fact to be true by dropping masses off the leaning tower of Pisa. But in fact it’s quite likely that Galileo never performed the experiment. Given the experimental apparatus at the time, it’s unlikely that such an experiment would be conclusive anyway. So why was Galileo convinced that objects fall at the same rate?

Galileo wasn’t the first to propose the idea, now known as the equivalence principle. Nearly a century before the publication of Galileo’s Two New Sciences, historian Benedetto Varchi writes of the idea in opposition to Aristotle’s claim that heavier bodies fall faster. A few years before Galileo’s claim, Simon Stevin performs an experiment with two lead balls, and from the sounds of their impacts concludes that they fell at the same rate. The reason we attribute the idea to Galileo is not because he was the first to claim it, but rather that he clearly outlined why it should be true.

A recreation of Galileo’s inclined plane. Credit: Museo Galileo

Galileo studied gravity by experimenting with inclined planes. He rolled balls down an inclined plane such that the ball would strike a bell at specific intervals. From this he showed that the distance a ball rolled with each unit of time followed a pattern of odd numbers. In one moment it would roll one unit of distance. In the next moment 3 units, then 5, 7 and so on. The unit of distance increased with successively higher inclines, but the pattern was always the same. In this way Galileo demonstrated that objects fell down an incline at a constant rate.

Credit: Gary Garber

Galileo also demonstrated the property of inertia. Aristotle claimed that objects fell to Earth because of their natural motion toward the Earth. A heavier object has a stronger natural motion, therefore should fall faster. But Galileo showed that rolling objects did not naturally move toward the Earth. If two inclined planes are placed against each other, a ball can roll down one then up the other one until it almost reaches its starting height. With a longer and shallower inclined plane, the ball still reaches about the same final height. In principle, if the second inclined plane were made flat, the ball would roll forever. From this Galileo argued that objects had a property of inertia, which meant that objects would continue to move unless something stopped it.

It is from the property of inertia that Galileo argued for the equivalence principle. To prove Aristotle wrong, he used a simple thought experiment. Suppose that heavier objects did, in fact, fall faster than lighter ones. If that were true, then if we dropped a small lead ball and a large lead ball from a tall tower, the large ball would strike the ground first. But suppose we then connected the two balls and dropped them alongside another large lead ball. According to Aristotle, the connected lead balls are heavier than the single large ball, so they should strike the ground first. But according to Aristotle, the connected lead balls should fall more slowly because the lighter ball is trying to  fall less quickly. From this contradiction Galileo concluded that all masses must fall at the same rate. Gravity must change the inertia of all masses equally.

Galileo was able to get the correct answer, and he did it by wondering about how things fall.

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Glass has the useful feature of being transparent at optical wavelengths. That, and the fact that light can refract (change direction) when it passes through curved glass is what made it useful as lenses, and eventually telescopes.

We usually think of Galileo as the inventor of the telescope, but this isn’t the case. Glass lenses have existed in Europe since the 1200s, and by the mid 1300s basic eyeglasses had appeared. These early lenses were pretty basic convex lenses. Convex means that they were thicker in the middle than at the edges, and as a result light passing through them would refract closer together. A magnifying class is a good example of a convex lens, and if you’ve ever let sunlight pass through one to make a concentrated point of light that can set things on fire (kids, don’t try this at home), then you’ve seen its effect.

In the mid 1400s a different type of lens known as a concave lens appeared. These are thicker on their edge than in their middle, and causes light passing through it to spread apart. If you are nearsighted and wear glasses, these are the type of lenses they use.

The first astronomer to use lenses seems to have been Leonard Digges. In the 1570 book Pantometria his son Thomas describes a “proportional glass” that might have been a telescope, though the description is vague. The first true telescope dates to at least 1608, when Hans Lipperhey applied for a patent for the device in the Netherlands. The device used a larger convex lens and a smaller concave lens place along either end of a tube. Light passing through the convex lens would be focused together, and would then pass through the concave lens, which spread the light into its original orientation. The net effect was to magnify the image entering the device.

In July of 1609 Thomas Harriot used such a telescope to look at the Moon. His telescope only had a magnification of 3, which isn’t much at all, so Harriot wasn’t able to determine the nature of the lunar features. That same year Galileo had a similar telescope constructed. Galileo’s first telescope had a magnification of about 9, which was powerful enough to distinguish features of the moon. Galileo’s later telescopes had magnifications of up to 30. So it is Galileo whom we now associate with the telescope.

Refracting telescopes such as Galileo’s are still used today, though they are less popular than reflecting telescopes which are easier to construct at larger sizes. But even reflecting telescopes require an eyepiece, which is typically a glass lens. So even when we aim a mirror to the night sky, we still raise a glass.

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Yesterday I mentioned that after discovering the moons of Jupiter, Galileo went on to observe the phases of Venus, which further reinforced the idea that the Earth moved about the Sun. So just how do phases of a planet prove it revolves around the Sun?

The phase of any planet or moon depends upon its position relative to the Sun and your position relative to it. This is because planets and moons don’t produce light themselves (at least not much in the visible spectrum), so only the side that is facing the Sun is illuminated. Basically that means that about half of any planet/moon is illuminated at any given time. But this doesn’t mean you always see half of it lit up.

A good example of this is seen in the phases of the Moon. The Moon orbits the Earth, and as it does we see different amounts of the Moon illuminated. A full moon occurs when the moon is positioned in the opposite direction of the Sun. With the Earth between the Sun and Moon, we see the fully lit moon, hence it appears full. Because of their orbits, the Sun, Earth and Moon aren’t in a perfectly straight line during most full moons, which is why lunar eclipses don’t happen every month.

When the Moon is aligned perpendicular to Earth and Sun, we see half the moon illuminated. These are called quarter moons because they occur at the first or third quarter of the lunar cycle. Quarter moons can be seen either in afternoon and early night (first quarter) or late night and early morning (third quarter).

When the Moon is between the Sun and Earth, then the illuminated side is away from us, so we don’t see it. It is known as the new moon. Again, since the Sun, Earth and Moon aren’t usually perfectly aligned, so solar eclipses are rare.

With Venus things are a bit different. You can see Galileo’s observations in the figure here. The phases of Venus are compared to the apparent sizes of Saturn, Jupiter and Mars. You’ll notice two interesting things about Venus. First, it has phases ranging from crescent to nearly full. Second, it changes dramatically in size, being large as a crescent and small when nearly full. Together these observations demonstrate that Venus orbits the Sun.

It was long known that Venus was never seen far from the Sun. It would appear in the morning sky before sunrise, or in the evening sky after sunset, but you’d never see Venus in the middle of the night. In the Earth-centered model it was thought that motion Venus was on an epicycle of a celestial sphere rotating about Earth in sync with the daily motion of the Sun. Since the epicycle of Venus was always aligned with the Sun, Venus never wandered far from the Sun. But this also meant that Venus would always be closer to the Earth than the Sun.

If that were the case, then Venus could never appear in a nearly full phase. If Venus is always between the Earth and Sun, then at most we would would only see half of it illuminated. When the Moon is in full phase it is farther from the Sun than Earth. So the fact that Galileo observed a nearly full phase of Venus meant it must have been further from the Earth than the Sun is from Earth. In other words, it must almost be behind the Sun.

When Venus is in its crescent phase it must be nearly in front of the Sun, since most of the illuminated side is away from us. This is where the change in size becomes important. Since Venus can be seen both as a crescent and as nearly full, it must sometimes be in front of the Sun and sometimes behind.

Now if you still support the Earth-centered model, you could argue that the Sun and Venus both lie on an epicycle, thus rotating about each other, but the epicycle still rotates about the Earth. But if that were the case, then either both the Sun and Venus would appear larger and smaller as they move closer and farther from the Earth. But only Venus changes its apparent size, and it does so exactly as you would expect for a planet orbiting the Sun.

So it must be that Venus orbits the Sun, not the Earth. The discovery of moons orbiting Jupiter just reinforces the fact that the Earth-centered model is wrong.

You could say the geocentric model was just a phase.

The post Just a Phase appeared first on One Universe at a Time.

In the first few months of 1610, Galileo Galilei pointed a small twenty-power telescope at Jupiter. What he observed changed the way we understood the universe.

With his telescope, Galileo saw what appeared to be three faint stars in a straight line near Jupiter. The next evening he saw what appeared to be the same three stars, but it seemed Jupiter had moved in the opposite direction to its expected motion. Within a few days it became clear that Galileo wasn’t observing the motion of Jupiter relative to some faint stars, but rather these stars were moving along with Jupiter.

It also became clear that these weren’t stars at all. They didn’t twinkle the way stars do, which was a long known method of distinguishing planets from stars. Stellar twinkling occurs because of small disturbances our atmosphere. Stars are so distant that they appear as points of light, so the atmospheric disturbances bend and deflect the starlight, causing it to twinkle. Planets are close enough that they appear as a small disk. They are too small to resolve as a disk with the naked eye, but they are wide enough that atmospheric disturbances don’t cause them to twinkle.

Galileo knew they must be more like planets than stars, so over the next two months he watched their motion, making a total of 64 observations of their positions. He discovered a total of four objects moving about Jupiter. Sometimes he could observe all of them, and other times he only observed one or two. It was clear, however, that these objects were moving about Jupiter in the same way that the Moon moves about the Earth. Galileo had discovered four moons of Jupiter.

Credit: Ernie Wright

Galileo’s discovery of Jupiter’s moons confirmed that the geocentric view of the universe was wrong. Copernicus had proposed a Sun-centered view of the cosmos nearly 70 years earlier, but this heliocentric model was still controversial when Galileo made his observations. Many supporters of Copernicus were still careful to distinguish the apparent motion of the planets around the Sun from the claim that planets actually moved about the Sun.

Galileo’s moons clearly did not move about the Earth, but instead orbited a mere planet. In the Fall of 1610, Galileo observed that Venus exhibited phases consistent a motion about the Sun rather than the Earth, which further supported the heliocentric model. Of course, claiming that this was evidence that the Earth and other planets actually orbited the Sun famously led him into a bit of trouble with the Church, but that’s a different story.

Perhaps the most amazing aspect of Galileo’s discovery is just how accurate his observations were. Ernie Wright has made a detailed study of Galileo’s moons as they appear in his Sidereus Nuncius (Starry Messenger), comparing them with the actual positions of the moons at the times of Galileo’s observations. You can see two examples of these in the figure above. The agreement is surprisingly good, particularly when you consider that Galileo made his observations with freehand sketches while observing them through a telescope less powerful than a cheap pair of modern binoculars.

The precision of Galileo’s observations more than justifies calling these four moons the Galilean moons of Jupiter.

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As I write this, it’s 10pm at the end of a long day.  I have an 8am class to teach tomorrow, and I would much rather be reading a book or watching Game of Thrones than writing yet another post on astrophysics.  So why do it?  Because it matters. Because if scientists don’t tell the story of science, someone else will.  With the rise of online media, it is increasingly easy for anyone to present scientific ideas in ways that are entertaining and engaging. This can lead to TV shows like Cosmos, and it can also lead to documentaries such as  The Principle.  If you haven’t heard of it, The Principle claims that we live in a geocentric universe.

By geocentric universe I really mean the idea that the Earth is the center of the universe and doesn’t move.  The idea that Galileo demonstrated was false 400 years ago.  This is not just a YouTube video someone edited in their bedroom.  The film was funded by Robert Sungenis, author of the book Galileo Was Wrong The Church Was Right, where he argues in favor of geocentrism. It features Michio Kaku and Lawrence Krauss, and has a slick trailer narrated by Kate Mulgrew.  Krauss and Mulgrew have issued statements that they disagree with the geocentric claims, but already the trailer has gone viral.

You might argue that such an incredulously ridiculous film should just be ignored.  Don’t feed the trolls, as it were.  Unfortunately it isn’t alone.  There’s the electric universe, young Earth creationism, anti-evolution, anti-vaccines, global warming skepticism, ancient aliens, mermaids are real, and the list goes on.  Presented to you by talented and beautiful people, often enhanced with slick computer graphics.

Central to all of these is the claim is that what you have been told about the universe is wrong.  That scientists don’t really know.  They don’t really understand the universe.  All they have is just a theory.

The phases of Venus. Credit: Chris Proctor.

There are lots of things scientists don’t know, but there is a great deal we do know.  We know, for example.  That the planets do not move around the Earth.  We know from the phases of Mercury and Venus that they orbit the Sun.  We know from a simple experiment you can do at home that the Earth rotates on its axis, and can even measure the rate of rotation with a simple pendulum.  We know that there is a universal law of gravity that holds the Earth and other planets in orbit with the Sun.  We know that the Earth moves around the Sun because we observe the parallax shift of nearby stars.  We know very, very clearly that geocentrism is wrong, and we’ve known this for centuries.

A recent study by the National Science Foundation found that 25% of Americans think the Sun moves around the Earth.  That’s 1 in 4 Americans. It is easy to write off more than 50 million people as just being stupid, but as the documentary A Private Universe demonstrated, even Harvard graduates held the misconception that the seasons are caused by Earth moving closer to and farther from the Sun, rather than being due to the tilt of Earth’s axis. Scientific ignorance can’t be blamed on a lack of intelligence. It is due to misconceptions that haven’t been broken.  Misconceptions that are fed by The Principle and other pseudoscience media.

Every time I see a slick pseudoscience video I’m reminded that scientists need to up their game.  We need to be more active in communicating science. We need to engage with the public and make it clear that we really can understand the universe.  We need to convey the wonder and awe of scientific understanding, and demonstrate how science can bring out the best in humanity.

So at the end of a long evening I’m writing a post about geocentrism and how it is provably wrong.  And about why communicating science clearly and honestly matters.  Because if scientists don’t tell the story of science, someone else will.

The post Engage appeared first on One Universe at a Time.

All of the outer planets (and at least one asteroid) have ring systems, but none are nearly so bright and extensive as those of Saturn.  Saturn will always be known as the ringed planet.  Saturn’s rings were a surprising mystery when first observed by Galileo in 1610, and in many ways they remain a mystery.

Early observations of Saturn by Galileo and others. Credit: Huygens’s Systema Saturnium

When Galileo observed Saturn through his telescope, he could clearly see it was not a circular disk like the other planets.  But his telescope wasn’t powerful enough to resolve Saturn as a planet with rings.  He speculated that Saturn perhaps had two large and close moons. As Galileo improved upon his telescopes, he was eventually able to resolve the rings.  Later observations by Huygens and Cassini showed that the ring system had a structure to it, including an inner and outer region separated by what is now known as the Cassini division.

Vertical spires in the rings cast shadows. Credit: NASA/JPL

Modern observations of Saturn’s rings show a complex system of thousands of ringlets within groups of rings.  This structure is mediated by small shepherd moons which, through a complex gravitational dance, keep these rings from dispersing. We see vertical structures rising from the rings, and spokes that move with the rotation of Saturn.

One of the biggest mysteries about Saturn’s rings that remains unsolved is their age.  The rings are largely composed of water ice, and are unusually bright.  This would suggest a rather young age of about 100 million years or so, since dust within Saturn’s moon system would gradually mix with the ice and darken the rings.  In this case the rings likely formed when two moons collided, or when a moon drifted too close to Saturn and was ripped apart by the planet’s gravity.

But recent analysis of the composition of the moons and rings of Saturn show that they have chemical similarities that vary with their distance from Saturn.  This suggests that the rings are much older, and likely formed at the same time as the moons about 4 billion years ago.  This would also explain the complex and subtle interactions that can occur between moons and rings.  The unusual brightness of the rings would still need to be explained, but one idea is that ice particles within the rings are continually clumping and breaking apart, which would help keep the rings looking “fresh”.

If the rings are truly old, then it is likely that Saturn’s ring will remain for much of the lifetime of the solar system, thus we can rest assured that future generations will always have a view of such an iconic planet.

Up next: Uranus

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We all know that many objects (atoms, cats, us) have mass.  What you probably don’t know is that there are multiple different types of mass, and this has real physical (and astrophysical) consequences.

The most familiar type of mass is probably the “quantity” version.  That is, an object like a car has a certain amount of “stuff” (metal, plastic, glass), and that quantity of matter can be measured by mass.  Of course, some things are small and dense, while others are large and light.   So with quantity mass it’s not just the size that matters, but the density.

In our daily lives, we usually determine mass by an object’s weight.  Heavier things have more mass.  The weight of an object depends on how it interacts with a gravitational field, such as the Earth’s gravitational field.  This interaction is due to a type of mass known as passive gravitational mass.  The more passive gravitational mass, the heavier an object will be in the Earth’s gravitational field.

Of course, a passive gravitational mass can interact with the Earth because the Earth has a gravitational field.  This field is produced by the active gravitational mass of Earth.  The active and passive gravitational masses are what allow gravitational interaction (at least in classical physics).

Another type of mass determines how easy or difficult it is to move an object.  This is known as inertial mass.  The inertial mass is the m in Newton’s second law (F = ma), and is why a baseball is easier to throw than a bowling ball.

So all massive objects have a certain quantity of matter, a passive gravitational mass that interacts with gravitational fields, an active gravitational mass that produces a gravitational field, and an inertial mass that determines how the object will move when forces act on it.  In Newtonian physics, all these different types of masses are the same.  When we use the term mass, we generally mean the Newtonian version, which is why we don’t distinguish between them.

But this simple, Newtonian view of mass can lead to some confusion when it comes to things like special relativity.  Special relativity is derived from the fact that the speed of light is a constant.  This means that if I’m travelling at 90% of the speed of light relative to you, and shine a flashlight ahead of me, I will see the light move from me at the speed of light, not 10% of the speed of light.  This strange behavior leads to things like time dilation.  From your vantage point my time appears to move more slowly.  But it also means that my mass appears to get larger.

This is sometimes referred to as relativistic mass, and is a very real effect.  For example, in the Large Hadron Collider protons are accelerated to *nearly* the speed of light.  They are moving so fast that their relativistic mass is much larger than their regular Newtonian mass.  This means we have to push them harder to keep them moving in a circular path. So as the protons are sped up, the magnetic fields used to keep them moving in a circle have to be strengthened.  The closer the protons get to the speed of light, the bigger their relativistic mass, and the harder they are to move.  This is also, by the way, why they can’t be accelerated to the speed of light.

In physics we tend to avoid the term “relativistic mass”, because it isn’t really the same as the other masses.  Relativistic mass is an object’s apparent inertial mass from your vantage point.  So if you see a proton zip past you at a large fraction of the speed of light, the proton would appear to have a large inertial mass.  But if I’m zipping along with the proton, I would say its mass is a normal proton mass.  Relativistic mass is dependent on who’s doing the observing, while the other masses are an inherent property of the object.

In Newtonian physics, the equivalence of the inertial and gravitational masses is why everything falls at the same rate.  Even though a baseball and a cannonball have different masses, they fall at the same speed (barring air resistance).  Bigger masses are harder to move, but they also feel a stronger gravitational force.  This has been known since the time of Galileo, but it was used by Einstein as the foundation of general relativity.

In order to formulate general relativity in terms of general covariance, Einstein later strengthened this argument to yield what is known as the strong equivalence principle:  The ratio between the inertial mass of a particle and its gravitational mass is a universal constant.

Einstein saw a parallel between the relative nature of motion in his theory of special relativity and the relative nature of gravity, and so he worked to generalize relativity to include both gravity and motion. This theory of general relativity is what he published in 1915. It was a radical proposal. In his theory Einstein argued that gravity was not a force in the way Newton had thought. Instead, gravity was an effect of a curvature of space and time.

In general relativity, the active gravitational mass of an object curves space around it.  But this leads to another type of mass, what you might call “curvature mass”.  As an example, consider the mass of a black hole.  A black hole is so dense that any matter it once had is now trapped forever behind its event horizon.  We can’t observe that matter, but we can determine the mass of a black hole by measuring the curvature of space around it.  So a black hole has mass even though it isn’t made of “stuff”.

In our daily lives we can treat mass as a single property of an object.  But in astrophysics that simple view can lead to massive problems.

Cartoon of (likely mythical) Galileo dropping objects from the tower of Pisa. Credit: San Diego State University

The post Massive Issues appeared first on One Universe at a Time.

A famous story in the history of science is that of the trial of Galileo Galilei.  Galileo believed that the Earth moved around the Sun, but this conflicted with the theological position of the Catholic Church, which held that the Earth was fixed in the center of the universe.  This conflict came to a head when Galileo was put on trial and was forced to renounce his assertion that the Earth moved around the Sun.  As the story goes, after making his public renouncement Galileo muttered under his breath “Eppur si muove!” which in Italian means “And yet it moves!”

There’s no contemporary evidence that Galileo actually said those words, but it makes for a good story.  It also exemplifies the frustration Galileo felt toward Church officials.  Galileo had good reason to believe the Earth moved around the Sun.  He had observed the phases of Venus, which showed that Venus moved around the Sun, and he had discovered four moons around Jupiter.  Both of these observations agreed with the heliocentric model of Copernicus, which held that the Sun was the center of the universe.

Heliocentrism was a huge theological problem for the Church.  It seemed unthinkable that God’s divine creation — humanity — would be placed upon a minor planet, rather than at the fixed center of the physical universe.  Besides, the Bible clearly states (in Chronicles 16 and Psalm 93 for example) that the Earth doesn’t move.

The central dispute between Galileo and the Church was whether Galileo could assert that the Earth really did move around the Sun (that is, as a scientific fact), or whether he should present the idea as merely a hypothesis.  Church officials admitted that Galileo’s observations gave the appearance of moving around the Sun, but argued that appearances could be deceiving.  Galileo, they argued, hadn’t completely proven his hypothesis.  Galileo, on the other hand, thought it was ridiculous to take poetic passages from the Bible literally.

This raises an interesting question: is there an experiment Galileo could have done to prove that the Earth actually moves?  It’s likely that nothing would have convinced the Church at that time, but there is an experiment Galileo could have done to demonstrate the motion of the Earth.  All he would have needed is a large pendulum.  The experiment was devised by Leon Foucault about 200 years after Galileo’s trial.

A simple pendulum consists of a mass hung from a wire or string.  Once released it will swing back and forth at a regular rate.  With friction and air resistance, the swing of the pendulum will die down over time, but this happens slowly for a large and heavy pendulum.  If the Earth were motionless, then a pendulum would swing back and forth in a perfectly straight line.  It’s orientation would never change.

But the Earth rotates, which means everything on the Earth moves around in a circle once a day.  If you are on the equator, you would travel the entire circumference of the Earth in 24 hours.  If you are near the north pole, you would travel only a small circle in 24 hours.  This means that while everything on Earth moves in a circle once a day, things closer to the equator move faster than things closer to the Earth’s poles.  Your speed depends upon your latitude.

As a pendulum swings, it will be slightly closer to the equator at one part of its swing, and slightly farther away at another part.  As a result, the motion of the Earth causes the orientation of the pendulum to shift slightly with each swing, an effect known as precession.  The effect is very small, but it builds up.  After several hours the orientation of the pendulum can be significantly changed.  Watching the precession of a pendulum you can see the direct effect of the Earth’s motion.  Galileo was right after all.

Foucault first demonstrated his pendulum in his cellar.  His experiment gained such popularity that he was soon asked to demonstrate his pendulum in the Pantheon in Paris, where there is a Foucault pendulum to this day.

If you happen to be in the Rochester area, there is a Foucault pendulum a little closer to home.  Simply take a trip to the science building at SUNY Geneseo. Included here are some pictures of their pendulum.

Their pendulum is featured in the video I did for WXXI a couple years ago about Foucault’s experiment.  (Try to look past my cringe-worthy performance.)

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