today we're going to talk about how we know what we know and for how long we've known it we've had a pretty good grasp of some things for a long time but have still had a lot of misunderstandings until much more recently two major ideas have centered around what object is at the center of the solar system or the universe prefix geo means earth geology is the study of the earth prefix helios means sun in fact the element helium was named for the sun because we actually identified it on the sun before we found it on this planet centric means center geocentric theories were common in history people thought that earth was the center of the universe other theories were heliocentric meaning that they placed the sun at the center of the solar system with the planets moving in orbits around the sun it should have been fairly obvious that the geocentric theory has some issues first take a look at jupiter can you see its orbit is not a smooth circle but it goes in some odd looping motion we do see planets change direction as they move across the sky but it's not exactly like they can just stop and turn around so there's a definite problem here now here's the heliocentric system sun at the center and all the planets move around it stars even farther beyond notice that uranus and neptune are not pictured here as they were not known to exist at the time they are so distant that you need a telescope to see them and telescopes had not been invented yet here we're going to start talking about some different individuals and groups of people and how they shaped our views over time if you haven't done so already make sure you've downloaded the unit 1 test review to see which groups and individuals to focus on for your test now let's go way back in time over 5 000 years ago ancient people were unaware that earth was a sphere they did know a lot and actually cared a lot about the patterns they saw in the sky the three pyramids at giza in egypt were aligned with the three stars in orion's belt the air shafts within the pyramids pointed toward orion where they thought their god osiris lived and toward the star thuban which was the north pole star at the time all of the other stars seemed to circle around thuben this was a sign of everlasting life for their pharaoh the stones in the stonehenge monument in great britain have a lot of different alignments with positions of the sun and moon at different times of year we don't know why it was built or what it's for but it's clear that they were paying very close attention to the sky here's a picture of the stonehenge monument you can see that it's in disrepair now but that some alignments can still be seen here such as the sun setting over the heelstone slightly more recently other groups were starting to put numbers to their observations they proved that the earth was round by the way it's a myth that people in christopher columbus's time thought the earth was flat well-educated people had known earth was round for quite some time though pockets of flat earthers have popped up throughout history babylonians also developed constellations the 24-hour day and a lunar calendar that modern months are modeled after notice that the word moon and month are very similar the ancient greeks did a lot for our mathematical understanding of celestial objects you've heard of the pythagorean theorem from geometry class a squared plus b squared equals c squared pythagoras's geometry helped early astronomers calculate very accurate distances in space aristotle came after pythagoras and aristotle is really important and you'll need to know about his beliefs though he also knew that earth was a sphere he still believed and taught the incorrect geocentric theory that everything in space orbits around the earth this is a model of aristotle's view earth in the center moon sun and other objects traveling on larger spheres around the earth then finally the largest sphere to which all the stars were attached as if space were just a bunch of russian nesting dolls additionally aristotle believed that earth could not be rotating because we don't feel like we're moving there's no constant whoosh of wind all the time and that parallax is not observed we'll get to parallax a little bit more in just a second but it does exist humans just didn't have the technology to measure it in aristotle's time now back to parallax look at the diagram when earth is in the position labeled january someone looking toward the star labeled a would see some background stars behind it six months later when the observer was in the july position that same star a appears to be in a different position with different background stars behind it though the star hasn't moved the background makes it look as if it has moved because the earth has moved you can do a simple activity right there in your chair to demonstrate hold one finger in front of your face about eight inches away close one eye and then switch go from right eye to left eye and back again though your finger isn't moving it looks like it's moving this is the idea of parallax aristotle could not observe parallax so he did not believe that the earth was revolving around the sun nearby stars actually do exhibit parallax they appear to shift somewhat over the six-month time frame but it's such a small amount aristotle couldn't detect it the shifts are very small only fractions of a degree aristarchus was the first person to propose a heliocentric theory he wasn't taken seriously at the time but he was able to use geometry to prove that the earth was not the center of our universe over two thousand years ago it wasn't until much later that people started to believe it when it was proposed by copernicus additionally geometry was used by eratosthenes to determine the earth's circumference hipparchus we mentioned in the second lecture about star magnitude he used geometry to determine the distance between earth and the moon seems pretty amazing to us but you can actually do these calculations with just a ruler a stick and shadows these early scholars had a much more intuitive understanding of geometry and relationships than most people have today imagine if you had to do those calculations to pass this course without a computer a calculator or any modern equipment these astronomers were pretty amazing i'm not going to ask you about these three guys specifically on your exam but i want you to see how even very early on we had a really good grasp of what was going on in space even though we went kind of backwards later ptolemy is another important guy who will pop up on your test ptolemy was a roman living at the end of the 1st century a.d he continued to believe in a geocentric model and even though he was completely wrong his theories were incredibly good at predicting where the sun moon and planets would be at any given time so people assumed they were correct have you ever gotten the right answer to a math problem and then only later realize that you did it totally wrong but somehow accidentally ended up with the right answer it's kind of what happened with this guy he was totally wrong about everything he assumed was true but as his guesses match the observations people didn't realize how wrong he was but again look at the circles around each of the planets ptolemy believed that as each planet moved along its orbit it also made these elaborate smaller circular patterns he called epicycles the picture on the left shows his epicycles see how the planet continually loops and loops around as it travels along its orbit doesn't make a lot of sense from a momentum point of view but it does explain why planets sometimes appear to travel backwards look at the second picture look at mars as it travels from right to left across the sky then moves back to the right and then again toward the left this is called retrograde motion epicycles really don't exist but they did explain though incorrectly why planets double back and move backwards it wasn't until the 1500s that we really understood why the planets sometimes move in retrograde orbits copernicus understood that the sun was the center of the solar system and that earth's rotation takes a day and revolution takes a year he was unwilling though to publish his book until he was on his deathbed he was concerned that he would be persecuted by the catholic church and rightly so as we'll see later copernicus is also super important this is a drawing from copernicus's text notice soul the sun in the center then mercury venus and the other planets moving around it the final circle represents the stars beyond saturn again uranus and neptune were not known yet at this time he believed that orbits were perfect circles so he wasn't correct about everything but he was a lot closer than anyone else had been in a while this is why the sun-centered theory makes things so much simpler the retrograde motion is easily explained by looking at the diagram on the right at position a earth looks toward mars seeing it in the west as earth moves to positions b and c we still see mars moving west but since mars travels more slowly than earth because it's farther away from the sun once earth gets to position d mars appears to be traveling backward it's similar to how sometimes if you're driving rapidly past a slower car sometimes that other car seems to be moving backwards epicycles were no longer needed to explain retrograde motion all that's happening is earth passes the other planet at a higher rate of speed and it makes the other planet appear to be moving backwards copernicus was pretty awesome he not only explained retrograde motion but he also knew the order in which the planets were positioned from the sun calculated the time that orbits take explained why the stars and sun appear to move throughout the sky and invented the idea of the au take a look at just how close copernicus's numbers are to the modern values given that he lived 500 years ago in a time with no telescopes or technology that's pretty amazing accuracy tico brahe lived around the same time as copernicus though he didn't openly believe in copernicus heliocentric theory he did make a lot of important observations he charted planet and star positions on star maps he explained how and why these objects moved the way they did this was his most important contribution he made such extensive observations of planetary movement and charted things so accurately that later astronomers like kepler were able to use his writings to determine how objects motion work mathematically he also observed a comet and a supernova which is an explosion that comes from a dying star he was such an astute observer that i think he probably did believe in heliocentrism but he was smart enough to not blab about it so the church didn't go after him and he was able to live his life to its natural end there were other astronomers like giordano bruno who were murdered by the church for their beliefs that contradicted church teaching this is why copernicus didn't release his work until he was dying anyway kepler was the best astronomer of his time he combined brahe's observations and copernicus's theories to develop a set of laws we still use today to explain motion in the sky you've probably heard of isaac newton and his laws of motions we'll talk about him in a bit kepler's laws are similar laws that explain planetary motion he found that if you didn't assume that orbits were perfect circles and you didn't assume that they always moved at exactly the same speed all the problems with copernicus's math went away planets move not along circular but elliptical oval-shaped orbits and their velocities change throughout their orbits remember our words perihelion and apelion planets are faster when they're close to the sun and slower when they're farther away these are kepler's three laws the first law says that planets orbit on elliptical paths the sun is not at the center but at one focus we'll see what that looks like on the next slide the second law is the law of equal area and the third law has to do with orbital time let's look at each one individually this is an exaggerated illustration of law one a circle has one center point an ellipse has two foci foci is the plural of focus you don't say focuses the sun is at one focus and the other is empty the distance across the long side evan and ellipse is called its major axis and the distance across the small side it's called the minor axis you can draw a perfect ellipse with just two tacks and a piece of string tie the ends of the string together and place the loop around the tacks then stick your pencil in and pull it around until the string is tight you can then trace all the way around the tacks keeping the string taut you can also calculate the eccentricity of your ellipse meaning how much it deviates from a perfect circle the part labeled a is the semi-major axis or half the major axis c is the distance from the center to the focus remember when talking about planetary orbits the sun is at one focus here are a variety of ellipses as the eccentricity number gets bigger the ellipse becomes more and more stretched out number one has a very low eccentricity while number five is a more eccentric orbit the first few may look like circles to you but they're very barely not you can probably definitely tell that the last one is squashed wider than it is tall more eccentric in our solar system the major planet's orbits are very close to being circular earth's eccentricity is very low quite close to a circular orbit a circular orbit would have an eccentricity of zero pluto's orbit is more eccentric and some of the other dwarf planets are even more eccentric still we don't know if this is standard for other solar systems or not kepler's second law is the law of equal area that means that these two blue segments are of equal area and it takes a planet equal amounts of time to move across that part of the orbit that means a planet traveling from a to b at apellion is moving very slowly because it doesn't travel a very long distance while it is so far from the sun however it must be moving very fast from a prime to b prime at perihelion as it has to move much farther in the same amount of time as it took to go from a to b in the case of the earth this means that northern hemisphere summer is somewhat longer than northern hemisphere winter we're closer to the sun and thus moving faster in the winter but since we're farther away from the sun in summer earth is actually traveling at a slower speed you can see how much longer this arch is in summer than this one during winter now think about the calendar february is a shortened month in the winter both july and august have 31 days making summer about a week longer than winter it's not a lot of difference but remember that earth's orbit is one of low eccentricity it's not a lot of difference but it is measurable the earth moves faster in winter when it's close to the sun at perihelion and slower in summer when it's farther from the sun at a app helion kepler's final law is an equation relating orbital period to distance from the sun earth's orbital period is one year it takes one year to travel around the sun earth's distance from the sun is 1au for earth this math is easy since we defined one au to be the distance from the sun to the earth that means if we know how long it takes the planet to go around the sun and we can see that just by watching objects in the sky we can calculate exactly how far from the sun it is the farther away a planet is from the sun the longer the orbit takes not just because it has farther travel but also because it's actually traveling at a slower speed many people think that galileo invented the telescope he did not hans lippershay actually invented it but galileo was the first person to turn it skyward you'll need to understand some of galileo's observations that he made with his telescope and how they supported copernicus's heliocentric theory galileo's telescope was not very powerful only 14 times magnification and it was very narrow meaning it didn't have a lot of light gathering capability magnifying power actually isn't the most important thing in making a good telescope a larger diameter is actually more important so it can conduct more light making far off dim objects appear brighter we'll look at a couple of equations that go along with that in the next lecture galileo was able to see many more stars with his telescope than could be seen with the naked eye with his telescope he could see mountains craters and relatively smooth areas on the moon he actually did these drawings on the right side picture he also noticed that venus has phases just like the moon note that the crescent venous phases are larger than the gibbous phases that's important galileo tried to use the phases of venus to prove that venus must be orbiting the sun rather than the earth to produce phases similar to the moon notice that the reason the crescent venus looks larger than the gibbous venus is because venus is closer to us when it's in its crescent phase this doesn't happen with the moon so venus cannot be orbiting the earth unfortunately the church was not convinced and it continued to persecute galileo the church didn't formally apologize until 1992 350 years after galileo died but better late than never i guess he also discovered that the sun was not a perfect sphere as it had been previously assumed to be it had sunspots the left picture is a photo through a telescope the picture on the right was a set of sunspots that galileo drew he even discovered four moons that orbit jupiter on the left is a photo through a telescope on the right you see notes galileo took while observing you see a circle representing jupiter each day and the two or three or four visible moons in different positions as they orbit the planet sometimes all four are not visible at once since one or more could be in front of or behind jupiter but as he could determine these moons were orbiting jupiter earth could not be the center of everything unfortunately for galileo claiming that objects orbit jupiter was contrary to the church's teaching the church still supported the old geocentric theory that the earth was the center of everything though copernicus's heliocentric theory had already been published many people still held on to the old view the church ordered galileo to stop teaching the heliocentric theory and not even talk or write about it he was found guilty of heresy by the church and lived under house arrest for the rest of his life though he continued to work on his theories isaac newton was another great physicist and astronomer you probably know him best for his laws of motion he not only invented calculus but he studied light and optics he invented a new type of telescope that uses mirrors instead of lenses that dramatically increased the power of telescopes at the time we'll talk about his telescope more in the next lecture newton is the last of our super important early historical astronomers though we'll talk about several more important astronomers in this course he worked on the math behind kepler's laws developed his own laws of motion and even figured out that other planets must exist beyond saturn because there were unexpected deviations in their orbits his laws of gravity and motion greatly extended our knowledge of how the cosmos works we're going to look at how the first and third laws apply in astronomy we won't deal with law number two in this course newton's first law is that an object in motion stays in motion unless it's acted upon by an unbalanced force this means that a planet should keep moving in a straight line the fact it follows a curved orbit is due to the gravitational attraction of the sun newton's third law is that for every action there is an equal and opposite reaction the force of gravity that earth exerts on the moon to hold it in orbit is the same force that the moon exerts on earth pulling the water during tides it's just in the opposite direction the third law also explains how we can use planet flybys to redirect spacecraft a spacecraft launched in a particular direction would just keep going straight but if it flies near enough to a planet it can actually be slingshot around it changing its direction and increasing its speed the craft exerts the same force on the planet but it isn't noticeable to the planet because it's so much more massive than the spacecraft kind of like punching a brick wall it affects you a lot more than it affects the wall here's an illustration of one of newton's thought experiments showing how orbits work the first cannonball launched with a certain amount of power will fall to the ground the second ball shot with more force will go farther but still crash the third with enough speed can stay in orbit a fourth cannonball that isn't shown if launched at a sufficient velocity could actually escape the earth altogether this is how a rocket is launched into space as i mentioned earlier there are several more important astronomers we'll be talking about but the rest of these won't be on your first test edwin hubble for whom the hubble space telescope is named george lemaitre father of the big bang theory albert einstein and his theories of relativity vera rubin and dark matter annie jump cannon and stellar classification and stephen hawking in his black holes will all be things we get into later in the course