Scale of the Solar System Scale of the Universe • • • • • How big is Earth compared to our solar system? How far away are the stars? How big is the Milky Way Galaxy? How big is the Universe? How do our lifetimes compare to the age of the Universe? Sizes and Distances: How Big is Big? • How big is the Earth compared to our solar system? • On a scale of 1-to-10 billion, the Sun is about the size of a grapefruit. The Earth is the size of a ball point about 15 m away. The distances between planets are Huge compared to their sizes. • How far away are the stars? • Alpha Centauri is 4.4 light-years (or approx. 4.4x1013 km) away from us. • On the same scale, the stars are thousands of km away. Scale of 1:1010 Sizes and Distances: How Big is Big? (Cont.) • How big is the observable Universe? • • • • • How big is the Milky Way Galaxy? 14 billion light-years in radius. 100 billion (or 1011) galaxies in the observable Universe. 100 billion stars per galaxy on average. Total of 1022 stars in the observable Universe – as many stars as grains of dry sand on all of the Earth’s beaches. • There are about 1x1011 stars in the Milky Way Galaxy. • It would take more than 3,000(!) years to count the stars in the Milky Way Galaxy at a rate of one per second. • The Milky Way Galaxy is about 100,000 light-years across; it takes light 100,000 years to travel across the galaxy! How do our lifetimes compare to the age of the Universe? Cosmic Calendar: a time-scale on which we compress the history of the Universe into one calendar year. • Big Bang took place on Jan 1 at 00:00:00am. • First galaxies were born in Feb. • Solar System formed around Sep 3; early life in Earth formed around Sep 22. • Modern humans evolved on Dec 31 at 11:58pm. • Our civilization is just a few seconds old! And a human lifetime is a mere blink of an eye. Spaceship Earth • • • • How is Earth moving in our solar system? How is our solar system moving in the Galaxy? How do galaxies move within the Universe? Are we ever sitting still? 1 How is Earth moving in our solar system? • Contrary to our perception, we are not “sitting still.” • We are moving with the Earth in several ways, and at surprisingly fast speeds… Earth orbits the Sun (revolves) once every year: • at an average distance of 1 AU ≈ 150 million km. • with Earth’s axis tilted by 23.5º (pointing to Polaris). • and rotating in the same direction it orbits, counterclockwise as viewed from above the North Pole. The Earth rotates around its axis once every day. Our Sun moves randomly relative to the other stars in the local Solar neighborhood… • typical relative speeds of more than 70,000 km/hr. • but stars are so far away that we cannot easily notice their motion. Are we ever sitting still? … And the Sun orbits the galaxy every 230 million years. • How do galaxies move within the Universe? Galaxies are carried along with the expansion of the Universe. But how did Hubble figure out that the Universe is expanding? Hubble discovered that: • All galaxies outside our Local Group are moving away from us. • The more distant the galaxy, the faster it is racing away. Conclusion: We live in an expanding Universe! 2 How many stars are in the observable Universe? A. B. C. D. One hundred billion 1011 1022 1030 A. B. C. D. 1022 stars are in the observable Universe    There are about 100 billion galaxies in the observable Universe. That’s 1011 galaxies. And there are roughly 100 billion stars in each galaxy (some more, some less, but that’s about the average). That’s 1011 stars per galaxy. 1011 stars 1011 galaxies x --------------1 galaxy = 1.5 x 1013 cm 1.5 x 108 km 1 light year (1 ly) 1 astronomical unit (1 AU) One hundred billion 1011 1022 1030 The Earth is 150 million kilometers from the Sun. What is another way of expressing this distance? A. B. C. D. 1.5 x 1013 cm 1.5 x 108 km 1 light year (1 ly) 1 astronomical unit (1 AU) 1022 stars The Earth is 150 million kilometers from the Sun. What is another way of expressing this distance? A. B. C. D. How many stars are in the observable Universe? 1 AU, 1.5 x 108 km, 1.5 x 1013 cm are all equivalent! • An Astronomical Unit is DEFINED as the distance from the Earth to the Sun • For the others, write out 150 million km… that’s 150,000,000 km. • Which is the same as 1.5 x 100,000,000 km… therefore, 1.5 x 108 km • To convert to centimeters: 103 m 102 cm 1.5 x 108 km x ---------- x ---------1 km 1m = 1.5 x 1013 cm 3 How is Earth moving? A. Rotates on its axis (spin). B. Revolves around the Sun (orbit). C. It doesn’t move. Imagine the Sun has the diameter of a grapefruit (14 cm). How large would Jupiter be? (real diameters: Sun - 1.4 x 106 km, Jupiter - 1.4 x 105 km) A. B. C. D. 1.4 km 1.4 cm 1.4 x 1011 km 1.4 x 1030 km 1.4 cm (size of a marble)!    Notice that the Sun’s diameter is about 10 times greater than Jupiter’s. So, if our model Sun has a diameter of 14 cm, divide that by ten… 1.4 cm How is Earth moving? A. Rotates on it’s axis (spin). B. Revolves around the sun (orbit). C. It doesn’t move. Imagine the Sun has the diameter of a grapefruit (14 cm). How large would Jupiter be? (real diameters: Sun - 1.4 x 106 km, Jupiter - 1.4 x 105 km) A. B. C. D. 1.4 km 1.4 cm 1.4 x 1011 km 1.4 x 1030 km Imagine the Sun has the diameter of a grapefruit (14 cm). How far away would Earth be? (real distance: 1.5 x 108 km Sun diameter: 1.4 x 106 km) A. B. C. D. 1500 cm 1500 m 15 cm 15 km 4 Imagine the Sun has the diameter of a grapefruit (14 cm). How far away would Earth be? (real distance: 1.5 x 108 km Sun diameter: 1.4 x 106 km) A. B. C. D. 1500 cm   Notice that the Earth-Sun distance is about 100 times greater than the Sun’s diameter! So, multiply the “grapefruit” Sun’s diameter by 100 to find a model Earth-Sun distance of about 1400 cm, which is close to the answer choice of 1500 cm. 1500 cm 1500 m 15 cm 15 km History of Astronomy: Babylonians and Greeks (pp. 52-61) Ancient Roots of Science • In what ways do all humans employ scientific thinking? • How did astronomical observations benefit ancient societies? • What did ancient civilizations achieve in astronomy? In what ways do all humans employ scientific thinking? • • • Scientific thinking is based on everyday ideas of observation and trial-and-error experiments… Our goal is to explain and predict the observed phenomena. A scientific model is a conceptual representation that provides both explanation and prediction of a given phenomenon. How did astronomical observations benefit ancient societies? • Keeping track of time and seasons – for practical purposes, including agriculture. – for religious and ceremonial purposes. • AND for aid in navigation 5 What did ancient civilizations achieve in astronomy? • daily timekeeping • • • • • • tracking the seasons and calendar monitoring lunar cycles monitoring planets and stars predicting eclipses navigate and more… South Pacific: Polynesians were very skilled in the art of celestial navigation Babylonia Astronomy of Babylonians • What were the purposes of Astronomy in ancient Babylonia? • What are the scientific breakthroughs of the Babylonians? • How did the western astronomy benefit from these breakthroughs? Purposes of Astronomy in ancient Babylonia • As in most ancient cultures, astronomy was actually practiced as astrology. • Astronomical events, whether they were every-day occurrences or rare incidents, had a deep religious meaning for the people. • It was believed that all things happened for a reason! Lives were lived according to the advice of these astronomers/astrologists! • Orientation of the constellations was used to mark seasons for harvesting or sowing crops. • Certain constellations were noted for their yearly rising or setting times, and provided an accurate clock by which time could be measured. Historically, two ethnic groups, the Sumerians and Akkadians, had dominated the Babylonia. An area rich in natural resources, and strategically located for trade routes and commerce, it was often under threat from outsiders throughout the region's history. The origins of western astronomy can be found in Mesopotamia, the "land between the rivers" Tigris and Euphrates. What are the scientific breakthroughs of the Babylonians? • Babylonians not only recognized Venus as the same object whether it appeared in the morning or evening, but they actually developed a method for calculating the length of the Venus cycle! • Babylonians were able to predict solar and lunar eclipses. They applied a simple method, which made future predictions based on past observations! • They almost certainly knew about the saros cycle; the general pattern of eclipses repeats every 18 years (and 11.3 days). • Babylonians created the zodiac–”the circle of little animals”–which marked the twelve constellations that the Sun, Moon, and planets travel between during their movements through the sky. 6 What are the scientific breakthroughs of the Babylonians? (Cont.) • Constellations that we still use today, such as Leo, Gemini, Capricorn, etc. were invented by the Sumerians between 2,000-3,000 BC. A great deal of astronomical mythology was handed down from the Sumerians. • Babylonian system of mathematics was sexagesimal, or a base 60 numeral system. From this we derive the modern day usage of 60 seconds in a minute, and 60 minutes in an hour. • Babylonians invented the degree system to distinguish positions in the sky (360 degrees, 60’ in 1 degree, etc.). How did western astronomy benefit from these breakthroughs? • The Babylonian underpinnings of western astronomy are extensive! • It was from the Babylonians that the Greeks gained their knowledge of the five visible planets and the constellations of the zodiac, and centuries of recorded astronomical observations. • Greeks also adopted the idea that the motions of the planets could be predicted with accuracy! • Greeks adopted the degree system and also many of the Babylonian constellations, which they renamed in Greek! Ancient Greek Science • Why does modern science trace its roots to the Greeks? • How did the Greeks explain planetary motion? • How did Islamic scientists preserve and extend Greek science? Why does modern science trace its roots to the Greeks? • Greeks were the first Artist’s reconstruction of Library of Alexandria. Established in 300 BC, this library existed for about 700 years! What is a scientific model? people known to make models of nature. • They used logic and geometry to explain patterns in nature without resorting to myth or the supernatural. Greek geocentric model (400 BC) • Scientific model is a conceptual representation whose purpose is to explain and predict observed phenomena. • They sought to understand the architecture of the Universe by constructing models of nature. 7 How did Islamic scientists preserve and extend Greek science? How did Islamic scientists preserve and extend Greek science? (Cont.) • Muslim world preserved and enhanced the knowledge they received from the Greeks (after the Library of Alexandria was destroyed). • While Europe was in its Dark Ages, Islamic scientists preserved and extended Greek science, later helping to ignite the European Renaissance. • Al-Mamun’s House of Wisdom in Baghdad was a great center of learning around 800 AD. Jews, Christians, and Muslims worked in synergy to advance science. More about the the European Renaissance at the next lecture! • With the fall of Constantinople (Istanbul) in 1453 AD, Eastern scholars headed west to Europe, carrying knowledge that helped ignite the European Renaissance. 8