The Scientific Method, or How Big is the Sun?

Transcription

1 The Scientific Method, or How Big is the Sun?

2 Learning Objectives! What is empiricism? What is objectivity?! What are the main principles of the scientific method? What is Occam s Razor?! Why do the principles of the scientific method make it more objective than other world views?! What were Eratosthenes and Aristarchus experiments?! How are cause and correlation different?! Do you know the basics from LAB 1: angles, powers of 10, prefixes, units. Do you know temperatures in Kelvin? What is a light year? Does it measure time or distance?! Note the [xtra] beside lectures 1-7 on the syllabus schedule. These may be helpful to learn hard concepts

3 Ancient Greeks! Introduced philosophical (non-supernatural) explanations for natural phenomena! Developed and applied a system of empirical observation and deduction! Aristotle (c BCE)! Knowledge comes from existence; facts are the only reality (empiricism)! Perhaps the birth of the scientific method Aristotle

4 Eratosthenes ( BCE)! A librarian from Alexandria! Learned that on June 20 in Syene, the noon Sun would reach to the bottom of a well! In Alexandria, an obelisk cast a shadow at local noon on the same day

5 Measuring the Size of the Earth Alexandria ~ 5,000 stadia Syene! Measured the circumference of the Earth to be 250,000 stadia (~25,000 miles)! Modern value? 24, miles (polar)

6 Aristarchus ( BCE)! First to argue for heliocentrism (Sun is at the center of our Solar System)! Correct but ignored for about 1800 years! Observed that when the Moon was half-full the angle between the Moon and the Sun was 3 o! Probably intended 3 o as a lower limit (it s a very hard measurement)

7 The Distance to the Sun A a (angle) B! Aristarchus suggested a > 87 o from observations (we now better measure the value to be o )! For a > 87 o it follows that A/B > 19! Thus, the Sun is at least 19 times farther away than the Moon (we now measure that it s more like 400 times)

8 Aristarchus ( BCE)! During a solar eclipse, the Sun and the Moon look the same size. Aristarchus concluded that the Sun was 19 times bigger than the Moon! During a lunar eclipse, the Earth s shadow is 3 times the Moon s size. Aristarchus deduced that the Sun s diameter was thus ~6.7 times the Earth s diameter

9 The Scientific Method Modify Theorize Observe Make a Test Predict

10 Hallmarks of Scientific! Experimental Reasoning! Scientific theories are based on observations (this is Aristotle s empiricism)! Falsifiable! Scientific theories make predictions that can be disproved by empirical observations by more than one person (this is objectivity)! Simple! Search for the simplest theory that explains the observations (this is called Occam s razor)

11 Astronomy is not Astrology because Cause is not Correlation!Do you agree with any of these statements?! I read my horoscope (astrology)! It s strange how sometimes when I think about somebody they text or call me! I was destined to be at the University of Wyoming! psychics sometimes have a feeling that lets them know things that other people don t

12 Cause is not Correlation

13 Astronomy as a Science! Astronomy is different than most other sciences! Unlike physics, chemistry, biology, and other sciences, astronomers often can t create our own experiments! Nature usually runs the experiments, we just get to observe the results from very far away! And many of the experiments take millions of years

14 Astronomical Numbers! In astronomy, numbers can get very large! Distance to the Sun: 93,000,000 miles! Number of stars in our Galaxy: 200,000,000,000! Mass of the Sun: 2,000,000,000,000,000,000,000,000,000,000 kg! Numbers can be very small, too:! Size of an atom: m

15 Examples of Prefixes Number Name Number Prefix Abbreviation Billion 1,000,000,000 giga G Million 1,000,000 mega M Thousand 1,000 kilo k One 1 One-thousandth milli m One-millionth micro µ One-billionth nano n

16 Examples of Scientific Notation Number Name Number Scientific Notation Billion 1,000,000, Million 1,000, Thousand 1, One One-thousandth One-millionth One-billionth

17 New Units for Astronomy! Often, even prefixes and notation feel cumbersome! Average Distance from Earth to the Sun = 1.5x10 8 km (or 93 million miles)! So, astronomers define a new unit!! Average distance from Earth to the Sun = 1 Astronomical Unit (AU) Useful for measuring distances in the Solar System

18 Distances Outside the Solar System! Light years (ly)!the distance light travels in one year (9.5 trillion km) Leaves Jul, 2017 Returns Jan, ly Arrives Oct, 2021! Light years measure distance not time!! Nearest Star: Proxima Centauri (4.2 ly from Earth)

19 Other Units in Astronomy! Astronomers create units for other measurements too! Usually, they use the Earth or the Sun as a reference point! Masses of planets: Earth-masses (M )! Masses of stars: solar-masses (M )! Sizes of stars: solar-radii (R )! Brightnesses of stars: solar-luminosities (L )

20 Measuring the Sky: Angles! Astronomers measure the sky using angles! To measure both the distances between objects in the sky and the sizes of objects in the sky! A circle, 360 o! The Sun and the Moon are each roughly half a degree (0.5 o ) across in the sky

21 Arcseconds! An arcsecond is an angle, but much smaller than a degree! 1 degree = 60 arcminutes! 1 arcminute = 60 arcseconds! So, 1 arcsecond = 1/3600th of a degree! Just how small is that?! The size of a dime at 2.3 miles away 2.3 miles

22 Astronomical Temperatures! Astronomers measure temperature in Kelvin (K)!Same degree of temperature size as for Celsius (1 C = 1K)!At 0K, C, F, stuff in the Universe (matter) reaches absolute zero, and can get no colder

23 Next Time A Sky Full of Stars - I.