Astronomy is the oldest science! Eclipses. In ancient times the sky was not well understood! Bad Omens? Comets

Transcription

1 Astronomy is the oldest science! In ancient times the sky was not well understood! Eclipses Bad Omens? Comets 1

2 The Ancient Greeks The Scientific Method Our ideas must always be consistent with our observations! In science observations are made with our five senses Laid the foundations of modern science Observations Hypotheses Ideas Predictions Experiments Hypotheses must be modified or even thrown out if they are found not to consistent with observations! At the turn of the 20 th century the astronomer Percival Lowell claimed to see canals on Mars! Reproducibility Valid experiments are those that are repeatable by other scientists working independently Observations could not be reproduced by other scientists! 2

3 Hypotheses that have been verified experimentally and shown to be reliable are then collected together to form theories and laws They are also subject to further testing Laws Summaries of observed behavior without explanation as to why the behavior occurs Laws predict WHAT will happen! Example: Kepler s Laws of Planetary Motion Theories (Models) Explanations of observed behavior Theories explain WHY something happens (and will also allow a prediction of what will happen!) Example: Einstein s Theories of Relativity Non-science Anything that cannot be measured with the 5 senses Example: religious belief Science without religion is lame, religion without science is blind Albert Einstein Astronomy is unique among the sciences! 3

4 Geology Chemistry Botany Common link? Astronomy Direct Measurement! The Universe is huge! 4

5 How can we know anything about the Universe when we can t make direct measurements? By collecting and analyzing the information carried across the Universe by light! light = electromagnetic (EM) radiation Carries information Not just visible Very fast travels at 300,000 km/s Numbers in Astronomy Angles Astronomy is a Physical Science it uses numbers to express things! 1 circle = 360 degrees (º), 1º = 60 arc minutes ( ), 1 = 60 arc seconds ( ) 5

6 Angular Separation Angles are subdivided just like time! Average time to run a marathon: 4 hours 32 minutes 8 seconds Separation between two stars in the sky: 4 degrees 32 arc minutes 8 arc seconds (4º 32 8 ) Angular and Linear Sizes In astronomy we have to deal with both very large and very small numbers. Distance to nearest star meters Size of a hydrogen atom meters Sun and Moon have same angular size (½º) but different linear sizes! Scientific Powers of 10 Notation 10 y 10 is called the base y is called the exponent it tells us how many times to multiply 10 by itself 10 1 = = 10 x 10 = = 10 x 10 x 10 = 1, = 10 x 10 x 10 x 10 = 10,000 etc. 6

7 At the end of a large number is a decimal point, even though it is not normally written: Converting Large Numbers into Scientific Notation Move decimal point y places to the left: Large # # between 1-10 x 10 y x Negative Exponents 10 -y 10-1 = 1 / 10 = = 1 / 10 x 10 = = 1 / 10 x 10 x 10 = = 1 / 10 x 10 x 10 x 10 = etc. Converting Small Numbers into Scientific Notation Move decimal point y places to the right: Small # # between 1-10 x 10 -y 7

8 x Write the following numbers in scientific notation: a b. 480 c d e f Converting from Scientific Notation to Standard Notation Move decimal point in opposite direction! Fill spaces with zeros! x

9 Write the following as standard numbers: a. 1.2 x 10 4 b x 10-2 c. 4 x 10 6 d. 5 x 10-3 Astronomical Distances normal units not meaningful! The Solar System The Stars much further! 1 Astronomical Unit (AU) = average Sun-Earth distance = 150 million km = 1.5 x 10 8 km Sun-Mercury = 5.79 x 10 7 km = 0.39 AU Sun-Neptune = 4.5 x 10 9 km = 30 AU Closest star (excluding Sun) > 270,000 AU away! Proxima Centauri the nearest star The Light Year 1 light year (ly) = distance light travels in one year at 3 x 10 8 m/s = 9.46 x km = 63,240 AU The light year is NOT a unit of time! Distance = 3.87 x km = 4.22 ly 9

10 Light Travel Time Light year = how many years light takes to travel from the object Example Light takes 4.22 years to travel from the nearest star! Significant delays! Result We see the nearest star not as it is now but how it was 4.22 years ago! Astronomy should also be considered a branch of history! The Celestial Sphere Observing the Sky 10

11 The Constellations star patterns Some well known constellations Which constellation is this? Ursa Major the great bear Orion the Hunter Which constellation is this? 11

12 What about this one? Leo the Lion Canis Major the Hunting Dog Cygnus the Swan Sagittarius the archer Scorpius the scorpion 12

13 State Boundaries Constellations Today 88 constellations cover the sky, each with its own boundary 12 Constellations are special! Astronomy is NOT Astrology! Constellations of the Zodiac The sky appears to be moving around the Earth! Really it is the Earth that is moving! Motions of the Sky 13

14 The Earth s Rotation and Revolution Diurnal Motion due to the Earth s rotation Star Trail Photography As the Earth revolves around the Sun, the constellations that can be seen a night shift! Northern Summer Constellations Northern Winter Constellations 14

15 Celestial Reference Points Celestial Coordinates GP = Geographical Poles GE = Geographical Equator Terrestrial Coordinates Declination and Right Ascension Los Angeles: lat 34ºN, long 118ºW Polaris is close to the North Celestial Pole Celestial Navigation 15

16 Stars appear to move in circles about the celestial poles! Polaris appears stationary Circumpolar Stars: stars which a permanently above the horizon from a particular location Finding Polaris in the sky Polaris from Los Angeles Polaris and navigation Polaris from London Special Places North Pole (lat 90º N): Polaris is directly overhead (the zenith) Equator (lat 0º): Polaris is on our horizon South Pole (lat 90º S): Polaris is directly beneath our feet (the nadir) Polaris is permanently below the horizon in the Southern Hemisphere 16

17 At what latitude was this picture taken? Precession The location of the North Celestial Pole slowly shifts over a 26,000 year period! The location of the South Celestial Pole also shifts over the same time period! Polaris is currently close to the NCP but will slowly shift away with time! There is currently no bright star close to the SCP but there will be in the future! The Earth s Axis Tilt This is caused by the Earth s axis of rotation precessing like a top 17

18 Precession is due to the pull of the Sun and the Moon s gravity on the bulge of the Earth! The Seasons Seasonal Variations Seasonal Variations 1. Changing temperatures The seasons are due to the axis tilt of the Earth, NOT its changing distance from the Sun! Seasonal Variations 1. Changing temperatures 2. Changing day lengths Seasons in N and S hemispheres are inverted! If the Earth had no axis tilt there would be NO seasons! 18

19 The Sun s rays are more concentrated in the Summer and less concentrated in the Winter! The Sun rises and sets in different directions during the year leading to varying day lengths! Mar 21, Sep 21: Sun rises due E, sets due W day = night Vernal Equinox (Mar 21) Autumnal Equinox (Sep 21) N Summer: Sun rises NE, sets NW day > night longest day Jun 21 (Summer Solstice) N Winter: Sun rises SE, sets SW night > day shortest day Dec 21 (Winter Solstice) The Phases of the Moon Properties of the Moon 1. Revolves around the Earth 2. Orbital period ~ 4 weeks = one month 3. Seen because it reflects light from the Sun 4. ALWAYS one hemisphere illuminated and one hemisphere in darkness The Synodic Month Every 29.5 days the Moon goes through a cycle of phases 19

20 As the Moon moves around the Earth the Sun- Earth-Moon Angle changes As the moon moves around the Earth, this angle changes causing us to see differing amounts of the illuminated hemisphere! Relationship between Sun-Earth-Moon Angle and Phase S-E-M Angle Illumination Phase Seen 0º 0% New 0-90º 0-50% Waxing Crescent 90º 50% 1 st Quarter º % Waxing Gibbous 180º 100% Full º % Waning Gibbous 270º 50% 3 rd Quarter º 0-50% Waning Crescent How do you tell whether the Moon is waxing or waning? When the Moon trails the Sun in the sky (is East of it) we have a waxing phase When the Moon is ahead of the Sun in the sky (is West of it) we have a waning phase When the moon is close to the horizon it appears orange and larger than normal The Moon appears red close to the horizon for the same reason that the Sun appears red close to the horizon 20

21 Atmospheric Scattering The Moon is not really larger close to the horizon! It is an optical illusion due to having landmarks close to the horizon to compare its size to! A Blue Moon The cycle of lunar phases is 29.5 days which is slightly shorter than the average calendar month If a new moon occurs right at the beginning of a month it is possible for a second full moon to occur in the same month A second full moon seen in the same month is called a Blue Moon Next Blue Moon: August 31 st 2012 Eclipses The orbits of the Earth and Moon are not in the same plane! The Two Types of Eclipses Lunar the Moon passes into the Earth s shadow at Full Moon (S-E-M angle 180º) Solar the Earth passes through the Moon s shadow at New Moon (S-M-E angle 0º) 21

22 At most new and full moon s the shadow miss each other no eclipse! Eclipses occur only when the Sun, Moon and Earth are aligned along the Line of Nodes There a minimum of 4 and a maximum of 7 eclipses per year The Shape of the Earth s Shadow Lunar Eclipses Moon passes into Earth s shadow at full moon There are a minimum of two and a maximum of five lunar eclipses each year During a total lunar eclipse the Moon becomes completely engulfed in the Earth s umbra The progression of a total lunar eclipse 22

23 The Moon turns orange during totality due to light bent by the Earth s atmosphere into its shadow! Other Types of Lunar Eclipses Maximum duration of total eclipse: 1 hr 42 min During a partial lunar eclipse the Moon is only partially engulfed in the Earth s umbra A partial lunar eclipse During a penumbral lunar eclipse the Moon misses the umbra and only enters the penumbra A penumbral lunar eclipse 23

24 Summary Next lunar eclipse visible from Southern California: December 20 th 2010 (total) The Moon s Shadow on the Earth Solar Eclipses Earth passes through Moon s shadow at new moon There are a minimum of two and a maximum of five solar eclipses each year The Moon s shadow traces out an eclipse path on the Earth as it rotates. The eclipse path is very narrow, having a width of only 250 km An Eclipse Diagram 24

25 A total solar eclipse is seen if you on the eclipse path (i.e. in the umbra of the Moon s shadow) Totality maximum duration only 7½ min! A Partial Solar Eclipse The Elliptical Orbit of the Moon Seen in the penumbra of the Moon s shadow Annular Eclipse: Moon too small to completely block Sun if eclipse occurs at apogee 25

26 The times of eclipses can be easily predicted! Next solar eclipse visible from Southern California: May 20 th 2012 (partial) Next total solar eclipse not until May 3 rd 2106! 26