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Introduction to the Sky Capella Sirius Aldebaran Orion Pleiades Comet Hale-Bopp Orion Mars Betelgeuse Rigel “Observation” is not “Looking” “Observation” is not “Looking” "You see, Watson, but you do not observe." --- Sherlock Holmes “Observation” is not “Looking” Instead: • • • • Study carefully, comprehensively Remember/record Try to make connections Identify systematic features Motivations for Observing the Sky • Curiosity • Practical applications – Navigation – Time-keeping – Calendar-keeping • Fear, religious belief – E.g. Astrology Polynesian Navigation Sky worship/astrology Egyptian Pharoh Akhenaton and family communing with Sun God (ca. 1350 BC) Astronomical Measurements Without Telescopes • Angles (quantitative) – Sky to Sky – Earth to Sky • Brightnesses (crude) • Colors, Shapes (crude) • Changes in above with time Isosceles triangle: Units of Angular Measure Naked Eye Instruments for Angular Measures 150 BC 1580 AD Naked Eye Instruments for Angular Measures Limiting accuracy ~ resolution of human eye ~ 1 minute of arc 150 BC 1580 AD "Hand-y" Angle Measuring (crude but useful for orientation) 10 degrees 5 degrees Ursa Major (The Big Dipper) The Magnitude System (a brightness ranking) Example: Range of magnitudes in Big Dipper Star Colors (prism-dispersed image) Puzzlah #9 If you go out at 9 PM on a clear night, turn to the south, and look up at the sky, you will see a certain group of bright stars. How will the location of those stars in the sky change if you come back at midnight or several weeks later? (A) The locations of stars in the sky are always the same (they never change). (B) The locations change during the night but are always the same at a given time of night. (C) The locations change during the night and also change at a given time of night from month to month. Puzzlah #9 If you go out at 9 PM on a clear night, turn to the south, and look up at the sky, you will see a certain group of bright stars. How will the location of those stars in the sky change if you come back at midnight or several weeks later? (A) The locations of stars in the sky are always the same (they never change). (B) The locations change during the night but are always the same at a given time of night. (C) The locations change during the night and also change at a given time of night from month to month. Puzzlah #10 During the day, the Sun moves from east to west across the sky. In which direction do the stars move after the Sun has set? (A) The stars are stationary; they don't move (B) West (C) East (D) North (E) South Puzzlah #10 During the day, the Sun moves from east to west across the sky. In which direction do the stars move after the Sun has set? (A) The stars are stationary; they don't move (B) West (C) East (D) North (E) South Main Actors in the Sky STARS Form background "reference frame" About 2000-5000 visible to naked eye over whole sky Patterns of bright stars seem "fixed", unchanging Move "in lockstep" from East to West and return to same position in sky after 23h56m Called the “diurnal motion” Positions of stars in night sky at given time change systematically during the year SUN Brightest object (by far!) Scattered sunlight masks stars during day Steady brightness Slow, eastward motion against stars, 1o per day Returns to same position after 365.25 days, or one year. Lunar Phases MOON Second brightest object in sky (but much fainter than sun) Faster eastward motion against stars, 13o per day Dramatic change in (illuminated) shape or "phase" during cycle Cycle takes 29.5 days to return to same phase. 12 cycles per year Our "month" is based on lunar cycles Daily motions of Moon are easy to track Planets 5 brighter starlike objects that move with respect to the stars. Mercury, Venus, Mars, Jupiter, Saturn Merc, Venus always near Sun. Others, up to 180o away. Motion is slower than Sun, Moon. Generally eastward, but loops to west "Retrograde Loop" in Mars Motion Eastward Other Phenomena Visible to Naked Eye • • • • • • } Meteors Transient Comets Star clusters Diffuse nebulae Milky Way External galaxies Constellations • Constellations are the patterns formed by brighter stars on the sky • Patterns seem fixed (i.e. don't change over years) • Recognized for millenia, by all cultures • Associated with mythological figures, animals, instruments, etc Stick-Patterns Official Names "Classical" figures added Greek amphora, ca. 400 BC, showing Leo, Aquila, Hercules, etc Orion, Taurus, Lepus in a classical celestial atlas. Hevelius, Firmamentum (1690) Cellarius, Harmonia Macrocosmica, 1661 More modern constellations Bode, 1801 "Asterism" "Asterism" Winter Hexagon Modern Constellations • "Official Constellations": 88 (est. 1930, IAU) • Boundaries of each well-defined • "Zodiac" = the 12 (13) constellations lying along the annual path of Sun through stars. (Names widely recognized but NOT all bright.) Significance of the Constellations? Significance of the Constellations? Not Much Significance of the Constellations? Not Much • Associations are arbitrary, man-made, culture-specific. • Not natural groupings: stars are not necessarily close in 3D space. Shapes are specific to Earth's present location in galaxy. • Fainter stars don't participate in pattern • Constellations are transient because stars are all moving with respect to each other. • Used as convenient "address" for roughly locating objects in sky. Modern star chart. Fainter stars don't participate in the pattern. Orion Projected View 3-D Distribution Motions in Big Dipper Over 100,000 Years Constellation Quiz • Two one-hour sessions offered M-R, 9 pm and 10 pm • Must reserve a place using on-line registration system • No special preparation needed • Bring a flashlight (red preferred), pencil, and clipboard (if you have one) • Cloudy or rainy weather will cancel the session • Check weather status after 6:30 pm at 924-7238 -- END --