Transcript Introduction to the Sky (PPTX)
Introduction to the Sky
Sirius Orion Capella Aldebaran Pleiades Comet Hale-Bopp
Mars Betelgeuse Orion 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
Egyptian Pharoh Akhenaton and family communing with Sun God (ca. 1350 BC) Sky worship/astrology
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 23 h 56 m 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, 1 o per day Returns to same position after 365.25 days, or one year.
MOON
Second brightest object in sky (but much fainter than sun) Faster eastward motion against stars, 13 o 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 Lunar Phases
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 180 o 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 • Comets } Transient • 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