Astronomy 1021 Chapter Notes - Chapter 2: Circumpolar Star, Celestial Equator, March Equinox

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16 Aug 2016
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Chapter 2: Discovering the Universe for Yourself
WATCH CRASHCOURSE ASTRONOMY
LOCAL SKY
Sky as seen from wherever you are standing
HORIZON
Boundary between earth and sky
ZENITH
Point directly overhead
MERIDIAN
Imaginary half-circle from north horizon to south horizon
Locate an object by specifying its altitude and direction (azimuth)
ANGULAR SIZE
The angle an object appears to span in your field of view (ex: sun is 1/2 degrees)
ANGULAR DISTANCE
Angle between two objects in the sky
1 DEGREE = 60 ARCMINUTES (60')
1 ARCMINUTE = 60 ARCSECONDS (60")
SOLAR DAY
Average time it takes the sun to make one circuit through the sky
SIDEREAL DAY
Time it takes any star to make one full circuit through the sky
CONSTELLATION
Region in the sky with well-defined borders
88 constellations cover the entire sky
CELESTIAL SPHERE
Used to map out the sky
An illusion, as we lack depth perception when we look into space
Ancient Greeks mistook this illusion as a great celestial sphere that surrounds earth
North celestial pole: point directly over North Pole
South celestial pole: point directly over South Pole
Celestial equator: projection of Earth's equator into space, complete circle in celestial sphere
Ecliptic: path of the sun, crosses celestial sphere at 23 1/2 degrees because of Earth's tilt
CIRCUMPOLAR
Stars near North celestial pole remain above the horizon, circling around the pole each day
Circumpolar star never sets
Stars near south celestial pole never rise above horizon
Naming stars
Names of easily visible stars are mostly historical
Stars named by their relative brightness, brightest = alpha, second brightest = beta, wtc
Greek letter star names include possessive form of constellation name
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STAR BRIGHTNESS
Brightness measured using refined version of magnitude system introduced by Greek astronomer
Hipparchus, 130 BC
He ranked visible stars from first to sixth magnitude, first = brightest (human eyes can only see up to 6
magnitude)
140 AD, Egyptian-Greek astronomer Ptolemy definitely used magnitude system in his own catalogue
Expressed in a system known as apparent VISUAL MAGNITUDE (mV), describes how stars look from
human eyes on earth
The magnitude system was made quantitative after it became possible to measure accurate relative
brightness of stars with telescopes
The system is logarithmic because biological systems like the eye respond in proportion to the logarithm
of the stimulus
FLUX - This is a measure of the light energy from a star that hits one square metre in one second.
We now know magnitudes with high precision (Theta Leonis: magnitude 3 vs. magnitude 3.34)
LIMITATIONS
Limitation 1: some stars are so bright that the scale must extend into negative numbers (Sirius: -1.47)
Limitation 2: with a telescope, it is possible to find stars much fainter than the limit for unaided eyes
Limitation 3: although some stars emit large amounts of infrared or ultraviolet light, those types of
radiation are invisible to human eyes (mVvisual magnitude)
Limitation 4: an apparent magnitude tells us only how bright the star is as seen from Earth
APPARENT BRIGHTNESS (MAGNITUDE)
1 magnitude difference in brightness corresponds to a factor of 2.51 in flux
For instance, a star with m = 5 produces 2.51 times less flux than a star with m = 4.
2.5 magnitudes correspond to a factor of 10 in flux
5 magnitudes correspond to a factor of 100 in flux
Smaller magnitudes correspond to brighter stars
Larger magnitudes correspond to dimmer stars
Brightest star: Sirius (Canis Majoris)
Luminosity
True brightness of a star
Changes in our sky are due to
The earth's orbit around the sun
The earth's rotation
Angle between horizon and celestial pole is equal to your latitude
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