AS101 Study Guide - Midterm Guide: 40Th Parallel North, Celestial Equator, Angular Distance

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Published on 20 Apr 2013
School
WLU
Department
Astronomy
Course
AS101
Midterm Review
What to focus on (for the final)
Chapter 1
- Familiarity with units (au, ly)
- Scientific notation
- Rough idea of sizes of things
Chapter 2
- Basic understanding of the cycles of the sky:
o Daily rising/setting
o Moon phases
o Seasons
o Eclipses
- How do daily cycles depend on your location on Earth?
- Constellations and Asterisms
Chapter 3
- Geocentric (Ptolemaic) vs. Heliocentric (Copernican) models
- Kepler: ellipses
- Some evidence that led to adoption of heliocentric model
o E.g. Galileo’s observations
- Newton’s idea of universal laws that apply both on Earth and off
1 AU = average distance between Earth and Sun (150 million km/ 1.5 x 108 km)
1 light year = distance light travels in one year (9.46x1012 km) nearly 1013 AU
1 pc (parsec) = 3.26 ly = 3.09x1013km
1Mpc (mega parsec) = 1 million parsecs
Distance to nearest star (besides sun) = 4.37 ly / 1.34 pc
Nearest star system is Alpha Centauri, 44 trillion km away
Nearest STAR is Proxima Centauri, 4.2ly away from us (textbook)
Distance to nearest star is 4.37ly (ppt)
RELATIVE SIZES
Sun > Earth by about 110 times in diameter
Moon’s diameter ¼ of Earth’s & Mercury a bit larger than the moon
Distance from Venus to Sun is about 0.7AU
Venus around Earth’s size
Mars, about 1.5 AU from the sun
Sun a bit larger than the average star
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Earth orbits the sun, inside a solar system in a galaxy, in a cluster in the universe
Our galaxy (Milky Way) includes ~100 billion stars
o Great cloud of stars, gas and dust bound by combined gravity of the matter
o Milky Way part of group of 40 galaxies called “Local Group”
Belongs to “Local/Virgo Supercluster”
o Size = approx. 80 000 ly
There are about as many galaxies in the universe as there are stars in the milky way
o Filament =
o Void =
PLANETS
Small nonluminous bodies, shine by reflecting light
Earth, Venus, Mercury are really close to the sun
Slightly elliptical orbits
Light from sun takes around 8 minutes
SUN
Rises in the east, sets in the west
STARS
Constellations
88 official ones
48 ancient constellations still in use (from ancient Mesopotamia, Babylon, Egypt, Greece
o Back then, just loose groupings of stars, faint ones not included
o Named based on gods, heroes, shapes; myths were associated
Constellation now represents a section of the sky, any star within the region belongs only to that
constellation
Like countries/provinces
Asterisms
Less formally defined grouping
Big Dipper, Ursa Major etc.
Like a landmark within a place
*the only thing that the stars have in common are that they are in approx. the same direction viewed
from earth
Naming Stars
Names derive from ancient Greek, Arabic or Latin
Originally descriptive or related to myths
Alternate naming:
o Greek letter + Latin possessive form of constellation’s name
o Letters in order from brightest -> faintest, but really faint ones don’t get a Greek letter
o E.g. alpha Orionis
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o *BETA ORIONIS IS SLIGHTLY BRIGHTER THAN ALPHA ORIONIS
Aka Rigel
ANGULAR DISTANCE
Angular distance/size measured in degrees
Full circle = 360O
1O = 60’ (arcminutes)
o Width of a finger at arm’s length
1’ = 60” (arcseconds)
   
  
Angular size & physical size proportional (double the size, double the angle)
Distance is inversely proportional
Object’s angular size = smaller if farther away
Approximate angular sizes for comparison
Size of moon/sun = 0.5O
Smallest feature most human eyes can see: 1 arcminute (1/60 of a degree)
Jupiter (from earth) about 0.8 arcminute
If sun 1ly away, angular size about 0.03 arcsecond
MAPPING THE SKY
Zenith: point directly overhead, 90 degrees
Horizon: all points 90 degrees away from the zenith
Meridian: a line passing through zenith and connecting N and S points on horizon
Sun crosses meridian around noon
Describing a point in the sky:
o Altitude: angular distance from horizon (negative = below horizon)
o Azimuth: direction, expressed as an angle (usually from south)
Celestial Sphere
Imaginary sphere surrounding earth
Mapping
Contains stars, planets, sun, etc.
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Document Summary

Basic understanding of the cycles of the sky: daily rising/setting, moon phases, seasons, eclipses. Some evidence that led to adoption of heliocentric model: e. g. Newton"s idea of universal laws that apply both on earth and off. 1 au = average distance between earth and sun (150 million km/ 1. 5 x 108 km) 1 light year = distance light travels in one year (9. 46x1012 km) nearly 1013 au. 1 pc (parsec) = 3. 26 ly = 3. 09x1013km. 1mpc (mega parsec) = 1 million parsecs. Distance to nearest star (besides sun) = 4. 37 ly / 1. 34 pc. Nearest star system is alpha centauri, 44 trillion km away. Nearest star is proxima centauri, 4. 2ly away from us (textbook) Distance to nearest star is 4. 37ly (ppt) Sun > earth by about 110 times in diameter. Moon"s diameter of earth"s & mercury a bit larger than the moon. Distance from venus to sun is about 0. 7au. Mars, about 1. 5 au from the sun.

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