Finding Your Way Around the Night Sky

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Finding Your Way Around the Night Sky

• Constellations The Celestial Sphere

Every night there are an abundance of interesting
objects to look at in the sky Whether you are an
amateur astronomer or a professional, you have to
know how to find objects from one night to the
next
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Relative Positions - The Constellations

• Constellations are simply groups of stars in the
  sky that appear to form a pattern
• Ancient cultures recognized these patterns from
  season to season
• Many constellations have various names from
  different cultures
• The Big Dipper is also known as
• The Wagon or Plough (Western Europe)
• The Great Bear (Greek)
• The Leg of an Ox (Egyptian)
• A Stag (Siberian)
• A Funeral Procession (Native American)

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Orion - Visible in Winter
Betelgeuse
Rigel
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Constellations

• Constellations serve many purposes
• They mark the seasons
• Orion in winter, the Summer Triangle in summer
• They serve as relative landmarks for finding
  stars
• Analogy Which is easier?
• Joe's house is in Atlanta
• Joe's house right next to the Big Chicken in
  Marietta
• Same applies to constellations
• Betelgeuse is that red star ... right there
  (points)
• Betelgeuse is the star that makes Orion's right
  shoulder
• They make for good storytelling!

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Constellations

• There are a total of 88 constellations in the sky
  (most of them relatively unknown)
• The twelve signs of the Zodiac are important
  constellations because they lie in the path of
  the Sun through the sky (more on this in a sec)
• The brightest stars in the sky are given names
  based on their constellation
• The brightest star in a constellation is
  (usually) called Alpha _____, the second
  brightest Beta ______, and so on
• For example, Betelgeuse is also known as Alpha
  Orionis, Rigel is also known as Beta Orionis
• Remember Alpha Centauri, the nearest star system?
  It is the brightest star in the constellation
  Centauris

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Constellations and The Celestial Sphere

• Always remember that how we see the stars on the
  sky has nothing to do with how they are grouped
  in space
• We have no depth perception when looking at the
  sky

Stars in a constellation that appear to be right
next to each other on the sky can actually be
100's or even 1,000's of light years away from
each other.
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The Celestial Sphere

• The Celestial Sphere is a simplistic way of
  thinking about the stars in the sky
• Looking up at the stars over the course of a
  night, it appears simply as a bunch of fixed
  points of light, moving uniformly across the sky
• One can think of this as the Earth being in the
  middle of a huge ball or sphere with lots of
  little lights painted on the inside
• As the sphere spins, we see the stars move across
  the sky

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Reality Check

• ALWAYS REMEMBER it is the Earth's rotation that
  makes the stars, Sun, Moon, and planets move from
  east to west in the sky
• ALWAYS REMEMBER each point of light is actually
  a huge star, all of which are at varying
  distances from us
• The Celestial Sphere is just a very handy way of
  thinking about the sky and is very useful for
  finding a specific object in the sky

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Earth Coordinates Review

• Lines of longitude are measured east-west around
  the globe relative to the prime meridian
  (Greenwich, England)
• 0-180o east or west of the prime meridian
• Lines of latitude are measured north-south on the
  globe relative to the equator
• 0-90o north or south of the equator
• For example
• Atlanta is at
• 84o 23' 16.8" W
• 33o 44' 56.4" S

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Celestial Coordinates

• Right ascension (RA) measures east-west on the
  celestial sphere
• Measured in hours, minutes, seconds since one
  rotation of the celestial sphere is equal to one
  sidereal day
• Remember, the four minute difference between
  sidereal and solar day will cause stars to not
  appear at the same exact time and place every
  night
• Declination (DEC) measures north-south of the
  celestial equator
• Measured in degrees, minutes, seconds like
  latitude
• For example, Betelgeuse is at
• RA 05h 55m 10.3s
• DEC 07o 24' 25.4"

RA 00h 00m 00s to 23h 59m 59s DEC 90o (north)
to -90o (south)
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The Ecliptic

• Because of the tilt of the Earth's rotation axis,
  the path of the Sun is tilted relative to the
  celestial equator
• The path that the Sun takes through the sky is
  called the ecliptic
• All of the planets (except for Pluto) also follow
  the ecliptic through the sky

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Solstices Equinoxes

• There are four special positions along the
  ecliptic
• The summer solstice is when the sun is highest in
  the sky for the northern hemisphere
• The winter solstice happens when the sun is
  lowest in the sky for the northern hemisphere
• The vernal (spring) and autumnal (fall) equinox
  occur when the sun is crossing the celestial
  equator

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What's Your Sign? - The Zodiac

• The ecliptic (and therefore the Sun) passes
  through the twelve signs of the Zodiac
• Your birth sign roughly corresponds to the when
  the Sun is located in your constellation
• Notice you cannot see your sign when it is your
  birth month, since the Sun outshines everything
  else
• This is about all astrology and astronomy have in
  common, no matter what my mom thinks

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Observing the Heavens

• At any given time, half of the celestial sphere
  is visible to you
• The point directly overhead in the sky is called
  the zenith
• The edge of the visible sky (where the sky meets
  the ground) is called the horizon
• An object's altitude is the vertical angle
  between the horizon and the object
• The zenith distance is the angle between the
  zenith and the object
• Note 90o - altitude zenith distance

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Observing the Heavens

• All objects in the sky (the Sun, the Moon, the
  planets, the star, everything) RISE IN THE EAST
  and SET IN THE WEST
• Not all objects stay in the sky as long as
  others, due to the tilt of the Earth's axis and
  your position on the surface
• Circumpolar stars are stars near the north
  celestial pole that neither rise or set

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On FridayBring a calculatorand a pencil!