The Nighttime Sky

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The Nighttime Sky
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Visible stars on a clear night

• Unaided human eye 6,000 stars
• At any one time 3,000 stars
• ½ above horizon, ½ below horizon
• Reality light pollution and other factors limit
  visible stars to a few hundred
• Suburban 500
• Inner-city 300

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Constellations
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In ancient times, constellations only referred to
the brightest stars that appeared to form groups,
representing mythological figures.
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Orion, the Hunter
Leo, the Lion
Scorpius, the Scorpion
Cygnus, the Swan
Antlia, the Air Pump
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Today, constellations are well-defined regions on
the sky, irrespective of the presence or absence
of bright stars in those regions.
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Constellation names are derived from the myths
and legends of antiquity.
Belt of Orion
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Looking at the Sky

• Constellations
• 88 official constellations
• Many origins
• Greeks, Southern European cultures, Middle East,
  Asia, etc.
• Traced back as far as 5,000 years ago
• Stars define the celestial sphere

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Constellations

• Constellations are patterns of stars that the eye
  picks out. The stars are usually not near each
  other they just lie in the same direction.

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The stars of a constellation only appear to be
close to one another Usually, this is only a
projection effect. The stars of a constellation
may be located at very different distances from
us.
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Orion
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Betelgeuze
Rigel
Stars are named by a greek letter (a, b, g)
according to their relative brightness within a
given constellation the possessive form of the
name of the constellation Betelgeuze a
Orionis, Rigel b Orionis
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Observing Focus Ursa Major (Big Bear, Big
Dipper)
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Finding stars Polaris, Arcturus, Spica using big
dipper
Take arc to Arcturus
Then spike Spica
Pole Star (Polaris)
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Winter constellations
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Real motion in the solar system

• Earth is spinning (rotating)
• Earth is orbiting (revolving around) the Sun
• The Moon is orbiting Earth
• The other planets are also orbiting the Sun

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Paths of Stars

• Earths counter-clockwise rotation
• Stars rise in the East, set in the West
• But some stars never set!
• Polaris stays nearly stationary in the sky
• Stars near Polaris move slightly, in circular,
  counterclockwise paths
• Some objects are never visible from the north
• Magellanic clouds (SH)
• Star positions change from night to night as the
  Earth revolves about the Sun rise and set 4
  minutes earlier each day.

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Risings and Settings

• The spin of the earth causes the stars to
  appear to rotate about the celestial pole. Some
  stars are therefore circumpolar and never set,
  while others dip below the horizon. Which stars
  are which depend on where you are on earth.
• Polaris happens to be very near the North
  celestial pole.

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Apparent motion Earths rotation

• Imagine standing at the North Pole
• The North Star is straight overhead
• All of the stars move in big circles along the
  horizon
• Stars never rise and never set!

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Rotation of CS
NCP
Star
Celestial equator
Earth
Celestial sphere
SCP
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Apparent Motion
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Circumpolar Stars
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Circumpolar Stars
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Circumpolar Stars - SH
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Apparent motion Earths rotation

• Imagine standing at the equator
• The North Star is on the horizon
• All of the stars rise in the East, pass straight
  overhead, and set in the West

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What you see depends upon where you are
NCP
Star
Horizon for observer at north pole
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Earth
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The Night Sky at the Equator North South
Celestial Poles are on the horizon
North Celestial Pole (North Star)
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Equator
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Most of us are in between these extremes we see
fixed Polaris (N. Celestial Pole star) over the
night, stars move WESTWARD about it
Looking North
E
W
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Mid-latitudes
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Apparent Motion
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Diurnal Motion

• Each day, the earth rotates once (west-to-east)
  on its axis. This causes us to face different
  directions and see different stars. The stars
  daily (diurnal) motion reflects the earths spin.

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The Yearly Motion
In addition to rotating, the Earth also revolves
about the Sun.
As the earth revolves the Sun is projected in
front of different constellations at different
times of year. The path the Sun takes across
heavens is called the ecliptic. The
constellations which the Sun passes through are
zodiac constellations.
Because the Sun is bright, we can only see some
constellations at certain times of year.
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Apparent motion Earths rotation

• The celestial sphere appears to rotate once every
  24 hours
• Polaris (the North Star) is very close to the
  North Celestial Pole
• Everything appears to move in circles around the
  North Star

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Apparent motion Earths orbitSeasonal stars
constellations

• To see the stars at night, you must look away
  from the Sun
• As Earth moves in its orbit, the stars you can
  see at night keep changing
• Constellations and stars are seasonal

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Apparent motion Earths orbitThe apparent
motion of the Sun

• The Sun is always in front of some stars
• As Earth moves in its orbit, the Suns apparent
  position in the sky changes
• The constellations that the Sun passes in front
  of are known as the zodiac