Motions of the Celestial Sphere

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Motions of the Celestial Sphere
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Constellations

• Sky looks like a dome with the star painted on
  the inside.
• 88 constellations such as Ursa Major, Orion, etc.
• Asterisms are not constellations for example the
  Big Dipper, the Teapot, etc.

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The Celestial Sphere

• Imaginary sphere around the Earth that denotes
  the location of the constellations.

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Descriptions of Celestial Sphere

• Zenith-directly overhead
• Meridian imaginary line running north south
  through the zenith
• Horizon where the sky appears to intersect the
  ground.
• Altitude height of a star above horizon
  (degrees).

The sky appear like a dome overhead.
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Descriptions of Celestial Sphere
Stars are not a same distances. So,
constellations are not real places.

• North Celestial Pole (NCP) extension of Earths
  rotation axis onto the sky.
• South Celestial Pole - ditto
• Celestial Equator extension of Earths equator
  onto the sky.
• Ecliptic apparent yearly path of Sun.

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The Sun and the Celestial Sphere

• As the Earth orbits the Sun we seen the Sun in
  different locations against the backdrop of
  stars.
• The Earth reaches the same location in its orbit
  on the same calendar date each year.

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The Sun and the Celestial Sphere

• As the Earth orbits the Sun we seen the Sun in
  different locations against the backdrop of
  stars.
• The path the Sun follows amongst the background
  of stars is nearly identical from year to year
  and is called the Ecliptic.

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The Sun and the Celestial Sphere

• As the Earth orbits the Sun we seen the Sun in
  different locations against the backdrop of
  stars.
• The set of constellations through which the Sun
  passes is called the Zodiac.
• The Sun lies in front of your birth sign
  constellation on your birthday.

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Rising and Setting of the Stars

• The stars, sun, moon and planets all appear to
  rise in the east and set in the west.
• Reflection of Earths rotation from west to east.

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Local Rising and Setting of the Stars

• We live on the side of the Earth.
• NCPs altitude equals your latitude.
• Circumpolar stars never rise or set but circle
  the NCP

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• Atlanta has a latitude of about 33 north degrees.
• The star Polaris is very close to the Celestial
  North Pole.
• What is the altitude of Polaris when viewed from
  Atlanta?
• 33 degrees
• 45 degrees
• 57 degrees
• 90 degrees

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Our view of Celestial Sphere

• We live on the side of the Earth.
• Sky appears tilted at an angle equal to our
  latitude.
• Stars appear to move in arcs across the sky that
  are not perpendicular to horizon.

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Our view of Celestial Sphere

• The sky appears to rotate around NCP.
• Altitude of NCP is always equal to your latitude
  on Earth's surface.

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This picture is a time exposure of the night sky
showing star trails as the stars appear to
rotate around the NCP. What is the name of the
bright star near the center of the rock
arch? Where are the circumpolar stars?
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Horizon coordinate system - coordinates are
measured with respect to horizon- change with
time and depend on observer

• Azimuth
• 0 to 360 degrees around horizon from north
  towards east
• 0 North, 90 East, 180 South,
  270 West
• Altitude
• 0 to 90 degrees up from horizon
• 0 Horizon, 90 Zenith

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Equatorial coordinate system - coordinates
fixed on the celestial sphere- time and observer
independent

• declination (dec)?
• Analogous to latitude, but on the celestial
  sphere it is the angular north-south distance
  between the celestial equator and a location on
  the celestial sphere.
• Measured in degrees
• 0 to 90 north from celestial equator
• 0 to -90 south from celestial equator
• right ascension (RA)?
• Analogous to longitude, but on the celestial
  sphere it is the angular east-west distance
  between the vernal equinox and a location on the
  celestial sphere.
• Measured in units of time hours, minutes,
  seconds
• 0 h 24 h from Vernal Equinox towards east
• Ex. Sirius has RA
• 6 h 45 m OR 645
• Dont confuse RA with time on your watch!

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Equatorial coordinate system
Comparing latitude and longitude to declination
and right ascension
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RA and Dec of the Cardinal Points on the
Ecliptic

• Vernal Equinox
• Sun appears on March 21
• RA 0h Dec 0
• Summer Solstice
• Sun appears on June 21
• RA 6h Dec 23.5
• Autumnal Equinox
• Sun appears on Sept. 21
• RA 12h Dec 0
• Winter Solstice
• Sun appears on Dec. 21
• RA 18h Dec -23.5

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Angular Measure

• 3600 circle
• 1800 visible sky
• 60 arc minutes/ degree
• 1/60th degree
• 60 arc seconds per minute
• 1/60th arc minute

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1/360th degree via Hubble(1 arc second)

• This new Hubble image shows galaxy cluster Abell
  1689. It combines both visible and infrared data
  from Hubbles Advanced Camera for Surveys (ACS)
  with a combined exposure time of over 34 hours