Coordinates, celestial sphere, and patterns in the sky

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Coordinates, celestial sphere, and patterns in
the sky

• Astronomy
• Mr. Buckert

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Appreciate subtleties !

• If you expect to see immediate results and flames
  in the telescope.you will be disappointed !
• The closest star to us (other than the sun) is
  approximately 4 years away if you were traveling
  at the speed of light.
• Anticipation of next question No, we cant
  travel at the speed of light.
• Even looking through a relatively large
  telescope, one needs to appreciate subtle
  features (color, brightness, fuzzy galaxies
  etc. )

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Mars through a telescope and a CCD camera (and a
good one too)
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General Star Info

• How many stars can one see on a clear night ?
• Approximately 3000.
• What is the closest star to Earth?
• The sun
• On what horizon do stars rise? Set?
• Rise from general east- Set in the general west
  directions (Around Polaris)the North Star

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Before we begin, a review of some basic
definitions

• The Universe
• the totality of all space, time, matter and
  energy
• Astronomy
• the study of the universe
• Light Year
• the distance traveled by a beam of light in one
  year (9.5 x 1013 km)
• (6 trillion miles)

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In order to study the universe, we need to locate
things using coordinate systems

• Coordinates--a set of numbers used to locate
  something
• terrestrial--latitude and longitude
• Equator 0 latitude
• Prime meridian 0 longitude

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Angular Measurements

• Degree 1/360 of a circle
• minute of arc 1/60 of a degree
• second of arc 1/60 of a minute (. Or 1/3600 of
  a degree)
• Hold finger out at arms length 2 degrees
• Hold fist out at arms length 10 degrees
• Moon and sun are only 1/2 (stays constant--if
  it looks bigger...its an illusion)

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Horizon system

• Horizon system--locating a star/planet using
  altitude (height) and azimuth (around)
• zenith--point directly above in the sky
• horizon where the land or treeline meets the
  sky
• Altitudefrom 0º on horizon to 90º at zenith
• Azimuthfrom 0º at North and going around 360º

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Horizon system

• Problems...
• 1. Stars change position over the course of the
  night because of rotation of Earthstars rise and
  set like the sun
• 2. Stars occupy different positions in the sky
  at different points on the earth (N vs. S
  hemispheres)

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Patterns in the Sky
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The Seasons
Earths axis of rotation is inclined vs. the
normal to its orbital plane by 23.5, which
causes the seasons.
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The Seasons
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The Seasons are a result of how direct the
sunlight is that strikes Earth.
Steep incidence ? Summer
Light from the sun
Shallow incidence ? Winter
They are not related to Earths distance from the
sun. In fact, Earth is slightly closer to the sun
in (northern-hemisphere) winter than in summer.
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The Seasons
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Northern summer southern winter
Northern winter southern summer
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Seasons are a result of Earths tilt on axis
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How the path of the sun changes over the course
of the year(this is at 40 degrees latitude)
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Ecliptic

• Definition 1the path of the sun in the sky
• Definition2the path of the sun on the celestial
  sphere over the course of the year

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Celestial Sphere

• Imaginary sphere onto which all stars are
  projected (imaginary sphere of what we see)
• North Celestial Pole is above North Pole of earth
• Polaris is North Star because practically on N.
  celestial pole

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Celestial Sphere

• Easier way to locate stars because they are
  fixed on this map that we call the celestial
  sphere

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Celestial Sphere

• Right ascension(like longitude) angular
  measurement around sphere (measured in hours)
• Declination(like latitude) angular measurement
  above or below celestial equator

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Sidereal Day vs. Solar Day

• Sidereal Day--23 hours, 56 minutes, 4 seconds
  (4 minutes shortertime it takes for stars to
  come back to same position)
• Therefore stars rise
• 4 minutes earlier each night
• Mean Solar Day 24 hrs (avg. time for Sun to
  return to same position)

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If you are standing in Atlanta, your view of the
celestial sphere looks like this
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If you are standing at the equator, your view of
the celestial sphere looks like this
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Diurnal Circles from Equator
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If you are standing at the North Pole, your view
of the celestial sphere looks like this
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Diurnal Circles from N. Pole
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Diurnal Circles from mid-latitude

• Circumpolar starsgroup of stars that do not set
  below horizon (includes Polaris, the North Star)
• The angle of Polaris above your horizon is the
  same as your latitude in degrees.

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From mid North latitude like ATL (or anywhere
the U.S.) it looks like this
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The Celestial Sphere
0
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The positions of the stars change over the course
of the year because the Earth orbits the Sun
Off-season constellations are up during the
daytime Notice that between June and August, the
Sun is directly in Gemini Notice that June to
August, Sagittarius is prime viewing during
summer nights
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Precession (1)
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At left, gravity is pulling on a slanted top. gt
Wobbling around the vertical.
The Suns gravity is doing the same to Earth. The
resulting wobbling of Earths axis of rotation
around the vertical w.r.t. the Ecliptic takes
about 26,000 years and is called precession.
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Precession (2)
As a result of precession, the celestial north
pole follows a circular pattern on the sky, once
every 26,000 years.
It will be closest to Polaris A.D. 2100. There
is nothing peculiar about Polaris at all (neither
particularly bright nor nearby etc.) 12,000
years from now, it will be close to Vega in the
constellation Lyra.
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The Motion of the Planets
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The planets are orbiting the sun almost exactly
in the plane of the Ecliptic.
Jupiter
Venus
Mars
Earth
Mercury
Saturn
The Moon is orbiting Earth in almost the same
plane (Ecliptic).
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Motion of the Planets

• The Sun, Moon and planets can always be found in
  a Zodiac Constellation on the ecliptic.
• Planets "wander" across the celestial sphere and
  through the stars in two ways
• 1. Direct Motion - normal eastward
  movement of planets
• 2. Retrograde Motion - occasional westward
  movement of planets

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The Motion of the Planets (3)
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Mercury appears at most 28 from the sun. It can
occasionally be seen shortly after sunset in the
west or before sunrise in the east.
Venus appears at most 46 from the sun. It can
occasionally be seen for at most a few hours
after sunset in the west or before sunrise in the
east.
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Remember the real reason we see retrograde motion.

• The retrograde motion of mars would occur over a
  couple of months.
• The movement is apparentmars doesnt really move
  like that in space. The position of mars shifts
  against the backdrop of distant stars.