Department of Physics and Engineering Astronomy 199

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Department of Physics and Engineering Astronomy
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• Night sky has been observed for thousands of
  years, revealing that
• Stars maintain their pattern relative to one
  another through the years
• Stars move from E to W across sky during the
  night, as if projected onto a domed screen
  (apparent diurnal motion)
• Stars near poles appear to move in concentric
  circles centered on a spot above the pole

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• Ancients concluded that stars reside on a huge
  sphere rotating around the Earth (Celestial
  Sphere)
• This sphere was thought to rotate about an axis
  passing through N S celestial poles.
• Stars appeared to form shapes, like clouds often
  appear to us today (constellations)

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• Ancient Sumerians, c. 2000 BC, had defined many
  constellations, including a bull and a lion.
  Many cultures had a bear.
• About half of the 88 constellations used today
  were identified by ancient Greeks, and many are
  still called by their Latin names.

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• The constellation patterns are random,
  accidental. Many times the stars are not even
  close to one another.
• Today, constellations are used to identify parts
  of the sky, and astronomers have listings of what
  stars are in certain constellations.

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• Suppose we are interested in determining how far
  apart two objects are in the sky.
• An Earth observer might find their angular
  separation.
• The angle from the horizon to the point on the
  celestial sphere directly above your head
  (zenith) is 90o.

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• To estimate angles
• Fist of outstretched arm is 10o.
• Width of little finger is 1o.
• Sun and moon are 0.5o.
• Some stars are so close together, their
  separation is measured in smaller units
• 1o 60 min 1 min 60 sec

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• A person with normal vision can detect 2 stars if
  they are separated by about 1 min or more of arc.
• Stars closer together must be measure by
  telescope or special detection equipment.

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• The apparent motion of the sky is due to the
  Earths rotation, or spin, about its axis passing
  through the geographic N-S poles.
• Earth completes 1 rotation from W to E every 24
  hrs. Celestial sphere then seems to move in
  opposite direction.
• Stars appear to follow curved paths (diurnal
  circles), with radius depending on how close they
  are to the celestial poles.

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• One star, Polaris, the North Star, has a very
  small circular path since it is near the N.
  celestial pole.
• Hence, Polaris appears fixed in the sky.
• Some stars are circumpolar.
• The largest diurnal circle is the celestial
  equator, and stars on this path are visible for
  12 hrs, gone for 12 hrs.
• There are some stars near S. celestial pole that
  we in Murray will never see.

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• If we were at N. geographic pole, the N.
  celestial pole is at our zenith, and all stars
  are circumpolar. They neither rise nor set, just
  move in circles.
• If we were at the Earths geographic equator, we
  get to see all the stars rise in the E and set in
  the W.

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• Just as the stars seem to move around the Earth,
  the Sun and Moon are seen to rise in the E and
  set in W.
• If you carefully observe the sky just after
  sunset, you will see that the stars behind the
  sunset are moving.
• As if the Sun is out among the stars, but not
  staying in same place on celestial sphere.
  Rather, the Sun seems to move to E among the
  stars.

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• Eventually Sun will return to its original spot
  among background stars. Takes 365 days.
• Ancient peoples realized that certain star
  patterns appeared in sky at same time every year.
• Suns motion and star patterns were associated
  with length of day/night, cold/warm weather,
  seasons of the year.

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• As the Sun moves among the stars, it traces out
  the same path year after year. The path the Sun
  follows on the celestial sphere is called the
  ecliptic.
• Some special dates
• June 21 Summer solstice
• December 22 Winter solstice
• March 21 Vernal equinox
• September 23 Autumnal equinox

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• The constellations that the Sun passes through as
  it follows the ecliptic are called the
  constellations of the zodiac. There are 12 such
  constellations.
• Zodiac means circle of animals
• The zodiac wasnt intended to be used as an
  astrological device, but ancient peoples
  attributed great powers to the heavens.

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• The Suns behavior is associated with the seasons
  of the year.
• Sun rises and sets farther N in summer than in
  winter. Actually rises truly in E only at
  equinox.
• Sun is in sky longer in summer than winter.
• Sun gets higher in sky in summer than in winter.
• Altitude height measured (in o) above horizon

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• The repeating positions of stars in the sky forms
  the basis for our present calendar.
• -Present calendar originated mainly with Romans
• -Originally only 10 months, and year began in
  March
• -To conform to solar year, 2 more months were
  added and the 12 alternated between 29 and 30
  days
• -This made only 354 days in a year, so every 3
  yrs another month was needed to catch up

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• In 46 BC, Julius Caesar reformed calendar for
  empire so that year had 365 days 1 every 4 yrs
• By 1582 calendar was out of synch again, with
  vernal equinox occurring Mar 11 instead of Mar 25
  as set by Caesar
• Pope Gregory XIII restored equinox to Mar 21, and
  skipped 10 days so Easter would fall at choice
  time

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• New rule An even century not divisible by 400
  will not be a leap year.
• Catholic countries complied with new calendar.
• England waited until 1752.
• Russia converted calendar in 1917.

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• Scientific Models
• Models are devices that represent or help
  visualize a physical system.
• There have been many models of the universe
  throughout history
• Sumerians (4000-3000 BC)
• Babylonians (2000-1000 BC)
• Greeks (600 BC- 200 AD)

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• Early Greek influence from Aristotle
• used observational approach to studying nature
• spoke of natural behavior of objects
• Earthly objects fall down, tend to slow down/stop
• heavenly objects stay suspended, move in circles,
  keep going forever.
• Early Greek model was geocentric, with celestial
  sphere for stars and a smaller sphere for sun.

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• Ptolomy (150 AD), writes Almagest, a
  comprehensive model that lasted for 1300 years.
• Stars stay on celestial sphere, but spheres for
  Sun, Moon, planets abandoned.
• Heavens were perfect, therefore all heavenly
  bodies moved in perfect circles around Earth
• Model worked for Sun and Moon, which always move
  E relative to distant stars.

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• Planets, however, sometimes didnt show this
  simple motion.
• They sometimes reversed direction in a backward
  loop.(retrograde motion).
• To explain this, Ptolomy gave some planets
  epicycles, circular orbits around their circular
  orbits.

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• The Greek model had features that we look for
  today in good scientific models
• Three criteria for a good scientific model
• 1. The model must fit the data available
• 2. The model must make predictions that allow
  it to be tested.
• 3. The model must have a scientific simplicity.
  (Occams razor)

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• When Ptolomys model had to add more and more
  epicycles, the 3rd criterion suffered.
• Ironically, 400 yrs before Ptolemy, Aristarchus
  had proposed a moving-Earth solution to explain
  celestial motion. Said it is simpler to say that
  Earth is rotating.
• Since Ptolomy felt no wind, he rejected this idea.

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• In order to study stars in detail, must know how
  to find them on celestial sphere.
• On Earth.latitude and longitude
• Celestial sphere is also divided by a gridwork,
  and stars position is given relative to fixed
  coordinates.
• Equatorial Coordinate System (ECS)

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• In ECS, position of star is given by declination
  (dec) and right ascension (RA).
• Dec is measured as angle above/below celestial
  equator
• RA is measure by hour circles running through
  celestial poles, starting where ecliptic crosses
  the celestial equator (vernal equinox) and
  measuring eastward. 1 hour 15o

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• So on Mar 21, sun is at dec 0o, RA00hr
• June 21, sun is at dec23.5o, RA 06 hr
• Sept 21, sun is at dec 0o, RA 12 hr
• Dec 22, sun is at dec -23.5o, RA 18 hr
• ECS coordinates will change over thousands of
  years due to Earths precession, which changes
  the alignment of the Earths rotational axis.

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• You may notice that a spinning top wobbles on its
  axis if it tips. Same happens to spinning Earth
  due to gravitational attraction of Sun and Moon.
  This wobble in spin is called precession.
• Earths period of recession is about 26,000 yrs.
• In 12,000 yrs, our North Star will be Vega.

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• A simpler coordinate system is the Horizon
  Coordinate System (HCS)
• Azimuth-angular distance CW from N to where
  object is located
• Elevation-objects angular position above horizon
• sometimes called altitude