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Knowing the Heavens


the study of the positions of objects in the sky and how these positions change ... Most stars in a constellation are nowhere near one another ... – PowerPoint PPT presentation

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Title: Knowing the Heavens

Knowing the Heavens
  • Chapter Two

Naked-eye astronomy had an important placein
ancient civilizations
  • Positional astronomy
  • the study of the positions of objects in the sky
    and how these positions change
  • Naked-eye astronomy
  • the sort that requires no equipment but human
  • Extends far back in time
  • British Isles Stonehenge
  • Native American Medicine Wheel
  • Aztec, Mayan and Incan temples
  • Egyptian pyramids

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Eighty-eight constellations cover the entire sky
  • Ancient peoples looked at the stars and imagined
    groupings made pictures in the sky
  • We still refer to many of these groupings
  • Astronomers call them constellations (from the
    Latin for group of stars)

Modern Constellations
  • On modern star charts, the entire sky is divided
    into 88 regions. Each is a constellation
  • Most stars in a constellation are nowhere near
    one another
  • They only appear to be close together because
    they are in nearly the same direction as seen
    from Earth

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The appearance of the sky changes during the
course of the night and from one night to the next
  • Stars appear to rise in the east, slowly rotate
    about the earth and set in the west.
  • This diurnal or daily motion of the stars is
    actually caused by the 24-hour rotation of the

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Animation of constellation movement
  • To represent what we have just discussed, follow
    this animation from the vantage point of our
    Californian observer.

Annual Motion
  • The stars also appear to slowly shift in position
    throughout the year
  • This is due to the orbit of the earth around the
  • If you follow a particular star on successive
    evenings, you will find that it rises
    approximately 4 minutes earlier each night, or 2
    hours earlier each month

It is convenient to imagine that the stars are
located on a celestial sphere
  • The celestial sphere is an imaginary object that
    has no basis in physical reality
  • However it is still a model that remains a useful
    tool of positional astronomy
  • Landmarks on the celestial sphere are projections
    of those on the Earth

  • Celestial equator divides the sky into northern
    and southern hemispheres
  • Celestial poles are where the Earths axis of
    rotation would intersect the celestial sphere
  • Polaris is less than 1 away from the north
    celestial pole, which is why it is called the
    North Star or the Pole Star.
  • Point in the sky directly overhead an observer
    anywhere on Earth is called that observers

The Celestial Coordinate System
  • Again, let us see what we have just determined in
    a more 3-dimension manner.

Positional astronomy plays an important role in
keeping track of time
  • Apparent solar time is based on the apparent
    motion of the Sun across the celestial sphere,
    which varies over the course of the year
  • Mean solar time is based on the motion of an
    imaginary mean sun along the celestial equator,
    which produces a uniform mean solar day of 24
  • Ordinary watches and clocks measure mean solar
  • Sidereal time is based on the apparent motion of
    the celestial sphere

  • Local noon is defined to be when the Sun crosses
    the upper meridian, which is the half of the
    meridian above the horizon

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Sidereal and Solar Days
  • Appreciating the difference between a solar day
    and a sidereal day is a challenging concept. See
    if this helps.

Circumpolar stars
  • At any time, an observer can see only half of the
    celestial sphere
  • The other half is below the horizon, hidden by
    the body of the Earth

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Last thought on coordinates
  • The Equatorial System of Coordinates is what most
    astronomers use when specifying the location of
    an object on the Celestial Sphere
  • Right Ascension (measured eastwards from the
    Vernal Equinox) goes from 0h to 24h
  • Declination (measured north or south from the
    celestial equator goes from -90 to 90 .
  • The hour angle (HA) of an object is the angle
    between the meridian on which the object is
    situated and the (observers) celestial meridian
  • ST RA HA

The seasons are caused by the tilt of Earths
axis of rotation
  • The Earths axis of rotation is not perpendicular
    to the plane of the Earths orbit
  • It is tilted about 23½ away from the
    perpendicular is called the obliquity.
  • The Earth maintains this tilt as it orbits the
    Sun, with the Earths north pole pointing toward
    the north celestial pole

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The Earths orbit
  • Seasons do NOT arise from the distance the Earth
    is from the Sun but rather as a result of the
    Earths annual motion and axial inclination the
    tip of our planet with respect to its orbital
    plane. As we move around the Sun, the
    orientation of our planet gives us seasons.

  • During part of the year the northern hemisphere
    of the Earth is tilted toward the Sun
  • As the Earth spins on its axis, a point in the
    northern hemisphere spends more than 12 hours in
    the sunlight
  • The days there are long and the nights are short,
    and it is summer in the northern hemisphere and
    winter in the southern hemisphere
  • The summer is hot not only because of the
    extended daylight hours but also because the Sun
    is high in the northern hemispheres sky
  • As a result, sunlight strikes the ground at a
    nearly perpendicular angle that heats the ground
  • This situation reverses six months later

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  • The Sun appears to trace out a circular path
    called the ecliptic on the celestial sphere
    tilted at 23 ½ degrees to the equator
  • The ecliptic and the celestial equator intersect
    at only two points
  • Each point is called an equinox
  • The point on the ecliptic farthest north of the
    celestial equator that marks the location of the
    Sun at the beginning of summer in the northern
    hemisphere is called the summer solstice
  • At the beginning of the northern hemispheres
    winter the Sun is farthest south of the celestial
    equator at a point called the winter solstice

Sept 21
June 21
Dec 21
March 31
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Landmarks on the Earths surface are marked by
the Suns position in the sky throughout the year
The Moon helps to cause precession, a slow,
conical motion of Earths axis of rotation
Precession causes the gradual change of the star
that marks the North Celestial Pole
Astronomical observations led to the development
of the modern calendar
  • The day is based on the Earths rotation
  • The year is based on the Earths orbit
  • The month is based on the lunar cycle
  • None of these are exactly the same as nature so
    astronomers use the average or mean day and leap
    years to keep the calendar and time consistent

The different types of year.
  • The sidereal year (year with respect to the
    stars) measured in solar time is 365d 6h 9m 10s
    (365.2564d) in length.
  • The tropical year (successive passages of the Sun
    through the Vernal Equinox) is 365d 5h 48m 46s
    (365.2422d) in length.
  • Due to precession, the tropical year is 20m 24s
    shorter than the sidereal year.

  • Caesar introduced the 365.25 days calendar and
    thus the Leap Year (February 29)
  • However, this is 11m 14s longer than the real
    tropical year. This accumulates to 3 days in 4
    centuries error.
  • To correct, October 4 was followed by October 15,
    in 1562 and the century rule was invoked.

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