Knowing the Heavens

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Knowing the Heavens

• Chapter Two

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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
  vision
• 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)

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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
  earth.

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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
  sun
• 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

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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.

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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

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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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• Who volunteers to be the class rep?

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• 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 observers zenith.

dec
RA
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Two more types of angle and time

• Hour angle (HA) of an object is the angle between
  the meridian on which the object is situated and
  the (observers) celestial meridian.
• Local Sidereal Time (LST) is the Right Ascension
  of an observers celestial meridian.
• LST RA HA

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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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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
  efficiently
• This situation reverses six months later

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What seasons are NOT

• Seansons are NOT! caused by the changing distance
  of the Earth from the sun during the course of a
  year!

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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 21
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Landmarks on the Earths surface are marked by
the Suns position in the sky throughout the year
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The Moon helps to cause precession, a slow,
conical motion of Earths axis of rotation
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Precession causes the gradual change of the star
that marks the North Celestial Pole
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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
  hours
• Ordinary watches and clocks measure mean solar
  time
• Sidereal time is based on the apparent motion of
  the celestial sphere

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• 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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You watch Sirius set at about 2 AM in the middle
of February. About what time will it set in the
middle of March?

• A) At 10 pm
• B) At midnight
• C) At 2 am
• D) At 4 am

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Astronomical observations led to the development
of the modern calendar

• Day is based on Earths rotation
• Month is based on the lunar cycle
• Year is based on Earths orbit

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Different types of day

• Apparent solar day time between two upper
  meridian transits of the sun.
• Mean solar day time between two upper
  meridian transits of the mean sun. (361deg
  rotation)
• Sidereal day time between two upper
  meridian transits of the vernal equinox.
  (360 deg rotation)

1 mean solar day 24 h 1 sidereal day 23
h 56 m 4.091s
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Different types of year

• Calendar year integer number of mean solar days,
  (365 or 366)
• Sidereal year time for the sun to return to
  the same position with respect to the stars
  (time of one 360 deg orbit of the earth around
  the sun).
• Tropical year time for the sun to return to
  the
  vernal equinox.

1 sidereal year 365.2564 mean solar days 1
tropical year 365.2422 mean solar days
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Calendars

• Caesar introduced the 365.25 days calendar and
  thus the Leap Year (an extra day, February 29,
  every year divisible by 4) .
• However, this is 11m 14s longer than the 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
  (Gregorian calendar).

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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
• (Local) Sidereal Time is the Right Ascension that
  is located on an observers local celestial
  meridian.
• LST RA HA

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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.

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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.

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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.

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The different types of year.

• Sidereal year (year with respect to the stars)
  365d 6h 9m 10s 365.2564d
  mean solar days
• 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.

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The Celestial Coordinate System

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