1445 Introductory Astronomy I

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1445 Introductory Astronomy I

• Chapter 1
• The Night Sky Motions of Sun, Earth and Moon
• R. S. Rubins
  Fall, 2008

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Introduction

• In the geocentric universe, the Sun and the Moon
  were the most important celestial objects.
• Behind the Moon were the fixed stars, which
  appeared to move together around the Earth in a
  regular motion.
• Among the stars were found the planets,
  following irregular paths, but never straying far
  from the Suns path.
• Now we know that the Moon is tiny, but only about
  240,000 miles away. Its importance lies in its
  proximity to the Earth.
• Over 1000 Earths could fit into Jupiter, the
  largest solar planet, while over 1000 Jupiters
  could fit into the Sun.
• The outermost major planet, Neptune, is
  about 3 billion miles from the Sun, but this
  distance is insignificant compared to the 25
  trillion miles to the nearest star, Proxima
  Centauri.

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How Many Stars?

• I will multiply thy seed as the stars of the
  heaven, and as the sand which is upon the
  sea-shore
  
• Genesis 1217
• A total of about 6000 stars can be seen by the
  unaided human eye, although only half at any one
  time.
• However, about one half of the stars we see as
  single are in fact binary pairs i.e. double
  stars, which are very close together.
• There are estimated to be about 200 billion (2 x
  1011) stars in our galaxy, the Milky Way.
• Since there are at least 50 billion galaxies in
  the visible Universe, there must be a total of
  more than 10 billion trillion (1022) stars.

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Practical Use of Astronomy

• The time to plant seeds was predicted from
• i. the positions of the constellations
• ii. the height of the noontime Sun.
• Planning sea travel often depended on the tides,
  which are influenced by the positions of the Moon
  and the Sun.
• The positions of the Sun in the day and the
  constellations at night were used for navigation
  at sea.
• In particular, the North Star, Polaris, was very
  important in navigation (in the northern
  hemisphere), because it closely marks the
  direction of due north, and its altitude in the
  sky gives the lattitude from which it is observed.

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Constellations

• In popular usage, the term constellation is used
  to denote a recognizable grouping of stars.
• Astronomers have redefined the constellations as
  88 regions of the night sky, while referring to
  the groupings as asterisms .
• The constellation Orion
• popular
  astronomical

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The Big Dipper as a Guide

• The two pointer stars furthest from the handle
  of the dipper point to Polaris (the North Star).
• The next two stars point in to Regulus, the
  brightest star in the constellation Leo.
• The curve through the handle passes close to
  Arcturus in Bootes, and ends at the Spica in
  Virgo.
• The pattern may appear upside-down because it
  rotates about Polaris.

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The Winter Triangle

• The Winter Triangle connects three bright stars
  Betelgeuse (in Orion), Procyon (in Canis Minor)
  and Sirius (in Canis Major).
• This triangle is almost equilateral, but slightly
  stretched in the direction of Sirius.

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The Summer Triangle

• The Summer Triangle connects three bright stars
  Vega (in Lyra), Deneb (in Cygnus) and Altair (in
  Aquila).
• This triangle is stretched in the direction of
  Altair.

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Celestial Sphere 1

• The celestial sphere is an imaginary hollow
  sphere, with the Earth at its center, to which
  all the stars seen in the night sky appear to be
  fixed .
• The motion of the stars in the night sky may
  be visualized as a rotation of the celestial
  sphere from east to west about a north-south
  axis.
• The rotation is from east to west because the
  stars rise in the east and set in the west.
• The fixed stars are actually at widely varying
  distances, all more than 4 light years (25
  trillion miles) away, moving relative to each
  other with motions that are not apparent to us.
• As a result, changes in appearance of the
  constellations are not apparent in a human
  life-span.

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Celestial Sphere 2

• Know the following
• North Celestial Pole
• South Celestial Pole
• Celestial Equator
• Declination (latitude)
• is measured from
• the Celestial Equator.
• Right Ascension (longitude)
• is measured from the Vernal
• Equinox (defined later).

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The Apparent Motion of the Night Sky
Equator USA
North Pole

• Because the Earth rotates from west to east, the
  stars appear to move from east to west.
• Thus, if one faces the west, the stars move as
  follows
• vertically downwards at the equator
• from left to right at the north pole (looking in
  any direction)
• from right to left at the south pole (looking in
  any direction)
• downwards and to the right the USA.

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Australian View of the South Celestial Pole
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Celestial Sphere 3

• Geocentric view
• The celestial sphere rotates from east to west,
  since the fixed stars all appear to move from
  east to west.
• This is the ancient view, in which the Earth
  was considered to be at rest at the center of the
  Universe.
• In the geocentric view, the ecliptic is defined
  as the annual path of the Sun around the
  celestial sphere.
• Heliocentric view
• The Earth revolves about the Sun and rotates (or
  spins) from west to east about a line joining
  the poles, so that we see the sunrise in the
  east, and the sunset in the west.
• In the heliocentric view, the ecliptic is defined
  as the plane of the Earths orbit around the Sun.
  

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The Ecliptic 1

• In the geocentric view, the plane of the ecliptic
  makes an angle of 23½o, with the celestial
  equator.

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The Ecliptic 2

• In the heliocentric view, the rotation (N-S) axis
  of the Earth is shown as tilted by 23½o from the
  plane of the ecliptic.

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Equinoxes

• Equinoxes (Latin equal nights) are those times
  of the year in which day and night are of equal
  length. These are the two points at which the
  ecliptic crosses the celestial equator.
• The vernal equinox occurs on about March 21, when
  the Sun crosses the celestial equator heading
  north.
• The autumnal equinox occurs on about September
  22, when the Sun crosses the celestial equator
  heading south.

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Solstices and the Seasons

• The summer solstice occurs on about June 21, when
  the Sun reaches the point on the ecliptic
  furthest north from the celestial equator.
• In summer, the Sun rises in the NE and sets in
  the NW.
• The winter solstice occurs on about December 21,
  when the Sun reaches the point on the ecliptic
  furthest south from the celestial equator.
• In winter, the Sun rises in the SE and sets in
  the SW.
• If the Earths rotation axis were not tilted,
  seasons (as we know them) would not exist, and
  every night would last 12 hours.

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Equinoxes, Solstices and the Seasons 3
Summer
Winter
The Suns daily path
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The Seasons and the Earths Axis

• The seasons result from both the 23½o tilt of the
  Earths rotation axis and its orbit about the Sun.

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Effect of Changing Distance of the Sun

• While the Suns distance from the Earth varies
  slightly throughout the year, becoming closest on
  about January 3, it has no noticeable effect on
  the climate.
• The effect of closer distance in the northern
  winter is reduced by the fact that the southern
  hemisphere has a higher percentage of oceans,
  which reflect heat and light back into space more
  efficiently than do forested land masses.
• If the Earths orbit were much more elliptical,
  then the effect would be more pronounced. If,
  in addition, the Earths axis were not tilted,
  then seasons would be produced only by the
  varying distance of the Sun.
• However, the seasons so produced, would occur
  at the same time for both hemispheres.

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Constellations Visible in the Spring

• At the vernal equinox, the Sun is in the
  constellation Pisces,
• so that Virgo is high in the night sky.

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Constellations Visible in the Fall

• At the autumnal equinox, the Sun is in the
  constellation Virgo, so that Pisces is high in
  the night sky.

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The Earths Precessional Motion 1

• The precessional motion of the Earths axis is
  its very slow conical motion caused by the
  combined gravitational pulls of the Sun and the
  Moon.
• The motion is analogous to that of a spinning
  top.
• Calculations have shown that without the presence
  of the Moon, the 23½o tilt of the Earths
  rotation axis would not be maintained, with wild
  swings in the tilt angle being the rule.

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The Earths Precessional Motion 2

• During the precessional period of about 26,000
  years, the Earths north-south axis traces out a
  circle in the sky.
• Presently, the celestial North Pole points to
  within a degree of Polaris, but in the year
  14,000, it will point roughly towards Vega.

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The Zodiac 1

• On its apparent eastward journey around the
  ecliptic, the Sun appears to pass through the
  twelve Constellations of the Zodiac.
• In 1930, astronomers added a thirteenth
  constellation Ophiuchus which the Sun passes
  through between December 1 and December 19 each
  year.
• Over 2000 years ago when the pseudoscience of
  astrology was introduced by the famous
  mathematician Euclid, a persons astrological
  sign was determined by where the Sun was in the
  Zodiac on his/her birthday.
• Because of the Earths precessional motion, our
  birthdays are now one sign later than they were
  2000 years ago.

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The Zodiac 2
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Traveling on Spaceship Earth

• The rotation or spin of the Earth is from W to E
  about a N-S axis, with a period of 24 hours.
• The rotational speed varies from 1650 km/hr (1030
  mi/hr) at the equator to zero at the poles.
• The Earth orbits the Sun with a 1 year period,
  and a speed of above 100,000 km/hr (60,000
  mi/hr).
• The precession of the Earths axis, with a period
  of about 26,000 years, causes the locations of
  well-known stars to change extremely slowly.
• Our solar system orbits the center of our galaxy
  with a 230 million year period, and a speed
  slightly of about 800,000 km/h (500,000 mi/hr).

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Siderial and Synodic Periods

• A siderial period is a period measured with
  respect to the distant stars.
• A synodic period is the period measured from a
  planet (or moon).
• The solar day is the synodic day measured from
  Earth, which is longer than the siderial day by
  about 4 min.
• The lunar month is the synodic month measured
  from Earth, which is longer than the siderial
  month by approximately 2.2 days.
• The tropical year is the synodic year, measured
  between successive vernal equinoxes, which is
  shorter than the siderial year by about 20
  minutes.

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Solar and Sidereal Days 2A

• The solar day is the average time (24 hours)
  between successive noon-times, as measured at 0o
  longitude in Greenwich, England (the prime
  meridian).
• The sidereal day is the time (23 hours 56 min.)
  taken for a planet to make one complete
  revolution with respect to the stars.

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Lunar and Sidereal Months

• The synodic or lunar month is the time
  (approximately 29½ days) between identical phases
  of the moon e.g. from full moon to full moon.
• The sidereal month is the time (approximately
  27.3 days) it takes the Moon to make one full
  orbit (360o) around the Earth.

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The Year and the Calendar

• Ancient astronomers realized that the year was
  roughly 365¼ days long.
• In 47 BCE, Julius Caesar added an extra day every
  4 years, thus creating leap years of 366 days.
• Pope Gregory XIII reformed the Julian calendar in
  1582, leaving out 10 days to get the seasons back
  on schedule, and decreeing that only those
  century years divisible by 400 were to be leap
  years.
• The average Gregorian year differs only by one
  day in 3300 years from the tropical year.
• With the modification that the years 4000, 8000,
  12,000 and 16,000 are not to be leap years, the
  Gregorian system will not have to be revised for
  20,000 years.

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Lunar Calendars

• Lunar calendars follow the Moons cycle, which
  averages 29½ days per month.
• Since the year would contain only 12 x 29.5 354
  days, an additional month was added usually every
  3 years.
• The Jewish calendar (now in the year 5767) is
  lunar, and is synchronized with the solar
  calendar by following the 19 year cycle,
  introduced by the Greek astronomer Meton in 432
  BCE.
• Easter has a partially lunar basis, being
  scheduled as the first Sunday following the first
  full moon on or after March 21.
• The Islamic calendar is purely lunar, so that 12
  months contain about 11 days fewer than a solar
  year.
• That is why the month-long fast of Ramadan begins
  about 11 days earlier each subsequent year.

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Phases of the Moon 1
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Phases of the Moon 2

• Note that the new moon is closer to the Sun
  than the full moon.

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Sky at Sunset

• The position of the Sun on 14 successive
  evenings, starting at the new moon and finishing
  at the full moon.
• Note that west is to your right, as if you were
  facing south, so that the Sun sets to your right.

1st quarter Moon rises at noon, sets at midnight.
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Sky at Sunrise

• The position of the Sun on 14 successive
  mornings, starting at the full moon and finishing
  at the new moon.

3rd quarter Moon rises at midnight, sets at noon.
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Moonrise, Moonset and Visibility
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Solar and Lunar Eclipses 1

• The plane in which the Moon orbits the Earth
  makes an angle of 5.2o with plane of the
  ecliptic.
• For an eclipse to occur, the Moon must be full or
  new at the same time as its path crosses the
  ecliptic.

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Solar and Lunar Eclipses 2

• The line of nodes is the hypothetical line
  joining the two points at which the Moons orbit
  crosses the ecliptic.
• Eclipses occur when the line of nodes points
  towards the Sun.

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Solar and Lunar Eclipses 3
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The Eclipse Seasons

• Eclipses are relatively rare, because for
  eclipses to occur, the Moon must be full or new,
  just as it crosses the ecliptic plane.
• There are just two short periods in a year, known
  as the eclipse seasons, when eclipses can occur,
  although there is no guarantee of eclipses
  occurring during a particular season.
• Between 2 and 5 solar eclipses can occur in a
  year, and a similar number of lunar eclipses.
  However, the total number of eclipses in a year
  cannot exceed 7.
• It was known to ancient astronomers that the
  basic pattern of eclipses repeats every 18 years
  11.3 days. This repetition pattern is known as
  the Saros cycle.

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Solar Eclipses 1 4A

• A solar eclipse occurs when the Moon blocks some
  or all of the Suns light, so that the Moons
  shadow falls on the Earth.
• The umbra, the central region of the Moons
  shadow, is surrounded by the penumbra .
• Only in the umbra is the sunlight totally
  blocked.
• A total solar eclipse occurs when the Moon is
  relatively close to the Earth, so that it appears
  large enough to totally blot out the Sun, thus
  allowing the faint solar corona to be seen.
• A partial solar eclipse occurs when only part of
  the Sun is blocked from the Earth.
• An annular solar eclipse may be seen as a thin
  ring encircling the Moons disk, if the Moon is
  far enough from the Earth for its umbra to not
  reach the ground.

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Solar Eclipses 2
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Solar Eclipses 3
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Total Solar Eclipse

• Only during a total solar eclipse is the solar
  corona visible.

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Annular Eclipse
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Solar Eclipse Tracks 2000-2020

• The width of the track depends both on the
  Earths latitude and the distance of the Moon
  from the Earth during the eclipse.

Saros cycle
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Lunar Eclipses 1
A lunar eclipse occurs when the Moon enters the
Earths shadow.
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Lunar Eclipses 2

• The Moon appears red in a total lunar eclipse
  because some light from the Sun is refracted by
  the Earths atmosphere.
• Since the blue end of the spectrum is scattered
  more than the red by the atmosphere, more red
  light reaches the Moon.

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Lunar Eclipses 3
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Lunar Eclipse Over Dallas