The Memphis Astronomical Society Presents A SHORT COURSE in ASTRONOMY

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TheMemphis Astronomical SocietyPresentsA
SHORT COURSEinASTRONOMY
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CHAPTER 2THE SKY AND THE EARTHDr. William J.
BuslerAstrophysical Chemistry 439
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SKY MAPS

• First, select the proper set of sky maps for your
  latitude.
• Then, using the date/time guide which should have
  come with your sky maps, select the proper
  monthly sky map for the current date and time.

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SKY MAPS

• Hold the map so that the direction you are facing
  corresponds to the edge (horizon) of the map
  which is down.
• What you see on that portion of the map should
  correspond to what is visible in that area of the
  sky.

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CONSTELLATIONS

• Scientifically meaningless.
• Mythologically interesting.
• Patterns are made up of relatively nearby stars
  in our Galaxy (less than 1000 light-years away).
• 88 constellations altogether.
• Learn the major constellations for each season
  first use them to find the others.
• Learn the names of all the first-magnitude stars,
  then other important stars.

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The ROTATION of the EARTH

• The Earth rotates eastward on its axis, making
  one rotation per day.
• This diurnal (daily) motion is what causes the
  Sun to appear to rise and set each day.
• Diurnal motion also causes the stars to exhibit
  the same motion during the night.

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CIRCUMPOLARITY

• The Earths axis of rotation points toward
  Polaris, the North Star as a result, Polaris
  remains stationary, while all the other stars
  wheel around the North Star during the night.
• Stars close to Polaris never set -- they are
  circumpolar.
• Those farther away from Polaris go below the
  horizon.
• The farther a star is from Polaris, the more time
  it spends below the horizon.

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• Circumpolar Star Trails over Kitt Peak

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The ROTATION of the EARTH

• A star located on the celestial equator is up for
  12 hours and down for 12 hours.
• (The celestial equator divides the heavens into
  northern and southern hemispheres.)
• Stars north of the celestial equator are above
  the horizon more than they are below it.
• Stars south of the celestial equator are below
  the horizon more than they are above it.

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• Equatorial Star Trails

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The Effect of LATITUDE

• The observers latitude affects the altitude of
  Polaris, i.e., its angular distance above the
  horizon.
• At the north pole, the observers latitude 90?
  the altitude of Polaris 90? (i.e., directly
  overhead).
• At the equator, the observers latitude 0? the
  altitude of Polaris 0? (i.e., on the northern
  horizon).
• At Memphis, the observers latitude 35? the
  altitude of Polaris is 35?.

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The Effect of LATITUDE

• At Memphis, the circumpolar constellations are
  those within 35? of Polaris.
• These include Ursa Minor, Ursa Major (Big
  Dipper portion), Cassiopeia, Cepheus, and Draco.
• At the north pole, the circumpolar constellations
  include all those within 90 of Polaris, i.e.,
  the entire northern celestial hemisphere.
• At the north pole, Polaris is stationary
  overhead all the other stars move along parallel
  to the horizon as the Earth rotates.

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The Effect of LATITUDE

• At the equator, the circumpolar constellations
  would include all those within 0 of Polaris,
  i.e., none.
• At the equator, Polaris is stationary on the
  northern horizon all the other stars rise
  vertically in the east, move across the sky for
  12 hours, and set in the west, remaining below
  the horizon for 12 hours.
• At the equator, all stars are eventually visible.
• At Memphis, all stars within 35 of the south
  celestial pole are never seen.

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The REVOLUTION of the EARTH

• The Earth revolves around the Sun once each year,
  moving in an eastwardly direction.
• This annual (yearly) motion causes the
  constellations seen at a given time each night to
  advance with the seasons.
• In other words, the diurnal and annual motions of
  the Earth have the same effect on what is seen in
  the sky.
• As a result, the constellations of the opposite
  season are seen just before sunrise.

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The REVOLUTION of the EARTH

• The annual and diurnal motions of the Earth have
  the same effect on what is seen in the sky.
• 12 months of revolution 24 hours of rotation.
• 1 month of revolution 2 hours of rotation.
• 2 weeks of revolution ? 1 hour of rotation.
• 1 day of revolution 4 minutes of rotation.
• In other words, the same stars rise 4 minutes
  earlier each night. (More later!)

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CELESTIAL COORDINATES

• Zenith The overhead point as seen from the
  observers position.
• Meridian The north-south line passing through
  the zenith.
• Celestial Equator An imaginary line around the
  sky directly above the Earths equator.
• The celestial equator is the projection of the
  plane of the Earths equator onto the celestial
  sphere, 90 from the celestial poles.
• The celestial equator divides the northern and
  southern celestial hemispheres.

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CELESTIAL COORDINATES

• The celestial equator appears to run from the
  eastern horizon to the western horizon.
• At the Earths equator, the celestial equator
  passes through the zenith.
• In the northern (terrestrial) hemisphere, the
  celestial equator does not pass overhead, but
  instead passes south of the zenith by an angular
  distance equal to the observers latitude.
• At Memphis, the celestial equator passes 35
  south of the zenith.
• At the north pole, the celestial equator runs
  around the horizon.

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INTERMISSION
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CELESTIAL COORDINATES

• The ecliptic is the plane of the Earths orbit
  projected onto the celestial sphere.
• The 12 constellations along the ecliptic are
  known as the constellations of the zodiac.
• As the Earth orbits the Sun, the Sun appears to
  move eastward along the ecliptic at the rate of
  about 1 per day, or one sign per month.
• The Moon and nearly all the other planets orbit
  in essentially the same plane as does the Earth.
• Therefore, the Moon and planets will also be
  found close to the ecliptic, against the
  background of the constellations of the zodiac.

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CELESTIAL COORDINATES

• The Earths axis is tilted 23.5 from the
  vertical (with respect to the plane of its
  orbit).
• As a result, the celestial equator is inclined to
  the ecliptic by 23.5.
• Therefore, the ecliptic and equator intersect at
  two points (the equinoxes), and are separated by
  a maximum of 23.5 at two points (the solstices).
• The vernal equinox is the intersection point at
  which the Sun (on the ecliptic) crosses the
  celestial equator going north. This event marks
  the beginning of spring.
• The vernal equinox is located in Pisces.

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CELESTIAL COORDINATES

• The autumnal equinox is the intersection point at
  which the Sun (on the ecliptic) crosses the
  celestial equator going south. This event marks
  the beginning of autumn.
• The autumnal equinox is in Virgo.
• When the Sun is at an equinox, i.e., on the
  celestial equator, it behaves like any other star
  on the celestial equator it is up for 12 hours
  and down for 12 hours. In other words, days and
  nights are 12 hours each all over the world at
  the time of the equinoxes.
• (The term equinox means equal night.)

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CELESTIAL COORDINATES

• The solstices mark the two points where the
  ecliptic is at its maximum distance (23.5) from
  the celestial equator.
• When the Sun is near the solstices, its
  north-south position (declination) remains nearly
  constant for several days before it heads back
  toward the celestial equator.
• Hence the word solstice, which means the Sun
  standing still.

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CELESTIAL COORDINATES

• The summer solstice is in Gemini, near Castors
  foot.
• When the Sun is at the summer solstice, 23.5
  north of the celestial equator, it is up much
  longer than it is down (i.e., days are longer
  than nights by the maximum amount).
• When the Sun reaches the summer solstice, this
  longest day marks the beginning of summer.

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CELESTIAL COORDINATES

• The winter solstice is in Sagittarius.
• When the Sun is at the winter solstice, 23.5
  south of the celestial equator, it is down much
  longer than it is up (i.e., nights are longer
  than days by the maximum amount).
• When the Sun reaches the winter solstice, this
  shortest day marks the beginning of winter.

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CELESTIAL COORDINATES

• Declination On the celestial sphere, lines of
  declination are parallels to the celestial
  equator, analogous to lines of latitude on Earth.
• Declination (Dec or ?) is measured from the
  celestial equator (0) to the north celestial
  pole (90), or to the south celestial pole
  (-90).
• Degrees of declination are subdivided into
  minutes and seconds
• 60' (minutes) 1.
• 60" (seconds) 1'.

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CELESTIAL COORDINATES

• Right Ascension Analogous to lines of longitude
  on Earth.
• Lines of right ascension (RA or ?) run from pole
  to pole, perpendicular to the celestial equator.
• The zero-hour (0h) line intersects the equator
  (and the ecliptic!) at the vernal equinox in
  Pisces.
• The 6h line passes through the summer solstice in
  Gemini.
• The 12h line passes through the autumnal equinox
  in Virgo.
• The 18h line passes through the winter solstice
  in Sagittarius.

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CELESTIAL COORDINATES

• Note that right ascension is measured eastwardly
  along the equator, starting at the vernal
  equinox.
• Note that 24h is the same as 0h.
• Hours of right ascension are subdivided into
  minutes and seconds. However, these are not
  equivalent to the minute and second subdivisions
  of degrees of declination! (Note that their
  symbols are different as well!)
• 360 of declination circle 24h right
  ascension.
• 15 of declination 1h of right ascension.
• 15' of declination 1m of right ascension.
• 15" of declination 1s of right ascension.

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• The Celestial Coordinate System

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SIDEREAL TIME

• The Earth rotates on its axis every 23h56m with
  respect to the stars this is called the sidereal
  day.
• Meanwhile, the Earths revolution carries it a
  little farther around its orbit.
• Consequently, the Earth must rotate a little
  longer (4 min) to bring the same point on the
  Earth into alignment with the Sun as on the
  previous day.
• I.e., the solar day is exactly 24h long, while
  the sidereal day is 23h56m long.
• This 4-minute difference causes each star to rise
  4 minutes earlier each night.

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In A, the spot on the Earth with the arrow is
facing the Sun -- it is noon there.The next
day, 23h56m later, the Earth has made one
complete rotation (to B) with respect to the
stars.However, the Earth must still rotate
another 4 minutes (to C) in order to face the Sun
(noon) again.
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SIDEREAL TIME

• Our clocks run on solar time, because it is
  convenient to have the Sun in approximately the
  same place in the sky at the same clock time each
  day throughout the year.
• Meanwhile, the stars keep sidereal time.
  Sidereal time is defined as the hour of RA on the
  meridian.
• Over the course of a year, this 4-minute daily
  interval adds up to another whole day.
• Thus, there are 366.25 sidereal days per year.
• A mechanical clock (or a telescope clock drive)
  can be made to keep sidereal time by setting it
  to run faster than normal by 4 minutes per day.

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THE SEASONS

• The Earths axis is tilted 23.5 from the
  perpen-dicular to the plane of its orbit around
  the Sun.
• The northern hemisphere of the Earth tilts toward
  the Sun in summer and away from it in winter.
• At the equinoxes, the Sun is up 12 hours and down
  12 hours.
• In summer, the Sun is north of the celestial
  equator, and is therefore up more than down.
• In winter, the Sun is south of the celestial
  equator, and is therefore down more than up.
• Besides the length of daylight, the angle of
  insolation leads to seasonal temperature changes.

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• The Seasons

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THE SEASONS

• The arctic circle is the parallel of latitude
  located 23.5 from the north pole i.e., 90 -
  23.5 66.5.
• Within (north of) the arctic circle, the Sun
  becomes circumpolar around the time of the summer
  solstice. (Land of the Midnight Sun.)
• Conversely, near the winter solstice, the Sun
  remains below the horizon, day and night.
• At the north pole, the Sun is constantly up from
  the first day of spring until the first day of
  autumn, then down again until the next spring.
• The Sun rises only because of its motion along
  the ecliptic, not the Earths rotation.

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PRECESSION

• The Earths axis is tilted 23.5 from the
  perpen-dicular to the plane of its orbit around
  the Sun.
• Due to the gravitational pull of the Moon on the
  Earths equatorial bulge, the Earths axis slowly
  wobbles.
• This causes the north celestial pole to trace out
  a circle in the sky 23.5 in radius, centered on
  the north ecliptic pole in Draco, every 26,000
  years.
• The north ecliptic pole is perpendicular to the
  plane of the ecliptic -- the axis of the Solar
  System.

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PRECESSION

• As a result of precession, therefore, there is a
  succession of North Stars.
• At the time of the construction of the pyramids,
  Thuban (? Draconis) was the pole star.
• After Polaris, the stars along the eastern edge
  of Cepheus will serve as pole stars.
• In 12,000 AD, Vega will be the north star.

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• The Precession Circle

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PRECESSION

• The equinoxes also precess westward the ecliptic
  remains fixed but the Earths wobble moves the
  equator westward.
• The vernal equinox spends about 2,200 years in
  each constellation of the zodiac before moving
  into the next one toward the west.
• 2000 years ago, the vernal equinox was in Aries.
  (It is sometimes still called the first point of
  Aries.)

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Ecliptic Present (Polaris) Celestial Equator
(V.E. in Pisces) Thuban Equator (V.E. in
Aries) Vega Equator (V.E. in Capri-cornus)

• Precession of the Equinoxes

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PRECESSION

• The vernal equinox is now in Pisces, the next
  constellation toward the west -- we are in the
  Christian era.
• Eventually, the vernal equinox will move farther
  westward into Aquarius -- the dawning of the Age
  of Aquarius.
• Precession slowly changes the right ascension and
  declination of every star consequently, star
  atlases must be drawn for a particular epoch.

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T H E E N D