What is Astronomy?

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What is Astronomy?
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Ancient Astronomy

• People have used the stars for ages.
• Used constellations to set planting times
• Used constellations to guide travels
• Some lights in the sky wandered through the
  constellations
• Called them planets which is Greek for wanderers

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The Mayan Calendar

• The Mayans were the first civilization to widely
  use a standard calendar. It consisted of 365
  days very much like the calendar we use today.

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Time

• Year Cycle through the seasons as the Earth
  revolves around the Sun.
• Month Based on cycle of the moon adjusted to
  make exactly 12 months in a year instead of the
  actual 12.4 cycles per year.
• Week 7 is chosen since there are 7 astronomical
  objects viewed by Ancient Astronomers besides
  the stars Sunday, Moonday, Thorsday (Thor
  Jupiter), Saturnday, in English can easily be
  identified. In Spanish you can easily identify
  Marsday (Martes Tuesday), Mercuryday (Miercoles
  Wednesday), and Venusday (Viernes Friday).

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Time

• Day Time from Noon on one day to Noon on the
  next. Noon is the suns highest point.
• Hour Day is broken into day and night, and the
  daytime is broken into 12 units, just like the
  year, and the night is broken into 12 units,
  also. Total is 24 hours in a day.
• Minute A minute (pronounced my-nute meaning
  tiny) part of an hour based on Egyptian idea of
  a minute (tiny) part 1/60th .
• Second A minute (tiny) part of a minute (tiny)
  part of an hour, or a second minute (tiny) part
  of an hour, or simply a second.

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Lunar Phases
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Lunar Phases
The moon goes through different phases during the
month because the illuminated (sun-facing) side
changes its orientation relative to the earth.
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Lunar Eclipses
When the moon passes through the Earths shadow,
a lunar eclipse occurs.
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Lunar Eclipses
A total eclipse of the moon.
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Solar Eclipses
Path of a total solar eclipse on the earth.
When the Earth passes through the Moons shadow,
a Solar eclipse occurs.
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Solar Eclipses
A total solar eclipse.
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Tilt of the Earths Axis

• The axis around which the Earth rotates is tilted
  by 23.5 degrees with respect to the ecliptic.

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• The seasons are due to the tilt of the Earths
  axis. Consider what happens on June 21 when the
  northern hemisphere of the Earth is tilted toward
  the Sun

Northern Summer
Northern Winter
Southern Summer
Southern Winter

• The sunlight strikes the ground more vertically
  than in December. The light is spread out over
  less ground and heats the ground better.

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Earth on June 22
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Earth on December 22
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Galileo Galilei (1564-1642)

• He used the telescope to look at astronomical
  objects. He discovered craters on the Moon,
  moons going around Jupiter, rings around Saturn,
  and the fact that Venus has phases that are
  related to its orbit.
• The heliocentric model can explain all of these,
  whereas the geocentric model fails.

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• Tycho Brahe (1546-1601) He collected great
  amounts of very precise data about the positions
  of the planets using instrumentation he
  developed.
• Johannes Kepler (1571-1630) He used Brahes data
  and mathematics to come up with three laws
• Planets follow elliptical instead of circular
  orbits.
• Planets sweep through equal areas in equal times.
• The square of the period is proportional to the
  cube of the semimajor axis of the ellipse.

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Planets

• Move in an elliptical orbit

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Planets

• Period of revolution
• the time it takes to go once around the sun
• One revolution is a year
• Mercury 88 days, Pluto 248 years

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What keeps them there?

• Law of inertia - objects motion wont change
  unless acted upon by an outside force.
• Wont change speed or direction
• Why do they curve?
• Gravity pulls them toward the sun

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What keeps them there?
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Rotation

• Planets spin on their axes
• One rotation is a day
• Mercury 58 days, Jupiter 10 hours

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A light wave is a light wave, no matter how
long...
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Refracting Telescopes
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Reflecting Telescopes
Refracting telescopes are very similar to
reflecting telescopes, except that the reflecting
telescopes use mirrors, instead of lenses.
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Same Space -- Different Light
Infrared Telescope
Radio Telescope
Optical Telescope
X-ray Telescope
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Light-Year

• To measure distances between stars we a distance
  measurement called the Light- year
• 1 light-year is the distance light travels in one
  year.
• Light moves at 300,000 km/sec
• Thats 186,000 mile/sec
• It would reach the sun in about 5 minutes
• How far would it go in a year?
• Nearest star is 4.3 light years away

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Distance to stars

• One method is parallax
• Apparent change in position as the earth goes
  around the sun

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Measure the angle to the star
Wait half a year
Measure the angle to the star
Triangle tells distance
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Rockets

• Rely on Netwons Third law of Motion
• For every action there is an equal and opposite
  reaction.
• Reward blast of hot gases causes rocket to shoot
  forward.
• First developed by Chinese in 1000
• Tube full of gunpowder with cap on one end.

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Satellites

• If a projectile has a large enough horizontal
  (tangential) velocity component (8 km/s), it will
  fall around the earth but never strike the
  surface.
• The result is a circular orbit around the earth.
• For the space shuttle or a satellite to orbit, it
  must be above the earths atmosphere to avoid air
  resistance (200 km above the surface).
• At an altitude of 200 km, the force of the
  earths gravity is only slightly reduced.

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Placing the Shuttle into Orbit

• The shuttle is given a vertical thrust from a
  rocket to push it up to an altitude of 200 km.
• At that point it is given a thrust from another
  rocket to increase its tangential speed to 8 km/s.

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Spacecraft

• Probes have gone past all the planets except
  Pluto
• Have sampled comets tails
• Have taken pictures of planets and moons.