Astronomy Stuff

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Astronomy Stuff

• Some Selected Topics

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How Big is the Universe?

• REALLY BIG!

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Our Position and Motion in the Heavens
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Are we ever sitting still?
Earth rotates on axis 1,000 km/hr
Earth orbits Sun 100,000 km/hr
Solar system moves among stars 70,000 km/hr
Milky Way rotates 800,000 km/hr
Milky Way moves in Local Group
Universe expands
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Planets in Order and To Scale
Build a scale model of solar system http//www.ex
ploratorium.edu/ronh/solar_system/
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Basic Properties of the Sun

• Radius is 100 times that of the Earth.
• 30,000 Earths would cover the surface.
• 1 million Earths would fit inside!
• It is a million times more massive than Earth
• 99.9 of mass of solar system
• Luminosity is 400 trillion trillion Watts.

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The Milky Way Galaxy
diameter of disk 100,000 l.y. (30,000 pc)
thickness of disk 1,000 l.y. (300 pc)
number of stars 200 billion
Sun is in disk, 28,000 l.y. out from center
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Planetary Motion

• Planets all orbit in same direction and nearly in
  same plane

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Johannes Kepler (1571-1630)
Trained as a mathematician and theorist. Needed
accurate data to make mathematical model of the
solar system
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What are Keplers three laws of planetary motion?
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Keplers First Law

• Each planets orbit around the Sun is an ellipse,
  with the Sun at one focus.

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What is an ellipse?
An ellipse looks like an elongated
circle
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Eccentricity of an Ellipse
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Keplers Second Law

• A planet moves along its orbit with a speed that
  changes in such a way that a line from the planet
  to the Sun sweeps out equal areas in equal
  intervals of time.

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Keplers Third Law

• The ratio of the cube of a planets average
  distance (a) from the Sun to the square of its
  orbital period (P) is the same for each planet.

a3 / P2 1
a3 P2
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So what is with the Zodiac Signs?

• Intro to the Sun Signs

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Annual Motion
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What is the cause of the seasons on Earth?

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The Cause of the Seasons
Note apparent change in altitude of Sun.
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Direct light causes more heating.
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The altitude of the Sun impacts both how much
energy is received on Earth and the duration of
daylight.
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Axis tilt causes uneven heating by sunlight
throughout the year.
Seasons Clip
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Suns Motion Across the Sky
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Why are the warmest days typically a month after
the beginning of summer?
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Typically

• It takes a while for the Earths surface to cool
  or to warm up.
• The hottest days of summer are about a month or
  two after the summer solstice.
• Likewise, the coldest days are about a month or
  so after the winter solstice.

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What would seasons be like if

• The Earth had no tilt?
• The Earths axis lay on the ecliptic plane?

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What is the Earths cycle of precession?
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Precession of the Earth
Earths axis precesses like the axis of a
spinning top once very 26,000 years.
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Changes in Axis Tilt

• Small gravitational tugs from other bodies in
  solar system cause Earths axis tilt to vary
  between 22 and 25
• Cycles every 41,000 years

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What do you notice going on here?
The Moon
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What is a constellation?
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A Constellation is
A region in the sky.
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Thought QuestionThe brightest stars in a
constellation

• All belong to the same star cluster.
• All lie at about the same distance from Earth.
• May actually be quite far away from each other.

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The brightest stars in a constellation

• All belong to the same star cluster.
• All lie at about the same distance from Earth.
• May actually be quite far away from each other.

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Constellations
Constellation stars need not be close together in
space.
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Why do we see phases of the Moon?
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Why do we see phases?
Phase Clip
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What are the different Moon phases?
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Phases of the Moon
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The Sun Milky Way