ASTRO 101

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ASTRO 101

• Principles of Astronomy

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Instructor Jerome A. Orosz
(rhymes with boris)Contact

• Telephone [phone]
• E-mail orosz_at_sciences.sdsu.edu
• WWW http//mintaka.sdsu.edu/faculty/orosz/web/
• Office Physics 241, hours T TH 330-500

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Text Perspectives on Astronomy First
Editionby Michael A. Seeds Dana Milbank.
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Astronomy Help Room Hours

• Monday 1200-1300, 1700-1800
• Tuesday 1700-1800
• Wednesday 1200-1400, 1700-1800
• Thursday 1400-1800, 1700-1800
• Friday 900-1000, 1200-1400
• Help room is located in PA 215

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Homework

• Assigned question due September 17 Question 9,
  Chapter 3 (Review Galileos telescope discoveries
  and explain why they supported the Copernican
  model and contradicted the Ptolemaic model.)
• OR
• Go to a planetarium show in PA 209
• Thu. Sep. 10 1000-1100, 1600-1700
• Fri. Sep. 11 1400-1500
• Mon. Sep. 14 1000-1100, 1500-1600
• Tue. Sep. 15 1100-1200
• Wed. Sep. 16 1100-1200
• Thu. Sep. 17 1400-1500
• Fri. Sep. 18 1600-1700

Sign up for a session outside PA 209 Hand in a
sheet of paper with your name and the date and
time of the session.
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Homework

• Assigned question due September 24 Question 2,
  Chapter 4 (Why do nocturnal animals usually have
  large pupils in their eyes? How is that related
  to the design of astronomical telescopes?)

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Dispatches from the Department of Anal Retention

• Is the Earth tilted or is the Earths rotation
  axis tilted (can you tilt a sphere?)
• The Earths axis is tilted relative to what?
• Does the Sun shine only on the Northern
  Hemisphere or only on the Southern Hemisphere at
  any given time?
• Does the total amount of heat from the Sun vary?
  Is the Sun more powerful in the summer?
• Why are the seasons opposite?

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Coming Up

• A Brief History of Astronomy (Chapter 3)
• Forces in Nature
• Gravity
• Electromagnetism
• Newtons Laws of motion, gravitation
• Orbits
• tides

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Question from Before

• Can you prove that the Earth orbits the Sun? Yes,
  use the phases of Venus. You can also can use
  stellar parallax.

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Questions for Today

• What is gravity?
• Why do we usually have two high tides per day?

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What is a model?

• A model is an idea about how something works.
• It contains assumptions about certain things, and
  rules on how certain things behave.
• Ideally, a model will explain existing
  observations and be able to predict the outcome
  of future experiments.

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Aristotle (385-322 B.C.)

• Aristotle was perhaps the most influential Greek
  philosopher. He favored a geocentric model for
  the Universe
• The Earth is at the center of the Universe.
• The heavens are ordered, harmonious, and perfect.
  The perfect shape is a sphere, and the natural
  motion was rotation.

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Geocentric Model

• The motion of the Sun around the Earth accounts
  for the rising and setting of the Sun.
• The motion of the Moon around the Earth accounts
  for the rising and setting of the Moon.
• You have to fiddle a bit to get the Moon phases.

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Geocentric Model

• The fixed stars were on the Celestial Sphere
  whose rotation caused the rising and setting of
  the stars.

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• This is the constellation of Orion

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• The constellations rise and set each night, and
  individual stars make a curved path across the
  sky.
• The curvature of the tracks depend on where you
  look.

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Geocentric Model

• The fixed stars were on the Celestial Sphere
  whose rotation caused the rising and setting of
  the stars.
• However, the detailed motions of the planets were
  much harder to explain

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Planetary Motion

• The motion of a planet with respect to the
  background stars is not a simple curve. This
  shows the motion of Mars.
• Sometimes a planet will go backwards, which is
  called retrograde motion.

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Planetary Motion

• Here is a plot of the path of Mars.
• Other planets show similar behavior.

Image from Nick Strobel Astronomy Notes
(http//www.astronomynotes.com/)
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Aristotles Model

• Aristotles model had 55 nested spheres.
• Although it did not work well in detail, this
  model was widely adopted for nearly 1800 years.

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Better Predictions

• Although Aristotles ideas were commonly
  accepted, there was a need for a more accurate
  way to predict planetary motions.
• Claudius Ptolomy (85-165) presented a detailed
  model of the Universe that explained retrograde
  motion by using complicated placement of circles.

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Ptolomys Epicycles

• By adding epicycles, very complicated motion
  could be explained.

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Ptolomys Epicycles
Image from Nick Strobels Astronomy Notes
(http//www.astronomynotes.com/).
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Ptolomys Epicycles
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Ptolomys Epicycles

• Ptolomys model was considered a computational
  tool only.
• Aristotles ideas were true. They eventually
  became a part of Church dogma in the Middle Ages.

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Next

• The Copernican Revolution

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The Sun-Centered Model

• Nicolaus Copernicus (1473-1543) proposed a
  heliocentric model of the Universe.
• The Sun was at the center, and the planets moved
  around it in perfect circles.

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The Sun-Centered Model

• Nicolaus Copernicus (1473-1543) proposed a
  heliocentric model of the Universe.
• These stamps mark the 500th anniversary of his
  birth.

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The Sun-Centered Model

• The Sun was at the center. Each planet moved on
  a circle, and the speed of the planets motion
  decreased with increasing distance from the Sun.

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The Sun-Centered Model

• Retrograde motion of the planets could be
  explained as a projection effect.

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The Sun-Centered Model

• Retrograde motion of the planets could be
  explained as a projection effect.

Image from Nick Strobels Astronomy Notes
(http//www.astronomynotes.com/)
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Copernican Model

• The model of Copernicus did not any better than
  Ptolomys model in explaining the planetary
  motions in detail.
• He did work out the relative distances of the
  planets from the Sun.
• The philosophical shift was important (i.e. the
  Earth is not at the center of the Universe).

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Tycho Brahe (1546-1601)

• Tycho was born in a very wealthy family.
• From an early age, he devoted himself to making
  accurate astronomical observations.
• He received a great deal of support from the king
  of Denmark, including the use of his own island.

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Tycho

• Tycho lived before the invention of the
  telescope.
• His observations of Mars were about 10 times more
  accurate than what had been done before.

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Johannes Kepler (1571-1630)

• Kepler was a mathematician by training.
• He believed in the Copernican view with the Sun
  at the center and the motions of the planets on
  perfect circles.
• Tycho hired Kepler to analyize his observational
  data.

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Johannes Kepler (1571-1630)

• Kepler was a mathematician by training.
• He believed in the Copernican view with the Sun
  at the center and the motions of the planets on
  perfect circles.
• Tycho hired Kepler to analyize his observational
  data.
• After years of failure, Kepler dropped the notion
  of motion on perfect circles.

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Keplers Three Laws of Planetary Motion

• Starting in 1609, Kepler published three laws
  of planetary motion

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Keplers Three Laws of Planetary Motion

• Starting in 1609, Kepler published three laws
  of planetary motion
• Planets orbit the Sun in ellipses, with the Sun
  at one focus.

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Ellipses

• An ellipse is a flattened circle described by a
  particular mathematical equation.
• The eccentricity tells you how flat the ellipse
  is e0 for circular, and e1 for infinitely flat.

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Ellipses

• You can draw an ellipsed with a loop of string
  and two tacks.

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Keplers Three Laws of Planetary Motion

• Starting in 1609, Kepler published three laws
  of planetary motion
• Planets orbit the Sun in ellipses, with the Sun
  at one focus.

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Keplers Three Laws of Planetary Motion

• Starting in 1609, Kepler published three laws
  of planetary motion
• Planets orbit the Sun in ellipses, with the Sun
  at one focus.
• The planets sweep out equal areas in equal times.
  That is, a planet moves faster when it is closer
  to the Sun, and slower when it is further away.

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Keplers Second Law

• The time it takes for the planet to move through
  the green sector is the same as it is to move
  through the blue sector.
• Both sectors have the same area.

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Keplers Three Laws of Planetary Motion

• Starting in 1609, Kepler published three laws
  of planetary motion
• Planets orbit the Sun in ellipses, with the Sun
  at one focus.
• The planets sweep out equal areas in equal times.
  That is, a planet moves faster when it is closer
  to the Sun, and slower when it is further away.

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Keplers Three Laws of Planetary Motion

• Starting in 1609, Kepler published three laws
  of planetary motion
• Planets orbit the Sun in ellipses, with the Sun
  at one focus.
• The planets sweep out equal areas in equal times.
  That is, a planet moves faster when it is closer
  to the Sun, and slower when it is further away.
• (Period)2 (semimajor axis)3

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Keplers Third Law
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The Keplers Law Simulator

• There are some animations on the web illustrating
  Keplers Laws
• http//www.astro.utoronto.ca/zhu/ast210/kepler.ht
  ml

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Heliocentric or Geocentric?

• The year is around 1610. The old school is
  Aristotle and a geocentric view. The new
  school is the heliocentric view (Copernicus and
  Kepler).
• Which one is correct?

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Heliocentric or Geocentric?

• The year is around 1610. The old school is
  Aristotle and a geocentric view. The new
  school is the heliocentric view (Copernicus and
  Kepler).
• Which one is correct?
• Observational support for the heliocentric model
  would come from Galileo.

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Heliocentric or Geocentric?

• The year is around 1610. The old school is
  Aristotle and a geocentric view. The new
  school is the heliocentric view (Copernicus and
  Kepler).
• Which one is correct?
• Observational support for the heliocentric model
  would come from Galileo.
• Theoretical support for the heliocentric model
  would come from Isaac Newton.

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Next

• Who Wins?

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Galileo Galilei (1564-1642)

• Galileo was one of the first to use a telescope
  to study astronomical objects, starting in about
  1609.
• http//www.pacifier.com/tpope/index.htm

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Galileo Galilei (1564-1642)

• Galileo was one of the first to use a telescope
  to study astronomical objects, starting in about
  1609.
• His observations of the moons of Jupiter and the
  phases of Venus provided strong support for the
  heliocentric model.

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Jupiters Moons

• The 4 objects circled Jupiter, and not the Earth!

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Jupiters Moons

• You can watch Jupiters moons move from one side
  of Jupiter to the other in a few days.

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Jupiters Moons

• Not all bodies go around the Earth!

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Venus

• Venus, the brightest planet, is never far from
  the Sun it sets at most a few hours after
  sunset, or rises at most a few hours before
  sunrise.

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Venus

• Venus, the brightest planet, is never far from
  the Sun it sets at most a few hours after
  sunset, or rises at most a few hours before
  sunrise.
• It is never out in the middle of the night.

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Venus

• Galileo discovered that Venus had phases, just
  like the Moon.

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Venus

• Galileo discovered that Venus had phases, just
  like the Moon.
• Furthermore, the crescent Venus was always larger
  than the full Venus.

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Venus

• Galileo discovered that Venus had phases, just
  like the Moon.
• Furthermore, the crescent Venus was always larger
  than the full Venus.
• Conclusion Venus shines by reflected sunlight,
  and it is closer to Earth when it is a crescent.

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Venus in the Geocentric View

• Venus is always close to the Sun on the sky, so
  its epicycle restricts its position.
• In this view, Venus always appears as a crescent.

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Venus in the Heliocentric View

• In the heliocentric view, Venus orbits the Sun
  closer than the Earth does.
• We on Earth can see a fully lit Venus when it is
  on the far side of its orbit.

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Venus in the Heliocentric View

• The correlation between the phases and the size
  is accounted for in the heliocentric view.

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• Galileos observations of Jupiter and Venus
  strongly favored the heliocentric view of the
  Universe.

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• Galileos observations of Jupiter and Venus
  strongly favored the heliocentric view of the
  Universe.
• Galileo was put before the Inquisition and forced
  to recant his views.

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• Galileos observations of Jupiter and Venus
  strongly favored the heliocentric view of the
  Universe.
• Galileo was put before the Inquisition and forced
  to recant his views.
• Pope John Paul II admitted in 1992 that the
  Church was wrong to denounce Galileo.

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Isaac Newton (1642-1727)
http//www-history.mcs.st-andrews.ac.uk/history/P
ictDisplay/Newton.html
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Isaac Newton (1642-1727)

• Isaac Newton was born the year Galileo died.

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Isaac Newton (1642-1727)

• Isaac Newton was born the year Galileo died.
• He was professor of mathematics at Cambridge
  University in England. (Steven Hawking currently
  holds Newtons Chair at Cambridge).

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Isaac Newton (1642-1727)

• Isaac Newton was born the year Galileo died.
• He was professor of mathematics at Cambridge
  University in England. (Steven Hawking currently
  holds Newtons Chair at Cambridge).
• He was later the Master of the Mint in London,
  where first proposed the use of grooved edges on
  coins to prevent shaving.

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Isaac Newton (1642-1727)

• Newton was perhaps the greatest scientist of all
  time, making substantial contributions to
  physics, mathematics (he invented calculus as a
  college student), optics, and chemistry.

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Isaac Newton (1642-1727)

• Newton was perhaps the greatest scientist of all
  time, making substantial contributions to
  physics, mathematics (he invented calculus as a
  college student), optics, and chemistry.
• His laws of motion and of gravity could explain
  Keplers Laws of planetary motion.

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Newtons Laws of Motion
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Newtons Laws of Motion

• A body in motion tends to stay in motion in a
  straight line unless acted upon by an external
  force.

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Newtons Laws of Motion

• A body in motion tends to stay in motion in a
  straight line unless acted upon by an external
  force.
• The force on an object is the mass times the
  acceleration (Fma).

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Newtons Laws of Motion

• A body in motion tends to stay in motion in a
  straight line unless acted upon by an external
  force.
• The force on an object is the mass times the
  acceleration (Fma).
• For every action, there is an equal and opposite
  reaction. (For example, a rocket is propelled by
  expelling hot gas from its thrusters).

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What is Gravity?
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What is Gravity?

• Gravity is a force between all matter in the
  Universe.

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What is Gravity?

• Gravity is a force between all matter in the
  Universe.
• It is difficult to say what gravity is. However,
  we can describe how it works.

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What is Gravity?

• Gravity is a force between all matter in the
  Universe.
• It is difficult to say what gravity is. However,
  we can describe how it works.

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What is Gravity?

• The gravitational force between larger bodies is
  greater than it is between smaller bodies, for a
  fixed distance.

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What is Gravity?

• As two bodies move further apart, the
  gravitational force decreases. The range of the
  force is infinite, although it is very small at
  very large distances.

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Newtons Laws

• Using Newtons Laws, we can

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Newtons Laws

• Using Newtons Laws, we can
• Derive Keplers Three Laws.

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Newtons Laws

• Using Newtons Laws, we can
• Derive Keplers Three Laws.
• Measure the mass of the Sun, the Moon, and the
  Planets.

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Newtons Laws

• Using Newtons Laws, we can
• Derive Keplers Three Laws.
• Measure the mass of the Sun, the Moon, and the
  Planets.
• Measure the masses of distant stars in binary
  systems.

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Laws of Physics

• The models of Aristotle and Ptolomy were based
  mainly on beliefs (i.e. that motion should be on
  perfect circles, etc.).
• Starting with Newton, we had a physical model of
  how the planets moved the laws of motion and
  gravity as observed on Earth give a model for how
  the planets move.
• All modern models in Astronomy are based on the
  laws of Physics.

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Newtons Laws and Orbits

• Newton realized that since the Moons path is
  curved (i.e. it is accelerating), there must be a
  force acting on it.

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Newtons Laws and Orbits

• If you shoot a cannonball horizontally, it
  follows a curved path to the ground. The faster
  you launch it, the further it goes.

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Newtons Laws and Orbits

• If you shoot a cannonball horizontally, it
  follows a curved path to the ground. The faster
  you launch it, the further it goes.
• If it goes really far, the Earth curves from
  under it

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Newtons Laws and Orbits

• Newton showed mathematically that the expected
  shape for a closed orbit is an ellipse (i.e. he
  explained the origin of Keplers first law).

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Newtons Laws and Orbits

• A geosynchronous satellite has an orbital period
  around the Earth of 24 hours (23 hours and 56
  minutes actually), which is the rotation period
  of the Sun.
• The net effect is that the satellite is always
  above the same spot.

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Newtons Laws and Tides

• If the tides are caused by the Moon pulling on
  the oceans, then why is there usually two high
  tides per day?

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Newtons Laws and Tides

• If the tides are caused by the Moon pulling on
  the oceans, then why is there usually two high
  tides per day?
• Actually tides are caused by differences in the
  gravitational forces.

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Newtons Laws and Tides

• Spring tides are when the Sun and Moon are
  roughly aligned (e.g. new and full moon). The
  tides tend to be higher at these times.
• Local conditions can also effect the height of
  the tides.

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Weight and Mass
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Weight and Mass

• In Physics, we distinguish between weight and
  mass

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Weight and Mass

• In Physics, we distinguish between weight and
  mass
• Weight is a force due to gravity.

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Weight and Mass

• In Physics, we distinguish between weight and
  mass
• Weight is a force due to gravity.
• Mass is a measure of the amount of matter in an
  object.

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Weight and Mass

• In Physics, we distinguish between weight and
  mass
• Weight is a force due to gravity.
• Mass is a measure of the amount of matter in an
  object.
• The units of weight are pounds in the British
  system or newtons in the metric system.

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Weight and Mass

• In Physics, we distinguish between weight and
  mass
• Weight is a force due to gravity.
• Mass is a measure of the amount of matter in an
  object.
• The units of weight are pounds in the British
  system or newtons in the metric system.
• The units of mass are stones in the British
  system or kilograms in the metric system.

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Weight and Mass

• Your weight depends where you are (e.g. on the
  Earth, on the Moon, in outer space, etc.).
• Your mass is the same no matter where you are.
• In most cases on Earth, we can use the terms
  weight and mass interchangeably.

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Weight and Mass

• The mass is used in Newtons Gravity formula

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Next

• Tides and Orbits
• Telescopes (Chapter 4)