ASTRO 101

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ASTRO 101

Principles of Astronomy

Instructor: Jerome A. Orosz (rhymes with “boris”)Contact:• Telephone: 594-7118

• E-mail: orosz@sciences.sdsu.edu

• WWW: http://mintaka.sdsu.edu/faculty/orosz/web/

• Office: Physics 241, hours T TH 3:30-5:00

“Discovering the Essential Universe, Fifth Edition”

by Neil F. Comins

Course WWW Pagehttp://mintaka.sdsu.edu/faculty/orosz/web/ast101_fall2012.html

Note the underline: … ast101_fall2012.html …

Also check out Nick Strobel’s Astronomy Notes:

http://www.astronomynotes.com/

Where: Room 215, physics-astronomy building.

No appointment needed!

Just drop by!

When: • Monday: 12-2, 4-6 PM• Tuesday: 12-1 PM; 4-6 PM• Wednesday: 12-2, 5-6 PM• Thursday: 4-6 PM

Homework

• Homework due September 18: Question 11 from Chapter 2 (In what ways did the astronomical observations of Galileo support a heliocentric cosmology?)

• Write down the answer on a sheet of paper and hand it in before the end of class on September 18.

Homework• Homework due September 25: Question 4

from Chapter 3 (What are the three main functions of a telescope?)

• Write down the answer on a sheet of paper and hand it in before the end of class on September 25.

Homework• Go to a planetarium show in PA 209:• Wednesday, September 12: 12:00 PM -- 12:30 PM• Thursday, September 13: 12:00 PM – 12:30 PM AND

12:30 PM – 1:00 PM• Friday, September 14: 12:00 PM – 12:30 PM AND

12:30 PM – 1:00 PM• Monday, September 17: 12:00 PM – 12:30 PM AND

12:30 PM – 1:00 PM• Thursday, September 20: 12:00 PM – 12:30 PM AND

12:30 PM – 1:00 PM AND 4:00 PM – 4:30 PM

• Friday, September 21: 12:00 PM – 12:30 PM AND 12:30 PM – 1:00 PM

• Get 10 points extra credit for homework part of grade.• Sign up for a session outside PA 209.• Hand in a sheet of paper with your name and the data and time of

the session.

The motion of the planets

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.

Who Wins?

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

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.

Jupiter’s Moons

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

Jupiter’s Moons

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

Jupiter’s Moons

• Not all bodies go around the Earth!

• 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.

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

• Furthermore, the crescent Venus was always larger than the full 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.

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.

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.

Venus in the Heliocentric View

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

• Galileo’s 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.

• Galileo’s 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.

Isaac Newton (1642-1727)

http://www-history.mcs.st-andrews.ac.uk/history/PictDisplay/Newton.html

• Isaac Newton was born the year Galileo died.

• He was professor of mathematics at Cambridge University in England. (Steven Hawking currently hold’s Newton’s 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.

• 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 Kepler’s Laws of planetary motion.

Newton’s Laws of Motion

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

2. The force on an object is the mass times the acceleration (F=ma).

3. For every action, there is an equal and opposite reaction. (For example, a rocket is propelled by expelling hot gas from its thrusters).

What is Gravity?

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

What is Gravity?

• It is difficult to say what gravity is. However, we can describe how it works.

What is Gravity?

• It is difficult to say what gravity is. However, we can describe how it works.

What is Gravity?

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

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.

Newton’s Laws

• Using Newton’s Laws, we can…

Newton’s Laws

• Using Newton’s Laws, we can… Derive Kepler’s Three Laws.

Newton’s Laws

• Using Newton’s Laws, we can… Derive Kepler’s Three Laws. Measure the mass of the Sun, the Moon, and

the Planets.

Newton’s Laws

• Using Newton’s Laws, we can… Derive Kepler’s Three Laws. Measure the mass of the Sun, the Moon, and

the Planets. Measure the masses of distant stars in binary

systems.

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.

Newton’s Laws and Orbits• Newton realized that

since the Moon’s path is curved (i.e. it is accelerating), there must be a force acting on it.

Newton’s 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.

Newton’s 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…

Newton’s Laws and Orbits• Why doesn’t the Moon fall down on the Earth?

Newton’s Laws and Orbits• Why doesn’t the Moon fall down on the Earth?• Because of angular momentum…

Angular Momentum

• Angular momentum is a measure of the spin of an object. It depends on the mass that is spinning, on the distance from the rotation axis, and on the rate of spin.

• I = (mass).(radius).(spin rate)

Angular Momentum

• For a given distance to the rotation axis, more mass means more angular momentum.

Angular Momentum

• For a given distance to the rotation axis, more mass means more angular momentum.

• For a given mass, a larger distance means more angular momentum.

Angular Momentum

• For a fixed mass and distance, a higher rate of spin means a larger angular momentum.

Angular Momentum

• I = (mass).(radius).(spin rate)

• The angular momentum in a system stays fixed, unless acted on by an outside force.

Conservation of Angular Momentum• An ice skater demonstrates

the conservation of angular momentum:

Conservation of Angular Momentum• An ice skater demonstrates

the conservation of angular momentum:

• Arms held in: high rate of spin.

• Arms extended: low rate of spin.

• I = (mass).(radius).(spin rate) (angular momentum and mass are fixed here)

Angular Momentum

• I = (mass).(radius).(spin rate)• The angular momentum in a system stays fixed,

unless acted on by an outside force.• An orbiting body will not move towards the other

body unless there is an external force.

Weight and Mass

• In Physics, we distinguish between weight and mass:

Weight and Mass

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

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.

Weight and Mass

an object. The units of weight are pounds in the British

system or newtons in the metric system.

Weight and Mass

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.

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.

Weight and Mass

• The mass is used in Newton’s Gravity formula:

• Chapter 3: Light and Telescopes

Coming Up:• The 4 forces of Nature

• Energy and the conservation of energy

• The nature of light– Waves and bundles of energy– Different types of light

• Telescopes and detectors

ASTRO 101

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