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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Coming Up

• Introduction to the Sky
• Constellations
• Stellar Brightness
• Stellar coordinates
• The clockwork of the sky
• Day/night
• Phases of the moon
• The seasons
• Solar and Lunar Eclipses
• A Brief History of Astronomy (Chapter 3)

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Question from Before

• Why dont we have eclipses every month?
• Because the plane of the Moons orbit around the
  Earth in inclined with respect to the plane of
  the Earths orbit around the Sun.

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Question for Today

• Can you prove that the Earth orbits the Sun?

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In Detail

• In most places on Earth, the weather patterns go
  through distinct cycles
• Cold weather winter, shorter daytime
• Getting warmer spring, equal daytime/nighttime
• Warm weather summer, longer daytime
• Cooling off fall, equal daytime/nighttime
• These seasons are associated with the changing
  day/night lengths.

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In Detail

• When it is summer in the northern hemisphere, it
  is winter in the southern hemisphere, and the
  other way around.

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What Causes the Seasons?
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What Causes the Seasons?

• Is the Earth closer to the Sun during summer,
  and further away during winter? (This was the
  most commonly given answer during a poll taken at
  a recent Harvard graduation).
• No! Otherwise the seasons would not be opposite
  in the northern and southern hemispheres.

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What Causes the Seasons?

• The Earth moves around the Sun. A year is
  defined as the time it takes to do this, about
  365.25 solar days.
• This motion takes place in a plane in space,
  called the ecliptic.
• The axis of the Earths rotation is inclined from
  this plane by about 23.5 degrees from the normal.

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What Causes the Seasons?

• The axis of the Earths rotation points to the
  same point in space (roughly the location of the
  North Star).
• The result is the illumination pattern of the Sun
  changes throughout the year.

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What Causes the Seasons?

• Here is an edge-on view, from the plane of the
  Earths orbit.

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What Causes the Seasons?

• Here is a slide from NASA and NOAA.

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What Causes the Seasons?

• A slide from Nick Strobel.

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What Causes the Seasons?

• Because of the tilt of the Earths axis, the
  altitude the Sun reaches changes during the year
  It gets higher above the horizon during the
  summer than it does during the winter.
• Also, the length of the daytime hours changes
  during the year the daylight hours are longer
  in the summer and shorter in winter.

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What Causes the Seasons?

• The altitude of the Sun matters when the Sun is
  near the horizon, it does not heat as efficiently
  as it does when it is high above the horizon.
• Image from Nick Strobels Astronomy Notes
  (http//www.astronomynotes.com/).

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What Causes the Seasons?

• Winter The combination of a short daytime and a
  Sun that is relatively low above the horizon
  leads to much less heating in the day, plus a
  longer period of cooling at night. Overall, it
  is colder.

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What Causes the Seasons?

• Summer The combination of a long daytime and a
  Sun that is relatively high above the horizon
  leads to much more heating in the day, plus a
  shorter period of cooling at night. Overall, it
  is warmer.

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What Causes the Seasons?

• Spring and Fall The number of hour of daylight
  is about equal to the number of nighttime hours,
  leading to roughly equal times of heating and
  cooling.

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Next

• The Moon

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What Causes the Phases of the Moon?
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What Causes the Phases of the Moon?

• The full Moon always rises just after sunset.
• The crescent Moon always points towards the Sun.
• A crescent Moon sets shortly after sunset, or
  rises just before sunrise.
• The Moon is illuminated by reflected sunlight.

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What Causes the Phases of the Moon?

• The full Moon always rises just after sunset.
• A crescent Moon sets shortly after sunset.

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What Causes the Phases of the Moon?

• The full Moon always rises just after sunset.
• A crescent Moon sets shortly after sunset.

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What Causes the Phases of the Moon?

• The lit side of the Moon always faces the Sun.
• Because of the motion of the Moon relative to the
  Sun, we see different amounts of lit and dark
  sides over the course of a month.

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What Causes the Phases of the Moon?

• The lit side of the Moon always faces the Sun.
• Because of the motion of the Moon relative to the
  Sun, we see different amounts of lit and dark
  sides over the course of a month.

Image from Nick Strobel (http//www.astronomynotes
.com/)
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NextLunar and Solar Eclipses
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Lunar and Solar Eclipses

• But first, lets discuss angular size and
  linear size

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Angular Size

• The physical size is measured in meters,
  light-years, etc.
• The distance is measured in the same units.
• The angular size is how large something looks
  on the sky, and is measured in degrees.

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Angular Size

• The angular size is how large something looks
  on the sky, and is measured in degrees.
• As you move the same object further, its angular
  size gets smaller.

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Angular Size

• The angular size is how large something looks
  on the sky, and is measured in degrees.
• If two objects are at the same distance, the
  larger one has the larger angular size.

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Angular Size

• Trick photography often involves playing with
  different distances to create the illusion of
  large or small objects
• http//www.tadbit.com/2008/03/06/top-10-holding-th
  e-sun-pictures/
• http//www.stinkyjournalism.org/latest-journalism-
  news-updates-45.php

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Angular Size

• This figure illustrates how objects of very
  different sizes can appear to have the same
  angular sizes. The Sun is 400 times larger than
  the Moon, and 390 times more distant.

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Lunar and Solar Eclipses

• A solar eclipse is seen when the Moon passes in
  front of the Sun, as seen from a particular spot
  on the Earth.
• A lunar eclipse is seen then the Moon passes into
  the Earths shadow.

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Shadows

• If the light source is extended, then the shadow
  of an object has two parts the umbra is the
  complete shadow, and the penumbra is the
  partial shadow.

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Lunar Eclipses

• During a total lunar eclipse, the Moon passes
  through Earths shadow.

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Solar Eclipses

• The umbral shadow of the Moon sweeps over a
  narrow strip on the Earth, and only people in
  that shadow can see the total solar eclipse.

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Solar Eclipses

• The umbral shadow of the Moon sweeps over a
  narrow strip on the Earth, and only people in
  that shadow can see the total solar eclipse.
• During totality the faint outer atmosphere of the
  Sun can be seen.

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Annular Eclipses

• The angular sizes of the Sun and Moon vary
  slightly, so sometimes the Moon isnt big
  enough to cover the Sun

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Lunar and Solar Eclipses

• Why isnt there an eclipse every month? Because
  the orbit of the Moon is inclined with respect to
  the orbital plane of the Earth around the Sun.

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How often do we see an Eclipse?

• Roughly every 18 months there is a total solar
  eclipse visible somewhere on the Earth.

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A Brief History of Astronomy
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Stonehenge (c. 2000 B.C.)
Stonehenge was probably used to observe the sun
and Moon. Image from FreeFoto.com
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The great pyramids of Egypt were aligned
north-south.
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A Brief History of Astronomy

• An early view of the skies
• The Sun it rises and sets, rises and sets
• The Moon it has a monthly cycle of phases.
• The fixed stars the patterns stay fixed, and
  the appearance of different constellations marks
  the different seasons.
• Keep in mind there were no telescopes, no
  cameras, no computers, etc.

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A Brief History of Astronomy

• But then there were the 5 planets
• These are star-like objects that move through the
  constellations.
• Mercury the fastest planet, always near the
  Sun.
• Venus the brightest planet, always near the Sun.
• Mars the red planet, slower than Venus.
• Jupiter the second brightest planet, slower
  than Mars.
• Saturn the slowest planet.

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A Brief History of Astronomy

• By the time of the ancient Greeks (around 500
  B.C.), extensive observations of the planetary
  positions existed. Note, however, the accuracy
  of these data were limited.
• An important philosophical issue of the time was
  how to explain the motion of the Sun, Moon, and
  planets.

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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

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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.

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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.

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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.

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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.

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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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The Middle Ages

• Not much happened in Astronomy in the Middle Ages
  (100-1500 A.D.).

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Next

• The Copernican Revolution