Download Lecture Two (Powerpoint format)

Science 3210 001 : Introduction to Astronomy
Lecture 2 : Visual Astronomy -- Stars and
Planets
Robert Fisher
Items
 Course adds
 The course had been booked to capacity, but I will be adding as many people
as the room can accommodate, in the order I have received requests.
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People who attended last week’s lecture
Natasha Shah
Amanda Mayfield
Simon Spartalian
 Add/drop day is February 6th. This means today is the last day where I am
generally available to sign add/drop cards.
 Course webpage has been updated with first week’s lectures, and the
first reading and homework assignment :
 http://flash.uchicago.edu/~rfisher/saic.html
 Questions -- seek and ye shall find!
Review of Lecture 1
 History of Astronomy
 Ancient Astronomy
 Advent of Natural Philosophy
 Medieval Astronomy in Arab World
 Birth of Modern Science
 Science Overview
 Scales in the Cosmos
 Cosmic Calendar
 Powers of Ten Video
Overview of Lecture 2
 I. The Celestial Sphere
 II. The Stars
 IiI. The Motion of the Planets
Important Lessons to be Learned Today
 Because the stars are very distant, their motion on the sky is welldescribed as if they revolved around the Earth
 The motion of the planets is significantly more complex, and
required elaborate geometrical constructions in the ancient
geocentric system due to Ptolemy
 Niklaus Copernicus simplified matters tremendously by putting
the sun at the center of the universe -- even though he lacked the
“smoking gun” evidence to prove his case
Motion of the Stars
 The foundation of all visual
astronomy is a simple fact :
the Earth is a Sphere
 While common knowledge
today, determination of the
shape of the Earth was a
significant challenge to
ancient peoples
 The most convincing
elementary argument comes
from the fact that the Earth’s
shadow (as seen in lunar
eclipses) is always circular,
as Aristotle correctly deduced
Earth Image, Apollo 17 Crew
Lunar Eclipse
The Earth as the Center of the Universe
 Looking up at the night sky, it appears as if the entire Universe
revolves around the Earth.
QuickTime™ and a
Sorenson Video 3 decompressor
are needed to see this picture.
Celestial Sphere, Zenith, Nadir, Horizon
 The distant stars appear to lie on a solid sphere, the celestial sphere.
 The zenith is the direction directly upwards.
 The nadir is the direction directly downwards.
 The horizon splits the celestial sphere in half along the zenith-nadir axis.
Zenith and Nadir Depend on Your Location
 The zenith and nadir directions depend on where one stands on the
Earth.
Rotation of the Earth
Motion of the Celestial Sphere
 The rotation of the Earth causes the celestial sphere to appear to
revolve.
 The north/south celestial poles correspond to the north/south poles of the
Earth’s rotational axis.
The Motion of the Sun
 At a given location, the sun rises towards the east and sets towards the
west.
 A sundial gnomon casts a shadow away from the sun, towards the west.
 The invention of the gnomon is attributed to the ancient Greek
philosopher Animaxander, successor to Thales
Determining North from the Sun’s Motion
 At noon, the sun reaches its highest point in the sky, directly north.
 This was a common method used by the ancients to determine North.
Clockwise
Clockwise
 Imagine one wanted to read a sundial. We are facing south.
 In the morning, the sun rises in the east, casting a shadow in the west.
 In the afternoon, the sun begins to set in the west, casting a shadow to the east,
following the same circular arc traced in the morning.
 The direction traced by the sun’s shadow in its arc, facing south, is clockwise.
 When mechanical clocks with hands were first made, they were constructed so as
to rotate in the same sense as the sundial -- clockwise -- not counter-clockwise.
Describing the Celestial Sphere -The Great Circle
Great circle
A great circle on a sphere divides the sphere into two hemispheres.
One can imagine the equator as an example of a great circle, but any
circle dividing the sphere is a great circle.
Describing the Celestial Sphere -- Great Circles
Great circles
Any of the circles in the figure above are examples of great circles.
Angles
 Separation between two points on the celestial sphere are
measured in terms of angles -- much like a clock.
 A full circle is 360 degrees.
 Each degree is 60 minutes.
 The full moon is roughly one-half degree in width.
 By remarkable circumstance, the width of the sun is also one-half
degree.
 Each minute is 60 seconds -- sometimes referred to as
arcseconds.
The Meridian
 The great circle on the celestial sphere found by connecting north and south and
passing through the zenith is referred to as the meridian.
 When a celestial body crosses the meridian, it is said to transit.
 When a body transits, it reaches its highest point from the horizon.
 The terms “AM” and “PM” derive their meaning from the meridian :
 AM = Ante-Meridian
 PM = Post-Meridian
The North Celestial Pole and Circumpolar Stars
 Looking north from Chicago at night, one can see the North Celestial
Pole.
 The North Celestial Pole is the direction along which the Earth’s axis is
aligned.
 The stars which immediately surround the pole never set beneath the
horizon. They are called circumpolar stars.
Star Trails Over Mauna Kea, Hawaii
Angles on a Familiar Sphere
 Before describing the celestial sphere in more detail, it helps to recall the
layout of a more familiar sphere -- the Earth.
 On the Earth, angle north or south of the Equator is marked off by
latitude.
 Angle around the Earth from West to East is marked off by longitude.
Daily Motion of the Stars
 The daily motion of the stars Is very simple.
 The celestial sphere makes one full circle about the Earth, once per day.
 The circle is determined by only angle -- the declination, directly
analogous to latitude on the Earth.
Question
 In the Northern hemisphere, the stars rise in the East, set in the
West, and revolve counter-clockwise around the North celestial
pole. In the southern hemisphere the stars rise in the
 A) East, set in the West, and revolve counter-clockwise around the
South celestial pole.
 B) East, set in the West, and revolve clockwise around the South
celestial pole.
 C) West, set in the East, and revolve clockwise around the South
celestial pole.
 D) West, set in the East, and revolve counter-clockwise around the
South celestial pole.
View from North Pole
 At the north pole, the zenith is
the north celestial pole.
 The nadir is the south celestial
pole.
 The horizon is the celestial
equator.
 Precisely half of the celestial
sphere is visible.
 All stars are circumpolar.
View from Equator
 The zenith is the celestial
equator.
 The north celestial pole always
appears directly north.
 The full sky is visible -- each
star rises for 12 hours each day.
View from Chicago
 The altitude of the north celestial
pole is equal to the latitude of
your position on the Earth roughly 42 degrees for Chicago.
 Stars within 42 degrees of the
north celestial pole are
circumpolar.
 Stars within 42 degrees of the
south celestial pole are not
visible.
Summary of Celestial Sphere Viewed fom Earth
Question
 The celestial equator is :
 A) The path of the sun compared with the stars.
 B) The path of the moon compared with the stars.
 C) The average path of planets on the sky.
 D) Always directly overhead at the Earth’s equator.
 E) Always along the horizon at the Earth’s equator.
Constellations
 Constellations are the “states” on maps of the celestial sphere.
 Each region of the sky belongs to precisely one constellation.
 Stars within each region are alphabetically named, starting with the
brightest stars, by a greek letter followed by the constellation name -- eg,
Polaris is Alpha Ursae Minoris.
The Ecliptic
 The sun appears to move along a plane in the sky referred to as the
ecliptic.
 The other planets also appear to move close to the ecliptic.
 Physically, the fact that all solar system bodies lie close to the ecliptic is
because the entire solar system lies within a flattened disk.
The Plane of The Ecliptic From Fire Island
The Solstices and Equinoxes
 The solstices occur when the sun reaches a maximum (solstice = sol
sistere or sun stops in Latin) distance away from the celestial equator -roughly June 21 and December 21.
 The equinoxes occur when the sun intersects the celestial equator -roughly March 21 and September 21. On this day, the sun appears
directly above the equator, and every point on earth has equal day and
night.
Earth on Equinoxes
Yearly Sky and Zodiac
 As the sun moves through the ecliptic, different portions of the night sky
become observable.
 The ecliptic falls into 12 constellations over the year -- the zodiac.
Angle of Inclination of Earth
 The ecliptic makes an angle of 23.5 degrees with the celestial equator.
 Physically, this means the Earth’s rotational axis is tilted with respect to
its orbit.
Angle of Inclination
 As the Earth orbits around the sun, the angle of inclination remains the
same.
Origin of Seasons
 The angle of inclination causes seasonal variation on Earth.
Question
 The ecliptic makes its smallest angle with the southern
hemisphere during the
 A) Summer
 B) Autumn
 C) Winter
 D) Spring
Private Universe
QuickTime™ and a
Animation decompressor
are needed to see this picture.
Lunar Phases
 The appearance of the moon varies over the course of the month.
Eclipses
 The lunar orbit is inclined by 5 degrees relative to that of the Earth/sun.
 Solar eclipses can occur during the new moon, but only when the sun,
moon, and Earth happen to line up.
 Similarly, lunar eclipses can occur during the full moon, but only when
the sun, Earth, and moon happen to line up.
Lunar Eclipses
 The moon passes through the shadow of the Earth.
 Light is fully blocked in the umbra, and only partially blocked in the
penumbra.
Types of Lunar Eclipses
 Three types of Lunar eclipses.
Lunar Eclipses
Question
 What would a total lunar eclipse look like to an observer standing
on the surface of the moon facing the Earth?
Lunar Eclipses from Moon
Solar Eclipses
 Solar eclipses occur when the
sun’s light is blocked by the
moon.
 In a sense, they are completely
serendipitous : the sun is 400
times larger than the moon, but
is also 400 times further away.
 Hence, the apparent angular
size of both the moon and the
sun are nearly identical.
Solar Eclipses
 Three types of solar eclispes can
occur.
August 11, 1999 Eclipse Viewed from Mir
The Eclipse of May 28, 585 BC
 Thales of Miletus is said to have
predicted a remarkable solar eclipse
on May 28, 585 BC.
 Of this occasion, Herodutus writes,
 ‘On one occasion [the Medes and the
Lydians] had an unexpected battle in
the dark, an event which occurred
after five years of indecisive warfare:
the two armies had already engaged
and the fight was in progress, when
day was suddenly turned into night.
This change from daylight to
darkness had been foretold to the
Ionians by Thales of Miletus, who
fixed the date for it within the limits of
the year in which it did, in fact, take
place…The Medes and Lydians,
when they observed the change,
ceased fighting, and were alike
anxious to have terms of peace
agreed on.’
 One must wonder -- how was it
possible for Thales to predict the
eclipse?
Solar Eclipses, 1999 - 2020
 Both lunar and solar eclipses recur with a frequency of 18 years, 11 days,
known as the Saros cycle.
 The Saros cycle was known to the ancient Babylonians, and may have
been used by Thales to predict the eclipse of May 28, 585 BC.
Why are Eclipses so Rare?
 For a total lunar or solar eclipse to occur, there must be a precise
alignment of the Sun, Earth, and moon.
 However, because of the inclination of the moon’s orbit with
respect to the plane of the ecliptic, such alignments are rare.
The Planets
The Motion of Planets
 Like the stars, the planets are generally seen to traverse the sky.
 Unlike the stars, occasionally the planets are observed to stop
and move from west-to-east in so-called retrograde motion.
 This behavior gave rise to the ancient greek name -- “planets”
comes from a Greek root meaning “wanderer”.
 A fully satisfactory explanation of this motion was not developed
until Newton.
The Earth as the Center of the Universe
 Looking up at the night sky, it appears as if the entire Universe
revolves around the Earth.
QuickTime™ and a
Sorenson Video 3 decompressor
are needed to see this picture.
Geocentric Model of the Universe
This observation led the ancients to formulate a geocentric model
of the universe, with the Earth at the center, and the Sun, planets,
and stars all revolving around the Earth along spheres.
Cacophony in the Celestial Harmony -The Problem of Retrograde Motion
 The geocentric model of the universe works very well for stars,
but there is a major problem for planetary motion.
 Occasionally, the outer planets will appear to slow down, stop,
then reverse their direction on the night sky -- retrograde motion.
Retrograde Motion
 The mystery of retrograde
motion can be simply explained
in a model with the Sun at the
center of the Solar system.
 An inner body (like the Earth) is
moving more rapidly than an
outer body (like Mars), and so
will “pass” it much like a faster
car on the expressway.
 During this passing, the outer
planet will execute retrograde
motion.
Retrograde Motion in the Geocentric Model -Epicyclic Motion
 Explaining retrograde motion in the geocentric model of the
universe, however, is almost impossible, unless one invents an
additional circular motion which each planet executes, called
epcicyclic motion.
QuickTime™ and a
MPEG-4 Video decompressor
are needed to see this picture.
Ptolemaic Model of the Solar System
 The ancient astronomer Ptolemy
(90 - 168 AD) created the most
complex version of the
geocentric model of the system,
which was used for almost one
and a half millenia.
 In the Ptolemaic model, the
moon, sun, and planets all
revolved in circles, which
themselves revolved around
circles around the Earth.
 And in fact, the Earth was not
quite at the center of this model,
either.
Why Did the Ancients Reject a Heliocentric
Model of the Solar System?
 In the heliocentric model, due to
the motion of the Earth about the
sun, the motion of the nearest
stars should appear to vary with
respect to the more distant stars.
 This effect is called parallax.
 The ancients attempted to
measure this effect, but failed. In
fact, because the stars are so
distant, it is only detectable with
telescopic measurements.
The Heliocentric World View
 Niklaus Copernicus was a 16th
century scholar and cleric, who
wrote treatises in a number of
fields.
 He is best remembered today for
his revolutionary astronomical
ideas.
Niklaus Copernicus (1473-1543)
The Copernican Model
 Copernicus summarized his model by the following bold (and remarkably
valid) assumptions :
 1.There is no one center of all the celestial circles or spheres.
 2.The center of the earth is not the center of the universe, but only of
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gravity and of the lunar sphere.
3.All the spheres revolve about the sun as their mid-point, and therefore the
sun is the center of the universe.
4….the distance from the earth to the sun is imperceptible in comparison with
the height of the firmament.
5.Whatever motion appears in the firmament arises not from any motion of
the firmament, but from the earth's motion.
6.What appear to us as motions of the sun arise not from its motion but from
the motion of the earth and our sphere, with which we revolve about the sun
like any other planet.
7.The apparent retrograde and direct motion of the planets arises not from
their motion but from the earth's. The motion of the earth alone, therefore,
suffices to explain so many apparent inequalities in the heavens.
Phases of Venus
 In 1610, Galileo used the
telescope to observe the phases
of Venus for the first time from
the Earth.
 The phases only made sense if
Venus orbited the Sun, not the
Earth.
 This proved to be a “smoking
gun” in favor of the heliocentric
model.
Next Week
 I) Planetary Motion
 A) Tycho Brahe and Johannes Kepler
 B) Kepler’s Laws
 II) Physics of Motion
 A) Galileo and the Physics of Kinematics
 B) Newton and Newton’s Laws of Motion
 II) Physics of Matter and Light