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Science 3210 001 : Introduction to Astronomy
Lecture 2 : Visual Astronomy -- Stars and
Planets
Robert Fisher
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 5 PM Tuesday
 5 PM Thursday
 12 Noon Friday
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Review of Lecture 1
 Astronomy is an ancient subject, passed down from Greek to
Islamic scholars, and transmitted back to the west.
 Our systems of thought evolve with time at an almost
imperceptibly slow pace, and continue to do so today.
 The universe is thought to have begun with a big bang, and is
expanding.
 The cosmic calendar varies over fantastically-long timescales.
We are very recent newcomers onto the cosmic scene.
 We are all stardust.
Overview of Lecture 2
 I. The Celestial Sphere
 II. The Stars
 IiI. The Motion of the Planets
Important Lessons to be Learned
 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
 Johannes Kepler’s three laws of planetary motion captures most
features of planetary motion extremely well in a heliocentric
model
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
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.
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
 In the afternoon, the sun begins to set in the west, following the same
circular arc traced in the morning.
 The direction traced by the sun in its arc, facing north, is clockwise.
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.
Angles
 Separation between two points on the celestial sphere are
measured in terms of angle.
 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
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.
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 anti-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 anti-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 everything lies within a flattened disk.
The Solstices and Equinoxes
 The solstices occur when the sun reaches a maximum (solstice = sol
sistere or sun stops in Latin) in declination -- 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 horizon
during the
 A) Summer
 B) Autumn
 C) Winter
 D) Spring
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
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
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 was
probably used by Thales to predict the eclipse of May 18, 584 BC.
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.
Retrograde Motion
 The mystery of retrograde
motion can be explained
relatively simply in a heliocentric
model of the solar system.
 An inner bod y (like the Earth) is
moving more rapidly than an
outer body (like Mars), and so
will “pass” it like a faster car on
the expressway.
 During this passing, the outer
planet will execute retrograde
motion.
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.
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.