Astronomy Picture of the Day
Venus goes through phases. Just
like our Moon, Venus can appear
as full as a disk or as a thin as a
crescent. Venus, frequently the
brightest object in the post-sunset
or pre-sunrise sky, appears so
small, however, that it usually
requires binoculars or a small
telescope to clearly see its current
phase.
The time-lapse sequence was
taken over the course of many
months and shows not only how
Venus changes phase but how it's
apparent angular size also
changes.
Homework #1 will be posted soon.
An announcement and email will be sent to
class members providing information and
instructions.
You will typically have 5 or more days to
complete a homework assignment.
DISTANCE SCALE
Seeing the past
Units of Distance in Astronomy
Meter: ~10% larger than 1 yard
Astronomical Unit (AU): Earth – Sun distance,
~ 150,000,000 kilometers
Light Year (LY):
distance light travels in one
year, ~ 9.46 x 1012 km
Parsec (pc): ~ 3.26 light years
T
i
m
e
Big Bang (~ 13.7 x 109 yrs ago)
Expansion & Cooling
Initially only Hydrogen & Helium
Growth of structures (gravity)
Stars and Galaxies form
Nucleosynthesis - “star stuff”
Planets possible - “heavier” elements
Solar System forms (4.6 x 109 yrs ago)
Which of the following best describes what we
mean by the “universe”?
(red) A vast collection of stars that number as
many as the grains of sand on all the beaches on
Earth.
(blue) All the galaxies in all the superclusters.
(yellow) The universe is another name for our
Milky Way Galaxy
(green) The sum total of all matter and energy.
Which of the following best describes what we
mean by the “universe”?
(red) A vast collection of stars that number as
many as the grains of sand on all the beaches on
Earth.
(blue) All the galaxies in all the superclusters.
(orange) The universe is another name for our
Milky Way Galaxy
(green) The sum total of all matter and energy.
Which of the following has your "cosmic address" in the
correct order?
You, Earth, solar system, Milky Way Galaxy, Local Group,
Local Supercluster, universe
You, Earth, Local Group, Local Supercluster, solar system,
Milky Way Galaxy, universe
You, Earth, solar system, Local Group, Local Supercluster,
Milky Way Galaxy, universe
You, Earth, solar system, Local Group, Milky Way Galaxy,
Local Supercluster, universe
Which of the following has your "cosmic address" in the
correct order?
You, Earth, solar system, Milky Way Galaxy, Local Group,
Local Supercluster, universe
You, Earth, Local Group, Local Supercluster, solar
system, Milky Way Galaxy, universe
You, Earth, solar system, Local Group, Local
Supercluster, Milky Way Galaxy, universe
You, Earth, solar system, Local Group, Milky Way
Galaxy, Local Supercluster, universe
“Spaceship Earth”
● Earth rotates about an axis (day)
“Spaceship Earth”
● Earth rotates about an axis (day)
●
Earth revolves about (orbits) the sun (year)
“Spaceship Earth”
● Earth rotates about an axis (day)
●
Earth revolves about (orbiting) the sun (year)
●
Earth’s rotation axis is tilted by 23.5 degrees to the orbital plane (seasons)
“Spaceship Earth”
● Earth rotates about an axis (day)
●
Earth revolves about (orbiting) the sun (year)
●
Earth’s rotation axis is tilted by 23.5 degrees to the orbital plane (seasons)
● The Sun and solar system orbit around the center of the Milky Way Galaxy
How do we know these
things?
It all begins with observations.
What are the “observed facts”?
How do we explain these observed facts?
Ultimately, what are the underlying laws of
nature that underpin these explanations?
The origins of astronomy and the physical
sciences stem from:
 observing the sky and nature
 finding repeatable patterns
 seeking natural explanations for these patterns
Celestial Sphere
Large imaginary spherical
surface centered on the
Earth.
Stars and other celestial
objects “fixed” on its
surface.
Conceptual Model, not a
physical model
The North Celestial Pole
(NCP) and the South
Celestial Pole (SCP) are
located at the intersection
of the earth’s rotation axis
with the celestial sphere.
The Celestial Equator is
the extension of the
Earth’s equator onto the
celestial sphere.
We view the
celestial sphere
in the context of
the “local sky”
Observer’s in
different locations
see a different
sky
Motions of the Earth (rotation,
revolution about the sun) cause the
portion of the celestial sphere visible
in the local sky to change with time.
“Local Sky” positions
(defined by observer's location)
Cardinal directions – NSEW
Zenith – point directly overhead
Nadir - point directly opposite zenith
Horizon – half way between zenith and nadir
Meridian – line from north point on horizon,
through zenith, to south point on horizon
(divides sky into eastern and western halves)
Local sky positions remain fixed:
horizon, zenith, cardinal locations,
meridian, are always in same location.
In general, most stars are seen to
 rise in the eastern half of the sky (i.e.,
east of the meridian),
 increase their altitude (angular distance
from the horizon) until they cross the
meridian ,
 set in the western half of the sky (i.e.,
west of the meridian).
However, some stars (called “circumpolar stars”)
are always above the horizon.
What about the Sun’s motion?
 Moves east to west, full circuit around the sky, each
“day”
 Moves slowly eastward each day (~ 1 degree/day),
relative to the stars: full circuit in one “year”
 Path through sky is a great circle called the “ecliptic”
 Constellations ecliptic passes through are referred to
as the “zodiac”
The sun follows the same path around the sky (celestial
sphere), repeating this journey once every year.
This path is called the “Ecliptic”
Why does the sun appear
to move like this?
A Geometrical aside…
Great Circles and Spheres
Great Circle
Intersection of a sphere with a plane
passing through the center of the sphere.
●
A great circle
divides a sphere
into two equal
hemispheres
Small Circle
●
Intersection of a sphere with a plane not
passing through the center of the sphere.
A small circle
divides a sphere
into two unequal
portions
Great Circles
Small Circles
The Ecliptic is a great circle
Locations of planets in the sky
Mercury: always close to Sun in sky
Venus: always close to Sun in sky
 Mars: no restrictions on distance from Sun in sky
 Jupiter: no restrictions on distance from Sun in sky
 Saturn: no restrictions on distance from Sun in sky
What causes these differences?
Motions of the planets
 On short term (diurnal motion), planets appear to move
with the stars, east to west, making a full circuit around the
sky (meridian to meridian) in approximately one day
 Most of the time, planets move slowly eastward each day
relative to the stars: different planets at different rates
What causes these motions?
 Planets are always
close to the “ecliptic”,
the apparent annual
path of the sun through
the sky.
Close grouping of five
planets in April 2002.
This is a pattern that
was well known to the
“ancients”
Why are the planets
restricted to these
locations?
Some planets occasionally reverse their motion
relative to the stars, moving slowly westward
relative to the stars, for a few days
apparent retrograde motion
What causes this?
What causes this?
The development of Astronomy as a Science
 Prehistory - development of myths to explain natural
phenomena - no physical model
 Later, many threads of physical models were developed,
most were lost
 We owe the origins of the physical sciences to the
Greeks, later threads came from many other cultures.
 Physical models are developed to explain observations
(everything starts with observations)
 These models then make further predictions that can be
tested.
 Understanding and explaining motions
observed in sky led to geometric models for the
geometry of the universe.
 These models, in turn, motivated the
development of the physical sciences: required to
explain this geometry.
 Ultimately, this led to an understanding of how
stars and planets form.
Understanding the visible sky and motions within
the sky is easily accomplished


using the concept of Celestial Sphere
understanding the rotational & orbital motions of
the Earth
As you move around on the Earth, the
portion of the celestial sphere visible in your
local sky changes.
1.
At what location on
the Earth will the
North Celestial Pole
appear directly
overhead?
Where on Earth is the
Celestial Equator
directly overhead?
How much of the
Celestial Sphere can
you see at any given
time?
Half of the sky is always visible above the
horizon.
Moving north or south on the Earth (changing
your latitude), leads to viewing a different portion
of the celestial sphere.
If you are located in the
northern hemisphere,
what is the point in the
sky around which
objects on the Celestial
Sphere appear to rotate
over the course of a
day?
Do they appear to rotate
clockwise or counterclockwise?
Discuss with your
neighbours
What is the relationship
between the altitude of the
North Celestial Pole and the
latitude of an observer on
Earth.
(hint: imagine observer’s at
the North Pole, at the
Equator, and at a midlatitude location, such as
Bloomington).
Today we divide the
celestial sphere into
88 regions, referred
to as constellations
(in a manner similar
to dividing the
Earth’s surface into
countries).
Circumpolar Stars:
never set
ConceptTest
As you move from the Earth's equator
toward the North Pole, the number of
circumpolar stars:
increases
decreases
stays the same
ConceptTest
As you move from the Earth's equator
toward the North Pole, the number of
circumpolar stars:
increases
b. decreases
c. stays the same
On the Earth any location can be given by two quantities:
Latitude (angular distance Longitude (angular distance
east or west from the
north or south of the
“Prime Meridian”)
equator)
Two quantities
(coordinates)
+
Locations that
define the origins
(“zero points”)
=
all that is needed
to locate anything
on a sphere
On the Celestial sphere
any location can be
given by its:
Declination (Celestial
Latitude, measured
north or south of the
Celestial equator)
 Right Ascension
(Celestial Longitude,
measured (angular
distance from the
“Vernal Equinox”)
Back to our story…
The north celestial pole (NCP) and
the south celestial pole (SCP) are
located at the intersection of the
earth’s rotation axis with the
celestial sphere.
The Celestial equator is the
extension of the Earth’s equator
onto the celestial sphere.
CONSTELLATIONS
The celestial sphere
is divided into 88
regions, referred to
as constellations,
in a manner similar
to dividing the
Earth’s surface into
countries.
On the Earth any location can be given by two quantities:
Latitude (angular distance Longitude (angular distance
east or west from the
north or south of the
“Prime Meridian”)
equator)
Two quantities
(coordinates)
+
Locations that
define the origins
(“zero points”)
=
all that is needed
to locate anything
on a sphere
On the Celestial sphere
any location can be
given by its:
Declination (Celestial
Latitude, measured
north or south of the
Celestial equator)
 Right Ascension
(Celestial Longitude,
measured (angular
distance from the
“Vernal Equinox”)
Local Sky: Defined by the observer
We view the celestial sphere in the
context of the local sky
So, we have a sphere (the Earth) within a sphere
(Celestial sphere). If both were fixed and
unchanging, we would always look to the same point
in the sky to see the same object. BUT…
 We can be located anywhere on the Earth
 The Earth is rotating
 The Earth is orbiting the Sun
 The Moon is orbiting the Earth
 The planets are also orbiting the sun
We need to take all of this into account to understand
changes in the sky, time, seasons, etc.
GROUP ACTIVITY
Schmooze: (from a Yiddish word meaning to chat)
 Assemble into groups of 5 or 6 students
converse
idly orfor
casually
(1)To
Select
a moderator
the dayand in a friendly manner,
especially in order to make a social connection.
 Introduce yourselves & “schmooze” for a couple minutes
(2)
Discuss
youinstance
are taking
the class
The actwhy
or an
of schmoozing.
 Put together a list of names of the group to turn in
 After a few minutes, we will have an exercise
Do the instructed exercise within the group have everybody do it but discuss within the
group)
Think about:
What changes with rotation?
What changes with “latitude”?
What doesn't change?
What can you see, what can’t you see?
“Local Sky” positions
(defined by observer's location)
Cardinal directions – NSEW
Zenith – point directly overhead
Nadir - point directly opposite zenith
Horizon – half way between zenith and nadir
Meridian – line from north point on horizon,
through zenith, to south point on horizon
(divides sky into eastern and western halfs)
Local sky positions remain fixed:
horizon, zenith, cardinal locations,
meridian, are always in same location.
Motions of the Earth (rotation, revolution
about the sun) cause the portion of the
celestial sphere visible in the local sky to
change with time.