Observing the Universe
Reading: Chapter 2: Section 1
The Night Sky
Questions:
• What does the universe look like from
Earth?
• What is the daily motion of stars as seen
from Earth?
• What is the seasonal motion of stars as seen
from Earth?
What the universe looks like
from Earth
With the naked eye,
we can see more
than 2,000 stars as
well as the Milky
Way.
1
The Milky Way
A band of light
making a circle
around the celestial
sphere.
What is it?
Our view into the
plane of our galaxy.
The Milky Way
A map of the sky
Just like a map of the earth is
divided into countries, a map
of the sky is divided into
constellations.
A constellation is a region of
the sky.
88 constellations fill the
entire sky.
Note: The star patterns that we often call constellations are actually
called asterisms. Ancient civilizations often had amazing stories about
asterisms. Eg: The Great Bear, Orion the Hunter….
2
The brightest stars in a constellation…
A. All belong to the same star cluster.
B. All lie at about the same distance from
Earth.
C. May actually be quite far away from each
other.
The brightest stars in a constellation…
A. All belong to the same star cluster.
B. All lie at about the same distance from
Earth.
C. May actually be quite far away from
each other.
The Celestial Sphere
When we look at the sky, we
cannot tell how far different
stars are from us
Ancient astronomers thought
that the sky was a giant
celestial sphere on to which
all the stars were attached.
Although we now know better, the celestial sphere is still
useful for describing the location (but not distance) of
stars in the sky. (We will see later in the course how to
measure distances to stars).
3
The Celestial Sphere
Stars at different distances all
appear to lie on the celestial
sphere.
Ecliptic is Sun’s apparent path
through the celestial sphere.
Why is it tilted from the
celestial equator ?
Note: The celestial sphere is a a useful tool for describing
the sky - it does not really exist.
The Celestial Sphere
The 88 official
constellations cover the
celestial sphere.
From any location on
Earth you can see half
the celestial sphere.
(Why?)
The Local Sky
An object’s altitude (above horizon) and direction
(along horizon) specifies its location in your local
sky
4
The Local Sky
Zenith: The point directly
overhead
Horizon: All points 90°
away from zenith
Meridian: Line passing
through zenith and
connecting N and S points
on horizon
Is the zenith the same as the north celestial pole ?
The Local Sky
Zenith: The point
directly overhead
Horizon: All points
90° away from zenith
Meridian: Line
passing through zenith
and connecting N and
S points on horizon
Note: Zenith is NOT the same as the north celestial pole
unless you are standing at the north pole.
Angular Size
Since we cannot accurately judge how far objects in the
sky are, we CANNOT tell their true size.
However, we CAN talk about the angular size of an
object.
The angular size of an object is the angle between two lines
from the edges of the object to your eye.
Try to estimate the angular size of an object for yourself…
5
Angular Measurements
• Full circle = 360º
• 1º = 60! (arcminutes)
• 1! = 60" (arcseconds)
The angular size of your finger at arm’s length is
about 1°. How many arcseconds is this?
A. 60 arcseconds
B. 600 arcseconds
C. 60 # 60 = 3,600 arcseconds
The angular size of your finger at arm’s length is
about 1°. How many arcseconds is this?
A. 60 arcseconds
B. 600 arcseconds
C. 60 # 60 = 3,600 arcseconds
6
Angular Size and Physical Size
• For objects with small angular sizes, if we know the distance to
the object we can estimate its actual physical size:
Size of the object = length of arc s
Distance to object = radius r of circle
Length of s/circumference of whole circle
= angular size $/angle around whole circle
s " !
= s= " 2# r
2! r 360! 360!
Conversely, if we know the physical size s, of an object, we can
estimate the distance r to it:
s " 360! s
= r= "
2! r 360! ! 2#
Angular Size
360 degrees
angular size = physical size !
2" ! distance
An object’s angular size
appears smaller if it is
farther away
Variation of the sky with position on earth
• When we describe the sky, we must specify where
on earth we are observing the sky from.
• We specify our location on earth using latitude and
longitude .
7
Variation of the sky with position on earth
• Imagine the Earth as a transparent sphere (actually the
because of Earth's rotation, its equator bulges out a little).
• Through the transparent Earth we can see its equatorial
plane. The point O is the center of the Earth.
• The angular elevation of a point P north or south of the
equatorial plane is its latitude.
• All points of equal elevation lie on the same circle of
latitude.
Variation of the sky with position on earth
• Meridians of longitude are semicircles going through the
north pole, south pole and crossing the equator.
• The meridian passing through Greenwich, England is
historically chosen to be 0 degrees. It is called the Prime
Meridian.
• The other longitudes are designated by the angle east or
west of the equator where they cross the equator.
Variation of the sky with position on earth
• The longitude of a point is determined by the meridian
passing through it.
• All points with longitude between 0 and 180 degrees west
of the equator lie in the Western Hemisphere
• All points with longitude between 0 and 180 degrees east
of the equator lie in the Eastern Hemisphere
Laser marking Prime Meridian in London
8
Variation of the sky with position on earth
• Latitude: position on Earth north or south of equator
• Longitude: position on Earth east or west of prime
meridian (runs through Greenwich, England)
What is the latitude and longitude of Waterloo?
Waterloo: 43.5 N 80.5 W
Go to
www.fourmilab.ch/cgi-bin/Earth
to view this location from space
Variation of the sky with position on earth
• Riddle: An astronomer travels 5 degrees of latitude
south, 5 degrees of longitude east, 5 degrees of
latitude north and returns to her starting point, where
she sees a bear. What colour was the bear ?
Ans: The bear was white.
The only place the astronomer
could take the described trip
and return to the same point is
the North Pole, so the bear was
a Polar bear !
Variation of the sky with position on earth
• Longitude helps us determine universal and local time.
• The earth rotates through 360 degrees of longitude in 24
hours. Thus the earth rotates through 15 degrees of longitude
in 1 hour.
• As the Earth rotates around its axis, at any moment one line
of longitude--"the noon meridian"--faces the Sun, and at
that moment, it will be noon everywhere on it.
• Thus when it is noon in Greenwich it is 11:00 at all points
at longitude 15 degrees W and 13:00 at all points 15 degrees
E. (What happens at the 180 degrees W/E longitude ?)
• Astronomers us Greenwich mean time (time at the Prime
Meridian) as the universal time to record events.
9
Variation of the sky with position on earth
Question
The North Star (Polaris) is 50° above your horizon,
due north. Where are you?
A. You are on the equator.
B. You are at the North Pole.
C. You are at latitude 50°N.
D. You are at longitude 50°E.
E. You are at latitude 50°N and longitude
50°E.
Variation of the sky with position on earth
Question
The North Star (Polaris) is 50° above your horizon,
due north. Where are you?
A. You are on the equator.
B. You are at the North Pole.
C. You are at latitude 50°N.
D. You are at longitude 50°E.
E. You are at latitude 50°N and longitude 50°E.
Variation of the sky with position on earth
Altitude of the celestial pole in your sky = your latitude
Since the North star (Polaris) and the Southern Cross
help to identifies the celestial poles, they are very
useful for navigation
10
Daily Motion of Objects in the Sky
Stars rise and set every night
Earth rotates from west to east so
stars appear to circle from east to
west.
Our view of daily motion of the stars
• Stars near the north celestial pole are circumpolar and
never set for observers in the Northern hemisphere (all
latitudes north of the equator).
• In the Northern hemisphere we cannot see stars near the
south celestial pole.
A circumpolar
star never sets
Celestial Equator
This star Your Horizon
never rises
Variation of the sky with position on earth
The night sky varies with latitude but not longitude.
• The night sky depends on latitude because your
position on Earth determines which constellations
remain below the horizon.
11
Question
What is the arrow pointing to?
A. the zenith
B. the north celestial pole
C. the celestial equator
Question
What is the arrow pointing to?
A. the zenith
B. the north celestial pole
C. the celestial equator
Yearly motion of constellations
Constellations we see depend on time of year:
They depend on time of year because Earth’s orbit
changes the apparent location of the Sun among the stars.
Why do we see the same asterisms as ancient
astronomers thousands of years ago ?
12
Summary
• What does the universe look like from Earth?
– We can see over 2,000 stars and the Milky Way with
our naked eyes,
– A map of the sky is divided into 88 regions or
constellations
– The celestial sphere is a convenient tool for
describing the sky
– We can specify the position of an object in the local
sky by its altitude above the horizon and its
direction along the horizon
– We can measure a stars angular size but not it actual
size with our naked eye.
Summary
• What is the daily motion of stars as seen from
Earth?
– Stars rise in the east and set in the west because of the
Earth’s rotation
– The sky varies with latitude but not longitude
– Local time of astronomical events varies with
longitude
– Universal time is taken to be Greenwich mean time
• What is the daily motion of stars as seen from
Earth?
– The constellations depend on time of year because
Earth’s orbit changes the apparent location of the Sun
among the stars.
Mathematical Concepts
• Angular Size
• Altitude, direction
13
