Download The Local Sky The Local Sky

The Local Sky
An object’s altitude (above horizon) and direction
(along horizon) specifies its location in your local
sky
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 ?
1
Specifying Locations 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 .
Specifying Locations 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.
2
Specifying Locations 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.
Specifying Locations 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
3
Specifying Locations 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
Specifying Locations 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 ?
4
Specifying Locations 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.
Specifying Locations on Earth
•  Longitude helps us determine universal and local time.
•  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 use Greenwich mean time (time at the Prime
Meridian) as the universal time to record events.
5
Stellar Coordinates
•  Declination: stellar ‘latitude’
•  Right Ascension: stellar ‘longitude’
Variation of the sky with position on
earth
Question
The North Star (Polaris) is 50° above your horizon,
due north. Where are you?
A. 
B. 
C. 
D. 
E. 
You are on the equator.
You are at the North Pole.
You are at latitude 50°N.
You are at longitude 50°E.
You are at latitude 50°N and longitude
50°E.
6
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
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. 7
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.
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.
8
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
9
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 constellations as ancient
astronomers thousands of years ago ?
Precession of Earth’s axis
•  Although the axis seems fixed on human time
scales, it actually precesses over about 26,000 years.
•  The gravity of the Sun and Moon cause the Earth’s
axis to precess in a slow circular sweep.
Earth’s axis
precesses like
the axis of a
spinning top
10
Precession of Earth’s axis
•  Hipparchus discovered the precession of the earth’s
axis over 2000 years ago !
•  Precession may affect the severity of the different
seasons and may cause extreme climate changes
such as ice ages
Earth’s axis
precesses like
the axis of a
spinning top
Precession of Earth’s axis
–  In about 12,000 years, the pole will have moved to the
apparent vicinity of the very bright star Vega (alpha
Lyrae).
–  The figure shows
the apparent path
followed by the north
celestial pole over
thousands of years.
11
Observing the skies for yourself
•  Find a dark spot far from light pollution of cities and cars
•  Use a sky chart (star map). There are some in your text.
•  Turn the chart until the horizon you are facing is at the bottom.
•  The stars on the bottom of the chart are now directly in front of you
•  The centre of the chart shows the overhead sky
•  The edges show the horizon around you
•  A pair of binoculars will be helpful to pick out faint objects
•  Try to find the brighter constellations and stars first
http://media.skyandtelescope.com/documents/GettingStartedNorth.pdf
Sky Chart: Northern Hemisphere
12
Sky Info: Northern Hemisphere
More resources
http://www.fourmilab.ch/yoursky/
http://www.planetarium.montreal.qc.ca/Information/ciel_du_mois_a.html
http://maps.seds.org/Const/constS.html
http://www.nrc-cnrc.gc.ca/eng/education/astronomy/constellations/index.html
http://hubblesite.org/explore_astronomy/tonights_sky/
http://www.skyandtelescope.com/observing/ataglance
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.
–  Many tar names have ancient Latin, Greek or Arabic roots
–  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.
13
Summary
•  What does the universe look like from Earth?
–  The brightness of stars is measured using the apparent
visual magnitude scale originally used by Hipparchus and
Ptolemy
–  Modern instruments measure the flux: the total light
energy hitting one square metre per second
–  Flux can then be used to calculate apparent visual
magnitude
–  Some stars are so bright they have negative magnitudes
–  Faint stars detected by telescopes have magnitudes larger
than 6
–  Apparent visual magnitude is based only on visible light.
–  Apparent visual magnitude does not say anything about the
stars actual energy output since their distance is not
included.
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.
14
Specifying Locations in the sky
•  Declination (DEC) and Right
Ascension (RA) are extensions of
latitude and longitude to the celestial
sphere.
•  Declination is measured in positive
or negative degrees going north or
south of the celestial equator
•  RA is measured in hours which
increase going east.
•  0 hrs corresponds to the crossing of
the ecliptic and the celestial equator
15