Download lecture 1 - University of Florida Astronomy

Daily Motions
• Sun:
Daily Motions
• Stars:
– stars travel on arcs
in the sky moving
from east to west.
– some stars rise and
set
– others do not and
are circumpolar
– due to Rotation of
Earth
• 23 hour 56 min
– rises in the east
– sets in the west
– travels on an arc
across the sky
– 24 hours
Solar Day = 24 hours
Solar and Sidereal Days
Why is the Solar Day longer than the Sidereal Day?
Sidereal Day = 23 hours 56 min
Annual Motions of the Sun
• The altitude of the Sun changes with season
– It reaches a maximum on the summer solstice
(June 21 in the northern hemisphere)
– Its minimum altitude is reached on the winter
solstice (December 21 in the northern
hemisphere)
• The sunrise and sunset points along the
horizon also change with season
– It rises and sets due east on the equinox (Sep 21,
and March 21)
– It rises north of east in the
summer south of east in the
winter
Annual Motion of the Stars
Coordinate system on Earth
• Every day a given star or
constellation rises and sets 4
minutes earlier with respect to the
Sun
• Therefore after one year it returns
to the same position relative to
the Sun
Gainesville: 29.6510N, 82.3250W
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Celestial Sphere
• You can imagine that the
stars and sun are attached to
the surface of a great sphere
• The earth appears to be at
the center of the sphere
• The sphere rotates from east
to west on an axis that points
to the north celestial pole
• To explain the daily motions
of the sky you can imagine
the sphere rotating once in
23 hours 56 minutes
Where are we?
• The position of a
celestial pole near
the horizon, tells us
we must be near
the Equator. Also
consider the angles
the star trails make
with the horizon: 90
degrees.
Parts of the Celestial Sphere
• Celestial pole - extends from
the rotation axis of the earth
– all stars appear to rotate
about this point
– some stars are
circumpolar
– some stars rise & set
• Celestial equator - extends
from the earth’s equator
• Your latitude on earth
determines the angle at
which stars will rise & set
with respect to your local
horizon
Question
• You are kidnapped, drugged and placed in
a dark room for an indefinite period of time!
• You escape and find a radio to call for help
but you don’t know where you are!
• Luckily, it’s a clear night. You watch the
stars for while and you see the stars rising
at a 20 degree angle with respect to your
horizon.
• What can you tell your rescuers about your
location?
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Annual Motion of Sun
• The sun moves along a
repeatable path on the celestial
sphere throughout the year. This
path is called the ECLIPTIC
– Sun moves east ward relative
to stars on celestial sphere
– It moves 360 degrees in a
year about 1 degree per day
– The constellations through
which we see the sun move
are the constellations of the
ZODIAC
• This apparent motion is due to
the Earth orbiting the Sun
Annual Motions of the Stars
Annual Motions
of the Stars
• The constellations that
are visible on a given
night are those opposite
from the Sun on the
celestial sphere
• Thus you see different
stars at different times of
the year as the Sun
moves along the ecliptic
relative to the stars
• The apparent motion is
due to the Earth orbiting
the Sun
Coordinate system on Earth
Gainesville: 29.6510N, 82.3250W
Coordinate Systems
• Altitude : angular
distance above
horizon
• Azimuth : angular
distance measured
along horizon to
location of object
• By convention azimuth
is measured from the
north going eastward
• Advantages
• Disadvantages
Equatorial Coordinate System
Declination – the angular
distance of a star along
a circle north or south of
celestial equator
(degrees:arcmin:arcsec)
Right Ascension – the
angular distance of a
object measured
eastward from the vernal
equinox along the
celestial equator (H:M:S)
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Seasons
• With the annual motion of the
Earth around the Sun, we notice
a cyclic pattern in the weather,
known as the seasons.
• Observations:
– Seasons are reversed in the Northern and
Southern Hemispheres
• For example, you’re at the beach at Cape Cod
in July, they’re skiing in New Zealand!
– The region near the equator is hottest; poles are
the coldest
– The Sun’s position in the sky changes
Reason for Seasons: Distance?
• Predictions:
Reasons for the Seasons?
• What causes this pattern?
• Two Possible Models:
– Distance of the Earth from the Sun?
– Tilt of the Earth’s axis with respect to the
Earth’s orbital plane (ecliptic)
Reason for Seasons: Distance?
• Observations:
– Relation between Earth-Sun distance and
season
• for example, the Earth should be closest to the
Sun in summer
– Same season every where on Earth
• since the Earth is tiny compared to the EarthSun distance
Reason for Seasons: Distance?
• Predictions don’t match Observations:
– Seasons are reversed in the Northern and
Southern Hemispheres
• For example, you’re at the beach at Cape Cod in
July, they’re skiing in New Zealand!
Reason for the Seasons: Tilt?
• The Earth’s axis of rotation is tilted by 23.5
degrees with respect to the ecliptic pole
• As the Earth moves around the Sun, the Earth’s
north pole points to a fixed position in space
– The Earth is closest to the Sun during
northern winter
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Reason for the Seasons: Tilt?
• Predictions:
– Summer occurs in hemisphere pointed
toward the Sun
– Winter occurs in the hemisphere pointed
away from the Sun
– Therefore: Seasons are reversed in
northern and southern hemisphere
Reason for the Seasons: Tilt
• The angle at which
the Sun’s rays strike
the surface of the
Earth is closer to a
right angle (90
degrees) in the
summer
• Therefore the Earth’s
surface intercepts
more rays of Sunlight
and hence more
energy
• This is the main
reason that it is
warmer in the
summer
Reason for the Seasons: Tilt
• Predictions match observations:
– The northern hemisphere and southern
hemisphere seasons are reversed
– The region near the equator is hottest; poles
are coldest
Reason for the Seasons: Tilt
• It’s warmer in the
summer and colder in
the winter for two
reasons:
– the Sun is higher in
the sky in the
summer
– the days are longer
– the Sun reaches it’s highest
point in the sky on the
summer solstice (June 21 in
northern hemisphere)
Reason for the Seasons
• Demonstration
Question
• When is the hottest time of year in Quito
Ecuador?
• If the Earth’s rotation axis was
perpendicular to it’s orbital plane
(ecliptic), what would you expect the
seasons to be like?
• If the Earth’s rotation axis was parallel
to it’s orbital plane (ecliptic), what daily
and annual effects would this have?
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Temperature in Ecuador
Gainesville: 29.6510N, 82.3250W
What is special about the Tropics (i.e. between
+23.5degrees and -23.5 degrees latitude)?
Assignment
Reference Reading – Astronomy & Astrophysics
by Zelik and Gregory
- chapter 1, chapter 3,
chapter 4-1 & 4-2
The Physical Universe, An Introduction to
Astronomy by Shu
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