Transcript Earth in Space 19-1

Earth in Space 19-1
 Astronomy is the study
of objects beyond the
Earth’s atmosphere.
 It is one of the oldest
sciences, as ancient
people studied the
heavens for agricultural
and religious reasons.
Rotation
 Ancient astronomers thought
that the sun, moon, and stars
were moving around the Earth.
 This is because of the Earth’s
rotation.
 The Earth spins on its axis, an
imaginary line that passes
through the center of the
planet and both poles.
Rotation at Night
Axis
 The Earth spins on its
axis at about 1,600
km/hour at the equator
towards the east.
 This is why the sun
rises in the east and
sets in the west.
 The axis is currently
pointing toward the
star Polaris, the North
Star.
Is the north star always Polaris?
 A slow wobble in the Earth's
 Polaris, the tip of the Little
daily rotation causes the
Earth's north pole to trace a
circle among the stars every
26,000 years.
Dipper's handle, currently
lies within three-quarters
of a degree from the polar
point, and will reach its
closest proximity--under
half a degree--in the year
2102 AD.
 Because of this motion,
called precession, the star
nearest the pole is not always
the same.
 Architects of the great
Egyptian pyramid used
Thuban (TOO-bahn), a star in
the constellation Draco the
Dragon, for their north star.
 No matter where you are in
the northern hemisphere,
when you face Polaris you
are facing north. Polaris
now points northward
more accurately than a
magnetic compass.
 In 14,000 years Vega will
become the pole star.
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This extremely slow but
steady cycle of precession
was discovered around 150
B.C by the ancient
astronomer Hipparchos.
Revolution
 Revolution is the movement
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of one object around
another caused by gravity.
One complete revolution is
called a year.
The path around the object
is called an orbit.
The Earths orbit is a slightly
flattened circle called an
ellipse. It travels at 30km/sec.
It takes the Earth about
365.25 days to make one
orbit of our Sun.
Day and Night
 Rotation causes the
Earth to have day and
night.
 It takes about 24 hours
for the Earth to rotate
on its axis once. We
call this 24 hour cycle a
day.
Tilt
 The Earth’s axis is
tilted 23.5° from the
vertical.
 The tilt is always
towards the North
Star, so part of the
year the north axis is
pointed towards the
sun, and part of the
year it is pointed
away.
 This causes the Earth
to have seasons.
Earth’s Seasons
 The Equator is warmer
because it receives
sunlight more directly than
the poles.
 Without the tilt Earth would
not have seasons.
 Summer and winter are
not affected by changes in
Earth's distance from the
sun. In fact, when the
Northern Hemisphere is
having summer, Earth is at
its greatest distance from
the sun.
Calendars
 The Earth’s year is
365.25 days. That is
why our calendar has a
leap year every four
years.
 Calendars were
originally made to make
astronomical
predictions.
Lunar Calendars
 The moon revolves
around the Earth every
27.3 days.
 This is why people that
use lunar calendars will
have years with 12 or
13 months.
Solstices
 Sun is directly at 23.5° north
or south latitude.
 Summer (June 21) solstice
has the longest
daylight/shortest night in the
Northern Hemisphere.
 The winter solstice
(December 21) is the
shortest daylight/longest
night in the Northern
Hemisphere.
 Of course these are reversed
if you live in Australia!
Equinoxes
 Means “equal night.”
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Days and nights are
equal in time.
Axis is not tilted towards
or away from the Sun.
Sun is directly over the
equator.
Vernal equinox (spring)
occurs around March
21st.
Autumnal equinox
occurs around
September 23rd.
Again if you lived in the
Southern Hemisphere
these would be
reversed!
Tropic of Cancer
 Northern most point where the sun can be
directly overhead at noon.
 Located at 23.5 degrees north latitude (pretty
much the same at the tilt of the Earth’s axis).
 Occurs on the summer solstice.
Tropic of Capricorn
 Southern most point where the sun can be
directly overhead at noon.
 Located at 23.5 degrees south latitude (pretty
much the same at the tilt of the Earth’s axis).
 Occurs on the winter solstice.
Arctic/Antarctic Circle
 Point where
at least one
day a year
the sun never
sets/rises.