Earth in Space 19-1

1
Earth in Space 19-1

• Astronomy is the study of objects beyond the
  Earths atmosphere.
• It is one of the oldest sciences, as ancient
  people studied the heavens for agricultural and
  religious reasons.

2
Rotation

• Ancient astronomers thought that the sun, moon,
  and stars were moving around the Earth.
• This is because of the Earths rotation.
• The Earth spins on its axis, an imaginary line
  that passes through the center of the planet and
  both poles.

3
Rotation at Night
4
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.

5
Is the north star always Polaris?

• A slow wobble in the Earth's daily rotation
  causes the Earth's north pole to trace a circle
  among the stars every 26,000 years.
• 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.

• Polaris, the tip of the Little 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.
• 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.

6
(No Transcript)
7

• In 14,000 years Vega will become the pole star.
• This extremely slow but steady cycle of
  precession was discovered around 150 B.C by the
  ancient astronomer Hipparchos.

8
Revolution

• Revolution is the movement 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 30-km/sec.
• It takes the Earth about 365.25 days to make one
  orbit of our Sun.

9
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.

10
Tilt

• The Earths 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.

11
Earths 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.

12
Calendars

• The Earths 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.

13
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.

14
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!

15
Equinoxes

• Means equal night. 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!

16
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 Earths axis).
• Occurs on the summer solstice.

17
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 Earths axis).
• Occurs on the winter solstice.

18
Arctic/Antarctic Circle

• Point where at least one day a year the sun never
  sets/rises.