Chapter 17   Earth's Cycles

 

I.    Chapter 17  Earth’s Cycles

A.     The Earth moves steadily and smoothly

1.   You are moving with it.

2.   You can’t sense the movement.

3.   Clues to the movement

a)   Sun and stars appear to move across the sky

b)   The seasons change during the year.

B.     Earth’s rotation

1.    The spinning of Earth around its axis

a)  Causes day and night

2.    An axis is an imaginary line that goes through its center from North Pole to South Pole.

a)    The Earth rotates from west to east.

b)    The sun and the stars appear to move from east to west in the sky.

3.    Shadows change their position and length as the Earth rotates

a)   Sunlight shines on an object from different angles.

b)    Shadows are longest in the morning and evening.

(1)  The sun is lowest in the sky.

c)    Shadows are shortest in the middle of the day.

(1)   The sun is overhead at noon.

C.     Earth’s Revolution

1.   The movement of an object around another object is revolution.

2.   The path the Earth takes around the sun is its orbit.

a)   It takes about 365 1/4 days to complete one orbit.

b)   The shape of its orbit is an ellipse.

(1)  An ellipse is a circle stretched out in opposite directions.

(2)  This causes the Earth’s distance from the sun to change as it revolves.

3. Gravity between the Earth and the Sun keeps Earth revolving.

D.     Earth’s axis is always tilted in the same direction as it revolves.

a)   The revolution and tilt of the Earth cause seasons.

b)  The Northern Hemisphere is tilted toward the sun in late June.

(1)  It gets more direct sunlight.

(2) Daylight lasts longer.

(3)  It is summertime.

(4)  The Southern Hemisphere is tilted away from the sun.

(a)  Less direct sunlight.

(b) Nights are longer than days.

(c)   It is wintertime.

c) In late December the seasons are reversed in each hemisphere.

d) During spring and fall the lengths of day and night are equal.

II.  Moon

A.     Shines by reflected light from the Sun.

B.     The moon revolves around Earth.

C.     The moon makes a complete revolution around the Earth in about 29 days.

D.     Gravity keeps it in its orbit.

E.      It also rotates once for every time it revolves around the Earth.

F.      The same side of the Moon is always facing Earth.

G.     Phases of the Moon

1. Phases are apparent changes of the Moon’s shapes.

a)   New Moon

b)   Crescent

c)  First quarter

d)   Full Moon

e)  Last quarter

2.   When the lighted half of the Moon directly faces Earth we see the Full Moon.

H.     Eclipses

1.  An eclipse occurs when one object in space gets between the Sun and another object, and casts a shadow on the other object.

2.  Lunar eclipse

a)  During some full Moons the Moon and Sun are on exactly opposite side of the Earth.

b)  The Moon passes through the Earth’s shadow.

c)   This can last as long as 100 minutes.

3.  Solar Eclipse

a)    Occurs when the Moon passes between the sun and the Earth and Casts its shadow on the Earth.

b)    The Moon’s shadow covers only a small part of Earth.

III.  Stars

A.     The Sun is the star closest to the Earth.

1.  It provides energy and life for all living things.

2.  The Sun is a medium-sized star.

3. During the day you can not see the other stars because the Sun is so bright.

B.     The star patterns are called constellations.