Download Topic 4: Earth-Moon-Sun

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Topic 4
Earth, Moon, Sun
 Celestial Observations
celestial sphere: a model of the sky, consisting of an imaginary sphere (with the Earth at the center) on
which the heavenly bodies are projected
celestial equator: a projection of Earth’s equator
celestial poles: a projection of the Earth’s North and South Poles
altitude: angular distance above the horizon
horizon: the edge, or rim, of the celestial sphere (everything at a 0 angle to the observer)
zenith: the highest point in the sky, or the point directly above the observer (90)
zenith
star with
90 altitude
star with
30 altitude
horizon
 30
observer
horizon
Rotation of the Earth
The Earth’s axis of rotation is an imaginary straight line through the Poles; tipped 23½ perpendicular to
the plane of Earth’s orbit
As Earth rotates west to east, celestial objects appear to rise in the east and set in the west.
Earth rotates 360 in 24 hours, so objects appear to move 15/hr.
The result of Earth’s rotation is our day and night cycle.
 Earth’s Revolution around the Sun
Because the Earth is tilted, the celestial equator is tilted……so sometimes the Sun appears above (north
of) the Equator, and sometimes the Sun appears below (south of) the Equator.
The Sun reaches its farthest point north of the Equator on June 21 st, the summer solstice.
The Sun reaches its farthest point south of the Equator on December 21st, the winter solstice.
The Sun crosses the Equator on March 21st, the vernal equinox, and on September 23rd, the autumnal
equinox.
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 Seasons
Seasons on the Earth are caused by:
1) the revolution of the Earth around the Sun
2) the tilt of the Earth on its axis
3) the parallelism of the Earth’s axis (every position of the Earth’s axis is parallel to every other position
of the Earth’s axis)
On about June 21st, the Northern Hemisphere is tilted toward the Sun so the Tropic of Cancer (23½
North latitude) is receiving the Sun’s vertical/direct ray. The Northern Hemisphere experiences more
hours of daylight and has its summer. The Southern Hemisphere is tilted away from the Sun causing it to
experience more hours of darkness and winter.
On about December 21st, the Southern Hemisphere is tilted toward the Sun so the Tropic of Capricorn
(23½ South latitude) is receiving the Sun’s vertical/direct ray and has it’s summer and longer days.
On about March 21st and September 23rd, neither hemisphere is tilted toward the Sun and the
vertical/direct ray is on the Equator, giving every place on Earth equal hours of daylight and darkness.
The locations of the Tropics are determined by the positions of the Sun……the maximum number of
degrees of latitude (north and south) that the noon Sun can ever be directly overhead.
The Sun is NEVER directly overhead at our latitude.
On June 21st, the Northern Hemisphere is tilted toward the Sun 23½; therefore, as the Earth rotates on
its tilted axis, every place within 23½ of the North Pole receives 24 hours of daylight. Every place
within 23½ of the South Pole never receives the Sun and has 24 hours of darkness (and vice versa for
December 21st).
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The locations of the Circles are determined by the positions of the Sun…….the maximum distance from
the Poles that can have 24 hours of daylight.
Arctic Circle = 66½ North latitude
Antarctic Circle = 66½ South latitude
On March 21st and September 23rd (the equinoxes), neither Pole is tilted toward the Sun so every place on
Earth has 12 hours of daylight and 12 hours of darkness.
Apparent Daily Motions of the Sun (at 42 North latitude)
The Sun’s apparent path is an arc -- it rises in the east, increases in altitude until it reaches its highest
point in the sky at solar noon (due south) and then decreases in altitude, setting in the west.
Because the Earth is tilted, sometimes the Sun is north or south of the Equator:
On June 21st, the Sun is north of the Equator at 23½ N, so the Sun rises north of east, reaches its
highest noon altitude of the year (71.5) and sets north of west, giving us approximately 15 hours of
daylight and 9 hours of darkness. [see “c” below]
On December 21st, the Sun is south of the Equator at 23½ S, so the Sun rises south of east, reaches its
lowest noon altitude of the year (24.5) and sets south of west, giving us approximately 9 hours of
daylight and 15 hours of darkness. [see “a” below]
On March 21st and September 23rd, the Sun is on the Equator, so it rises due east, reaches a noon altitude
of 48 and sets due west, giving every place on Earth 12 hours of daylight and 12 hours of darkness. [“b”]
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 Motions of the Moon
The Moon appears to rise in the east and set in the west, moving at a rate of 15/hr, due to Earth’s
rotation.
However, the Moon is also orbiting Earth at a rate of 13/day. Since it takes the Earth 50 minutes to
“catch up” with the Moon’s orbit, the Moon rises 50 minutes later each day.
Moon’s period of rotation = 27 days
Moon’s period of revolution = 27 days
Because its period of rotation equals its period of revolution, there is only one side of the Moon always
seen from Earth.
Moon phases are the changing appearances of the Moon as seen from Earth, caused by the Moon’s
revolution around the Earth.
The period between one cycle of phases = 29½ days, which is longer than the period of revolution because
the Earth has also been revolving around the Sun during that time and the Moon needs to “catch up.”
Phases of the Moon
The photographs show what the moon looks like from Earth. The
drawings show what the moon would look like from space.
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 Tides
A tide is the periodic rise and fall of sea level, caused by a giant wave formed by the gravitational
attraction between the Earth, Sun, and Moon. Because the Moon is so much closer, it plays more of a role
than the Sun.
The tidal range is the difference between the level of the ocean at high and low tide.
 Eclipses
Eclipses occur when the Earth or Moon temporarily blocks the sunlight reaching the other.
Eclipses can occur only when the Sun, Earth, and Moon are perfectly lined up (at New and Full phases) and
along the same plane (which is rare because the Moon’s orbit it tilted 5).
Solar eclipse: When the noon Sun gets blocked out by the passing New Moon.
Lunar eclipse: When the Full Moon gets blocked out by the passing Earth