8.5: Motions of Earth, the Moon, and Planets
pg. 320
Key Concepts:
1. Careful observation of the night sky can offer clues about the motion
of celestial objects.
2. Celestial objects in the Solar System have unique properties.
3. Some celestial objects can be seen with the unaided eye and can be
identified by their motion.
4. The Sun emits light and other forms of radiant energy that are
necessary for life to exist on Earth.
5. Satellites have useful applications for technologies on Earth.
6. The study of the night sky has influenced the culture and lifestyles of
many civilizations.
Earth’s Rotation
- The motion of the Sun in our sky is created by the rotation of Earth around
its Axis.
- A complete rotation takes 24 h.
- Rotation causes the sun to travel from East to West.
- Day and Night are created by this rotation.
Earth’s Revolution and Planetary Orbits
Orbit Radius: The average distance between an object in the Solar System
and the Sun
- Earth spins on its axis (24 h. cycle).
- At the same time Earth is orbiting the Sun (365.25 days).
- Earth’s orbit is elliptical. At times it is closer to the Sun and other times
further away.
- A planets Orbital Radius is the average of the distance of the complete
elliptical orbit.
- Note: Earth is farthest away on July 4 and closest on January 3.
Figure 1: Earth’s axis is an imaginary line that goes through the planet from
pole to pole.
- The shape and size of a planets orbit will impact the time it takes to ravel
around the Sun.
- The further the distance away a planet is from the Sun the longer the time it
takes to complete its orbit cycle.
- Mercury 88 days, Earth 365.25 days, and Mars 687 days.
- The planets orbit the Sun in a counter clockwise direction.
Effects of Earth’s Revolution
- Earth’s revolution also impacts our view of celestial bodies we can see.
Motions of the Moon
- The moon also rotates on an axis.
- The moon revolves around the Earth in its own orbit, held in place by
gravity.
- Moons rotation is equal to the time of its orbit of Earth. Therefore we
always see the same surface of the moon.
The Force of Gravity
Gravitational Force: the force of attraction between all masses in the
Universe.
- The force of attraction between objects in the Universe is called Gravity.
- The greater the mass the greater the force of gravity.
- The mass of the Sun keeps all planets in their orbits.
- The mass of the planets keeps their moon in their orbits.
- The speed of a planet revolving around the Sun is equal to the force of
gravity.
- Therefore a planet gets no closer or further away, will not leave its orbit.
Figure 3: Earth stays in a stable orbit because of the balance between its
forward speed and the Sun’ gravitational pull.
Explaining Motion in the Solar System
- Claudius Ptolemy (Greek astronomer 87 -150 CE) believed the Earth was
the centre of the Universe and the Sun and planets revolved around it.
- This model is called a geocentric model.
- Nicholas Copernicus (1473 – 1543) proposed a model which had the Sun
in the centre and all the planets revolved around it.
- This model is called a heliocentric model.
- Galileo research using a telescope in the 1600’s finally convinced the
scientific community that The Earth orbited the Sun.
a) Geocentric
b) Heliocentric
Figure 4- 5: a) A geocentric model of the Solar System. b) A heliocentric
model of the Solar System.
Earth’s Tilt
- The Earth is tilted on its axis at a 23.5 o from vertical.
- This tilt determines the seasons and daytime temperatures.
Figure 6: Earth’s axis is tilted 23.5o from the vertical.
The Reason for the Seasons
Solstice: an astronomical event that occurs two times each year, When the
tilt of Earth’s axis is most inclined toward or away from the Sun, causing the
position of the Sun in the sky to appear to reach its northernmost or
southernmost extreme.
Equinox: The time of the year when the hours of daylight equal the hours of
darkness.
- Seasons occur between the Northern and Southern Hemispheres.
- When it is winter in the Northern Hemisphere it is summer in the Summer
Hemisphere.
- The 23.5 o tilt is maintained as Earth orbits the Sun.
- In the summer the Northern Hemisphere is tilted closest to the Sun and the
Sun sits high within the sky. Yet the Earth is furthest from the Sun, in orbit.
- In winter the Northern Hemisphere is tilted furthest away from the Sun and
the Sun sits low within the sky. Yet the Earth is Closest to the Sun, in orbit.
- This is vice versa for the Southern Hemisphere.
Figure 7: a) the northern hemisphere receives more direct sunlight than the
southern hemisphere when Earth is tilted toward the Sun. b) the reverse
effect occurs when Earth is tilted away from the Sun.
- The solstice occurs when the tilt of Earth Axis is at it most extreme.
- June 21 is the Summer solstice (longest day), and December 21 is the
Winter solstice (shortest day).
- There are two days of equal time that occur, this is called the equinoxes.
Figure 8: The Seasons in the Northern Hemisphere
Earth’s Wobble
- The North Pole axis, when extended into space will align with the star
Polaris.
- Polaris is known as the North Star.
- Over time the Earth’s North Axis had not always pointed at Polaris.
- The change in the Earth’s rotational axis is called precession.
- In 12 000 years it will return to point to Vega. It will follow its circle over
26 000 years.
Figure 10: Precession of Earth’s axis is similar to the wobble of a slowly
spinning top.
Phases of the Moon
Lunar Cycle: all of the phase of the Moon.
- The moon is illuminated by the Sun. It does not produce its own light.
- The illuminated side does not always face the Earth. This creates what is
known as the phase of the Moon.
- The phases of the Moon will follow a four week predictable pattern.
- During these patterns, there will be eight phases, known as the Lunar
Cycle.
First Half of the Lunar Cycle:
- The first phase is the New Moon. The Moon is not visible (not illuminated).
- As the phase progresses, more of the Moon is illuminated.
- Second phase is the waxing Moon, just a curved sliver of light illumination.
- Third is the Half Moon or first quarter phase.
- Then the fourth Moon, Waxing Gibbous where more then half is
illuminated or three quarters of the face.
- The fifth phase is the Full Moon; the full face of the moon is illuminated.
Second Half of the Lunar Cycle:
- The illuminated portion of the moon begins to wane, get smaller.
- Sixth phase, the Waning Gibbous shows three quarters of the face.
- Seventh phase, the third quarter shows half of the face of the moon.
- The last phase is the waning crescent moon which shows a sliver again.
Figure 11: the phases of the Moon as seen from Earth.
Eclipses – In the Shadow
- Astronomical events like a Solar or Lunar eclipse occur when one celestial
objects blocks the view of another celestial object from Earth.
Eclipse: a darkening of a celestial object due to the position of another
celestial object.
Solar Eclipse
- The diameter of the Sun is 400 times greater than the Moon.
- The Sun is 400 times farther from Earth the Moon.
- Therefore the Sun and Moon appear to be the same size.
- A solar eclipse occurs when the Moon aligns between Earth and the Sun.
- This blocks our view of the Sun and darkens our sky during the day.
- A total solar eclipse of the Sun, only allows the corona to be seen from
Earth.
- A partial solar eclipse, the Moon only covers a portion of the Sun.
- When viewing a solar eclipse, a person should not view it directly.
- Should use a suitable solar filter, or view its shadow, because the direct
Sun rays can damage your eyes.
Figure 13: This view of the sun and Earth, looking down from the North
Pole, shows a total eclipse. A total eclipse occurs somewhere on Earth once
every two years.
Lunar Eclipse
- A Lunar eclipse occur when Earth is placed between the Sun and the Moon.
- A total lunar eclipse occurs when Earth total blocks the Moon within its
shadow.
- A partial lunar eclipse occurs when the Moon passes through only part of
the Earth’s shadow.
Figure 15: this view of the Sun and Earth, looking down from the North
Pole, shows a total lunar eclipse. A total lunar eclipse can last up to an hour.
Tides – The Pull of the Moon
Tide: the alternate rising and falling of the surface of large bodies of eater,
cause by the interaction between Earth, the Moon and the Sun.
- Tides are the rising and falling of the Ocean’s water levels.
- The gravitational pull of the Moon pulls the surface water towards it.
- The side of the Earth facing the Moon will see its oceans bulge towards it.
- On the opposite, there will also be a bulge. Moons gravitational pull is the
weakest.
- Every six hours there is a change in tide, two high and two low tides.
Figure 16: When the Moon is directly overhead, coastal areas experience
high tide. At the same time, coastal areas on the opposite side of Earth also
experience high tide. The regions on Earth between the two tidal bulges
experience low tides.
- The Sun also impacts the tides. But it is very minimal.
Figure 17: a) When the Moon, the Sun, and Earth are aligned, spring tides
occur. b) When the Moon and the Sun are on perpendicular sides of Earth,
weaker neap tides occur.