Download Performance Benchmark E

Performance Benchmark E.8.B.7
Students know regular and predictable motions of Earth around the Sun and the Moon
around the Earth explain such phenomena as the day, the year, phases of the Moon, and
eclipses. E/S
Many ancients believed that Earth was static and the heavens moved around it. However,
the Earth-Moon system is in constant motion, resulting in the celestial motions we
perceive.
Motion of Earth Around the Sun
Half of Earth faces the Sun and is illuminated by the Sun’s light. Because Earth spins
(rotates) on its axis about once every 24 hours, the illuminated half is constantly
changing, resulting in periods of light (daytime) and dark (nighttime). A day is equal to
this 24 hour rotation period. Earth orbits (revolves) around the Sun about once every 365
days. This period is called a year.
For more details about how days and years are defined, go to
http://www.astronomynotes.com/nakedeye/s7.htm.
Earth’s axis of rotation in not perpendicular to the plane in which the planet orbits the
Sun (this plane is called the ecliptic). Rather, the axis is tilted about 23.5 degrees from the
perpendicular (this tilt is called obliquity). The axis’ tilt generally points in the same
direction, with only a slight, very slow, and periodic wobbling. At its northern end, the
axis of rotation currently points to the star Polaris (otherwise known as “the North Star”).
To learn more about this subtle wobbling of Earth’s rotation axis, called precession, go to
http://astro.wsu.edu/worthey/astro/html/lec-precession.html.
Because Earth’s axis is tilted, the number of daytime hours varies depending on (1) your
position on Earth and (2) Earth’s position in its orbit around the Sun. In the northern midlatitudes, the daytime period increases from its minimum around December 21 (called the
winter solstice) to it maximum around June 21 (called the summer solstice).
Correspondingly, daytime hours decrease from around June 21 to December 21. Midway
between each solstice, daytime and nighttime hours are equal (about 12 hours each).
These days are called equinoxes, with the vernal (spring) equinox occurring around
March 21 and the autumnal equinox occurring around September 21.
The seasons directly result from Earth’s obliquity and the variation of sunlight received at
different latitudes at different times of the year. During the northern hemisphere’s winter,
Earth’s position in its orbit around the Sun results in this hemisphere being tilted away
from the Sun. During this time, the sunlight also hits Earth’s surface at a greater angle
from the perpendicular. Therefore, less energy is received per unit area at Earth’s surface
than during other times of the year. With less solar energy being received, winter is a
relatively cold season. The opposite is true in the summer months, where the number of
daytime hours is greater and sunlight hits Earth’s surface closer to the perpendicular.
Therefore, more solar energy is received at the surface.
Figure 1. Comparison of solar energy received at
Earth's surface from the summer (top), when
given amount of solar energy is concentrated in a
smaller area, to the winter (bottom), when the sun
angle is lower and solar energy is spread over a
wider area
Motion of the Moon Around the Earth
The Moon also rotates on its axis, but much slower than Earth. The Moon rotates about
once every 28 days. Interestingly, this is almost exactly the period of the Moon’s orbit
around Earth. This is not a coincidence, but results from the fact that the Moon is
“gravitationally locked” to Earth, meaning that one side of the Moon always faces Earth.
To learn more about gravitational locking, go to
http://csep10.phys.utk.edu/astr161/lect/moon/tidal.html.
Just like Earth, half of the Moon is always illuminated by the Sun. The Moon has daytime
and nighttime periods, with each (day and night) lasting about 14 Earth days. Depending
on the Moon’s position in its orbit around Earth, varying amounts of the illuminated half
are visible to us. These differing amounts of visible illumination are called moon phases.
These moon phases repeat themselves in a cycle about once every Moon orbit around
Earth. A full moon occurs when the entire illuminated half is visible from Earth and a
new moon occurs when the entire illuminated half is facing away from Earth.
Find more details about the phases of the Moon at
http://www.lpi.usra.edu/education/skytellers/moon_phases/about.shtml.
The plane of the Moon’s orbit around Earth is tipped by about 5 degrees with respect to
the ecliptic. Therefore, the Moon is almost never directly between Earth and the Sun;
likewise, Earth is almost never directly between the Moon and Sun. However, twice per
month, the Moon’s orbit crosses the ecliptic. If at these exact times the Moon is in either
a new or full phase, an eclipse will occur. A lunar eclipse occurs during these crossings if
the Moon is full. In this case, Earth is directly between the Sun and Moon. A solar eclipse
occurs during these crossing if the Moon is new. In this case, the Moon is directly
between the Sun and Earth.
To learn more about eclipses, go to http://sunearthday.nasa.gov/2006/facts.php.
Performance Benchmark E.8.B.7
Students know regular and predictable motions of Earth around the Sun and the Moon
around the Earth explain such phenomena as the day, the year, phases of the Moon, and
eclipses. E/S
Common misconceptions associated with this benchmark
1. Students incorrectly believe daytime and nighttime hours are always equal, and
also, may believe that day and night are caused by the Sun orbiting Earth.
Historically, the Sun was thought to orbit Earth. This belief was grounded in casual
observations that the Sun would rise above the eastern horizon in the morning, move
across the sky through the local meridian, and sink below the western horizon in the
evening. However, even in ancient times, more careful observations noted that the Sun
would rise above and set below the horizon at different locations each day. Today, most
people are casual observers.
The language of the Sun orbiting Earth persists today. We still refer to sunrise and sunset.
This implies that the Sun is in orbit around Earth, rather than Earth spinning on its axis.
Indeed, it is not readily apparent that Earth is in motion because of our miniscule size in
relation to the planet.
To learn more about student’s ideas about day and night, see Section 2.3 at
http://aer.noao.edu/cgi-bin/article.pl?id=63.
2. Students incorrectly believe seasons are caused by a changing distance between
Earth and the Sun.
In their everyday experiences, students observe that when they are closer to a warm
object, it feels hotter. Science education research suggests that this phenomenon may be
the root cause to students believing that summer occurs when Earth is closest to the Sun.
In fact, Earth’s orbit around the Sun is only slightly eccentric (i.e., Earth’s orbit around
the Sun is almost a perfect circle). Consequently, the distance from Earth to the Sun
varies less than 1%, with only a small amount of differential heating that does not
account for seasonal changes in Earth’s climate. Interestingly, Earth is closest to the Sun
during the northern hemisphere’s winter.
More details about this season misconception can be seen in Section 2.8 at
http://aer.noao.edu/cgi-bin/article.pl?id=63.
3. Student incorrectly think the Moon is up only at night.
In mythology, the Moon is often depicted as the Sun’s opposite and is incorrectly
described to be visible when the Sun is not (i.e., only during the night). This myth is
perpetuated because the Moon is more difficult to see during the daytime. Although the
Moon is often visible during the daytime hours, its light can be overpowered by the
brighter Sun. Also, when the Moon is present in the daytime sky, it is often in the
crescent or new Moon phases causing the Moon to be appear dim or not visible to the
naked eye.
To learn more about students’ misconceptions about the Moon, go to
http://aer.noao.edu/cgi-bin/article.pl?id=138.
4. Students incorrectly think moon phases are caused by Earth’s shadow and that
eclipses occur every month.
Science education research shows that Moon phases are caused by the shadow Earth (or
other objects) on the Moon. In other words, students incorrectly believe that Moon phases
are essentially eclipses. As discussed earlier, eclipses occur only rarely and half the Moon
is almost always fully illuminated by the Sun. Depending on the Moon’s position in its
orbit around Earth, we see some percentage of the illuminated half.
To get more details about Moon phase and eclipse misconceptions, and how to teach for
understanding in these topics, go to http://aer.noao.edu/cgi-bin/article.pl?id=138.
5. Students incorrectly think that moon creates its own visible light.
All objects emit electromagnetic radiation and the type of electromagnetic radiation
emitted is dependent on the object’s temperature. The average temperature of the Moon is
about 250 K, which means that it emits infrared light, not visible light which is only
emitted by objects in the thousands of Kelvins.
The visible light we see from the Moon is sunlight reflected from the Sun. To learn more
about this reflected light, go to
http://www.windows.ucar.edu/tour/link=/kids_space/qmoon_phas.html.
Performance Benchmark E.8.B.7
Students know regular and predictable motions of Earth around the Sun and the Moon
around the Earth explain such phenomena as the day, the year, phases of the Moon, and
eclipses. E/S
Sample Test Questions
1. The cycle of daytime and nighttime is created by what motion?
a. Earth’s orbit around the Sun
b. Sun’s orbit around Earth
c. Earth’s rotation on its axis
d. Sun’s rotation on its axis
2. On Earth, a year is defined by which motion?
a. Earth’s orbit around the Sun
b. Sun’s orbit around Earth
c. Earth’s rotation on its axis
d. Sun’s rotation on its axis
3. Earth’s seasons are the result of
a. the greenhouse effect, where the atmosphere is thicker in the summer and
thinner in the winter.
b. solar flares, which occur more frequently in the summer than in the winter.
c. its elliptical orbit, causing Earth to be closer to the Sun in the summer and
farther in the winter.
d. Earth’s tilt, causing the number of daylight hours to decrease from
summer to winter.
4. When is it possible to observe the Moon?
a. The Moon might be visible in the day or night, depending on the season.
b. The Moon might be visible in the day or night, depending on its phase.
c. The Moon is visible only at night, regardless of its phase or the season.
d. The Moon is always visible, regardless of its phase or the season.
5. Which of the following is the best explanation for the cycle of moon phases?
a. Different amounts of the Moon’s illuminated side are visible from Earth at
different times.
b. The Moon passes in and out of Earth’s shadow as the Moon orbits around
Earth.
c. Different amounts of the Moon are illuminated at different times in the
cycle of phases.
d. Our ability to see the Moon from Earth changes as the Moon orbits the
Sun.
6. Which of the following is true when we see a lunar eclipse?
a. The Moon is in its new phase.
b. The Moon’s path crosses the ecliptic.
c. It occurs every month.
d. The Moon’s shadow covers Earth.
7. Which of the following is true when we see a solar eclipse?
a. It occurs every month.
b. Earth’s shadow covers the Moon.
c. The Moon is in its full phase.
d. The Moon’s path crosses the ecliptic.
Performance Benchmark E.8.B.7
Students know regular and predictable motions of Earth around the Sun and the Moon
around the Earth explain such phenomena as the day, the year, phases of the Moon, and
eclipses. E/S
Answers to Sample Test Questions
1. (c)
2. (a)
3. (d)
4. (b)
5. (a)
6. (b)
7. (d)
Performance Benchmark E.8.B.7
Students know regular and predictable motions of Earth around the Sun and the Moon
around the Earth explain such phenomena as the day, the year, phases of the Moon, and
eclipses. E/S
Intervention Strategies and Resources
The following list of intervention strategies and resources will facilitate student
understanding of this benchmark.
1. The Skytellers Website
The Lunar and Planetary Institute has developed a comprehensive site that covers
scientific understanding of day and night, seasons, moon phases, and other items
in the context of Native American myths and legends. There are several activities
associated with each of the concepts. The entrance portal to the site is located at
http://www.lpi.usra.edu/education/skytellers/intro.shtml.
2. Moon Phase Journal
One of the best ways for students to learn about moon phases is to have them keep
a journal of Moon observations for an extended period. It is recommended that
students keep a journal for at least one full lunar cycle; two cycles is even better,
but requires significant follow up.
An excellent Web site that includes an outline for a Moon journal lesson, with
assessment strategies and other resources, is found at
http://www.teachersdomain.org/resources/ess05/sci/ess/eiu/lp_lunarcycle/index.ht
ml.
3. Eclipse Lesson Plans
NASA’s Sun-Earth Day site contains several lessons concerning eclipses. The site
also has a large selection of resources, some of which cover the motions of the
Moon and Sun in the sky. The portal for these lessons is found at
http://sunearthday.nasa.gov/2006/educators/lp_912.php.