11 Sun, Earth, and the Moon

Formative Assessment: Sun, Earth and the Moon

Heliocentrism vs Geocentrism

In early times, humans believed in geocentrism–the theory that Earth is at the center of the solar system, and the Sun and other planets revolve around it. During the Renaissance in the 1500s, Copernicus popularized the concept of heliocentrism–the theory that the Sun is at the center of the universe and Earth orbits the Sun.

Throughout Copernicus’ lifetime, the scientific community widely denied the theory of heliocentrism. A generation later, the Sun-centered theory became more commonly accepted when Galileo invented the telescope in 1609, making it easier to observe space. Additionally, Galileo made a variety of discoveries about our solar system that disproved the geocentric model of the universe. Despite these new discoveries, however, there was still significant pushback against heliocentrism, particularly from the Catholic Church.

At the time, the Church defended its stance on geocentrism because it believed Galileo’s discoveries left too many questions unanswered and did not explicitly prove heliocentrism. During this period, a case could still technically be made for geocentrism until technology advanced enough for scientists to discover more evidence supporting heliocentrism.

Additionally, the Church had certain clergy who interpreted parts of the Bible very literally, as if it were a science textbook rather than a theological work. Galileo’s claims were scandalous in their eyes because heliocentrism directly conflicted with certain biblical passages. For these reasons, the Church put Galileo on trial, convicted him of heresy, and sentenced him to house arrest for the remainder of his life. In 1822, the Church eventually accepted the theory of heliocentrism once there was enough scientific evidence to claim it as truth.

Key Takeaway

To remember the theories of heliocentrism and geocentrism, break down the names and look at the etymology.

  • The root geo– means “earth”, and centr– means “center”. So, geocentrism is the theory in which Earth is at the center of the solar system.
  • The root helio– means “sun”, and centr– means “center”. So, heliocentrism is the theory in which the Sun is at the center of the solar system.

Phases of the Moon

The Moon orbits around Earth once every 28 days, or about once a month. Depending on where the Moon is in its orbit, it appears different from Earth. However, everyone on Earth sees the same phase of the Moon on the same day.

The phases of the moon are:

  • New: The Moon’s face is not visible from Earth
  • Crescent: Between a new moon and a quarter moon
  • Quarter: From Earth, we can see half of the moon’s face which is a quarter of the entire moon
  • Gibbous: Between a quarter moon and a full moon
  • Full: All of the Moon’s face is visible from Earth

For the first half of this cycle, the visible part of the Moon waxes or grows larger. After reaching a full moon, the Moon wanes or grows smaller for the second of the cycle.

The image, below, shows the Moon’s phases.

Moon Phases from Earth” by pmonaghan is licensed under CC-By-NC-ND 2.0

Where are the Moon, Sun, and Earth in relation to each other for the different Moon phases?

For further explanation of the Moon’s phases, watch the following video.

Video credit: “The Moon” by Khan Academy is licensed under CC BY-NC-SA 3.0. Note: All Khan Academy content is available for free at khanacademy.org.

Equinox & Solstice 

There are 2 equinoxes and 2 solstices per year – Spring Equinox, Autumn Equinox, Winter Solstice, and Summer Solstice. Equinoxes (which sounds like the word equal) mark the day in which all of Earth receives an equal amount of sunlight–12 hours. This equal amount of sunlight occurs when the Equator is directly in line with the Sun. The Spring Equinox happens around March 20th and the Autumn Equinox happens around September 23rd each year.

Illumination of Earth by Sun on the day of equinox” by Przemyslaw “Blueshade” Idzkiewicz is licensed under CC BY SA-2.0

Solstices mark the days of the year in which a hemisphere receives the least amount of sunlight (aka the shortest day of the year) and the most amount of sunlight (aka the longest day of the year). These days occur when one of the tropic lines are directly in line with the Sun. In the Northern Hemisphere, the Winter Solstice (the day with the least sunlight, usually around December 21) occurs when the Tropic of Capricorn (the southern tropic line) is in line with the Sun. The Summer Solstice (the day with the most sunlight, usually around June 21) occurs when the Tropic of Cancer (the northern tropic line) is in line with the Sun.

Winter Solstice
Illumination of Earth by Sun on the day of summer solstice in the northern hemisphere” by Przemyslaw “Blueshade” Idzkiewicz is licensed under CC BY SA-2.0
Summer Solstice
Illumination of Earth by Sun on the day of summer solstice in the northern hemisphere” by Przemyslaw “Blueshade” Idzkiewicz is licensed under CC BY SA-2.0











Eclipses happen when light is blocked. There are two types of eclipse that we can see on Earth: solar eclipses and lunar eclipses.

To understand each type of eclipse, you must determine whether the Sun or Moon is being blocked.

Solar Eclipse

  • In a solar eclipse the Sun is being blocked–this happens when the Moon perfectly crosses between Earth and the Sun. A solar eclipse always occurs during a new moon.
Solar Eclipse
Image credit: “Total Solar Eclipse” by NASA

For more explanation of solar eclipses, watch the video below:

Video credit: “What Creates a Total Solar Eclipse?” by Andy Cohen/TED-Ed is licensed under CC BY-NC-ND 4.0

Lunar Eclipse

  • In a lunar eclipse, the Moon is blocked when it passes through Earth’s shadow. When the Moon is in this position, the Sun’s light cannot reach it. A lunar eclipse always occurs during a full moon.
Lunar Eclipse
Lunar Eclipse” by NASA’s Marshall Space Flight Center is licensed under CC BY-NC 2.0

For more explanation of lunar eclipses, watch the video below:

Video credit: “Lunar Eclipse Essentials” by NASA is public domain


Earth orbits in the same plane as the other planets in our solar system: the Plane of the Ecliptic. However, Earth’s is also tilted on it axis. This tilt never changes in relation to space, so different areas of Earth are tilted toward the Sun at different times of year. This is why we have seasons.

  • If a hemisphere is tilted towards the Sun it gets more sunlight and it warms up–aka summer.
  • If a hemisphere is tilted away from the Sun it gets less sunlight and it cools down–aka winter.

Additionally, the Northern and Southern Hemispheres have opposite seasons due to the tilt of Earth’s axis. When the Northern Hemisphere is tilted towards the Sun it is summer (this is winter in the Southern Hemisphere). When the Southern Hemisphere is tilted towards the Sun it is summer (this is winter in the Northern Hemisphere).

Earth’s seasons are explained in the image below.

Image credit: Seasons by NASA Space Place is public domain


Watch the video, below, to learn more about the tropic lines and Earth’s shadows. The following practice questions will be explained in the video:

  1. It is the Summer Solstice in the Southern Hemisphere and you are standing on the Tropic of Capricorn. It is noon, so the Sun is directly above you. Which direction does your shadow point?
  2. If you are in Iowa, what will happen to the length of your shadow as we approach the Spring Equinox?

Key Takeaways

  • As Earth orbits around the Sun, different areas receive direct or indirect sunlight due to the tilt of Earth on its axis. This is what causes seasons.
  • Earth’s distance away from the Sun does not cause the seasons.
  • The tropic lines, equator, arctic circle all mark areas of specific direct sunlight.
Next Generation Science Standards: NGSS

Performance Expectations

1-ESS1-1 Use observations of the sun, moon, and stars to describe patterns that can be
1-ESS1-2. Make observations at different times of year to relate the amount of daylight to the time of year. [Clarification Statement: Emphasis is on relative comparisons of the amount of daylight in the winter to the amount in the spring or fall.] [Assessment Boundary: Assessment is limited to relative amounts of daylight, not quantifying the hours or time of daylight.]
5-ESS1-2. Represent data in graphical displays to reveal patterns of daily changes in length and direction of shadows, day and night, and the seasonal appearance of some stars in the night sky. [Clarification Statement: Examples of patterns could include the position and motion of Earth with respect to the sun and selected stars that are visible only in particular months.] [Assessment Boundary: Assessment does not include causes of seasons.]
MS-ESS1-1. Develop and use a model of the Earth-sun-moon system to describe the cyclic patterns of lunar phases, eclipses of the sun and moon, and seasons.  [Clarification Statement: Examples of models can be physical, graphical, or conceptual.]

Disciplinary Core Ideas: DCI

First grade: ESS1.B: Earth and the Solar System

Fifth grade: ESS1.B: Earth and the Solar System

Middle School

This model of the solar system can explain eclipses of the sun and the moon. Earth’s spin axis is fixed in direction over the short-term but tilted relative to its orbit around the sun. The seasons are a result of that tilt and are caused by the differential intensity of sunlight on different areas of Earth across the year. (MS-ESS1-1)

Crosscutting Concepts: CCC

First grade: Patterns

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Connections to Nature of Science

Scientific Knowledge Assumes an Order and Consistency in Natural Systems

  • Science assumes natural events happen today as they happened in the past. (1-ESS1-1)
  • Many events are repeated. (1-ESS1-1)


Cause and Effect

Systems and System Models

Lesson ideas:

  1. Create a daily journal of sky observations: where the Sun is, what time sun rise and sun set, Moon position. Can be part of daily calendaring. Compare to seasons.
  2. Play with shadows outside and inside: shadow tag, tracing shadows with sidewalk chalk and seeing how they move throughout the day; compare shadows to where the Sun is. Play with shadows inside: shine flashlights on sticks indoors, have kids move the flashlight up and down to see how the lenght of shadow moves. Draw the shadow and write where the flashlight was.
  3. Experiment with Sun/Moon/Earth system as we did in class to understand equinoxes, solistices, eclipses, seasons, etc.




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