Clarification of Key Idea

Expectation for Students

The expectation is that students will know that the Earth has a spherical shape. Moreover, they are expected to understand how the Earth can appear flat to someone standing on the Earth’s surface while it appears spherical to an observer in space. Many students at this grade level will have encountered a globe or seen pictures of the Earth from space and correctly identified it as representing the Earth. However, they are less likely to understand how their experiences on the Earth’s surface, which from their vantage point appears flat, can be reconciled with the idea that the Earth is a sphere. It is important to keep in mind here student commonly held ideas about the existence of two Earths, a flat Earth based on their everyday experiences and a spherical Earth based on ideas they learn in school and elsewhere (see Ideas Students Have).

The concept of a spherical Earth includes understanding that objects are arranged all over its surface. Because things are pulled toward the Earth everywhere on the Earth, and because the Earth is spherical, there is no universal up and down direction. Students are expected to know that the everyday use of the terms “up and down” is not an indication of universal direction but really a case of “toward the Earth” and “away from the Earth” (see Ideas Students Have). Moreover, students are expected to know that people live all over the surface of the Earth (e.g., places on opposite sides of the Earth including both the Northern and Southern hemispheres) without any risk of falling off of it, their feet are on the ground and when they drop something, it falls toward the Earth, not away from it. (see Clarification of Key Idea 4B/E1 and 4G/E1). This is in contrast to commonly held ideas that people live inside a spherical Earth or only on the top of a spherical Earth and that when something is dropped it falls in a universal “down” direction.

Students should understand that the term “Earth” in this key idea refers to the entire planet on which we live, not the ground or soil. For accuracy, the term “approximately spherical” is in the benchmark because the Earth is not perfectly spherical. However, it is not necessary for students in this grade span to understand this subtlety or to describe the shape using terms such as “oblate spheroid.” In fact, the tremendous size of the Earth means that variations on the order of several dozen miles are tiny compared to the radius of the Earth. [Note: Actually, the Earth is closer to being a perfect sphere than a basketball is. The diameter from the North Pole to the South Pole (the shortest diameter) is approximately 12,714 km. The equatorial diameter (the longest diameter) is approximately 12,756 km.]

This key idea also calls for students to know that like the Earth, the sun and planets are spherical in shape. The idea in the benchmark that stars are spherical is not included in this key idea because there are no phenomena accessible to students in this grade range to support the idea that the stars are spherical. Once students understand that stars are like the sun (see benchmark 4A/E5) students can be asked to infer that since the sun is spherical, stars are also spherical.