Unit 5: Exploring the Nature of Astronomical Phenomena in the Context of the Sun
Unit 5: Exploring the Nature of Astronomical Phenomena in the Context of the Sun/Earth/Moon System
XI. Pondering Additional Issues
This section explores several additional issues: how things move and interact, what keeps the Moon and the Earth in their orbits, why the oceans have tides, and what happens when objects fall here on Earth and on the Moon?
A. Reviewing understandings about the Sun, Earth, Moon, and Stars
It may be helpful to step back and review before heading into new territory. So far this course has developed the following understandings about light and shadows as well as about the Sun, Earth, Moon, and stars:
What causes shadows? Some shadows occur when an object blocks light and casts a shadow behind it on something else. Other shadows occur when an object blocks light from shining on its own back-side. If you stand facing the Sun on a sunny day, for example, your shadow forms on the ground or a wall behind you; a shadow also forms on your back because your body blocks sunlight from shining there.
Why does it get dark at night? The Sun seems to rise in the morning, move high across the sky during the day, and set in the evening. It gets dark when the Sun appears to sink below the horizon in the western sky and stays dark until the Sun appears to rise above the horizon again in the eastern sky. According to this conceptual model, it gets dark at night because the source of light, the Sun, has moved below the horizon during its apparent daily journey revolving around the fixed Earth. This Fixed Earth, Revolving Sun model is easy to envision and believe because that is what one sees: the Sun, as well as the Moon and many stars, seeming to move daily around a flat motionless Earth.
An alternative conceptual model, however, is that the Sun, Moon, and stars only appear to be moving daily around the Earth; instead it is the Earth that actually is moving, a spherical Earth rotating daily on its axis. According to this conceptual model, it is day on the front side of the spinning Earth, the side that is facing toward a fixed Sun; it is night on the back side of the spinning Earth, the side that is facing away from a fixed Sun. It gets dark at night because the back side of the rotating Earth is in the Earth’s own shadow. According to this Rotating Earth, Fixed Sun model, the body of the Earth itself is blocking the Sun’s light from shining on the side of the Earth that is facing away from the Sun.
The Rotating Earth, Fixed Sun model is not as easy to envision as the Fixed Earth, Revolving Sun model. Acting out these two conceptual models for explaining day and night, however, can clarify how these models are alike and different. Also seeing a physical model can help, such as a spinning globe near a lamp in a dark room, particularly if a tiny flag or stick figure stuck to the globe represents a familiar location on the rotating Earth. Understanding and accepting the Rotating Earth, Fixed Sun model requires, however, more detailed observati