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
V. Using Central Ideas to Develop an Explanatory Model for the Phases of the Moon
As noted in II.B.4, Joseph’s Moon, even very young children are aware of the Moon and able to communicate what they are observing. That the Moon often is visible during the day is a surprise to many adults. After starting to notice the Moon during the day, many also become puzzled that the Moon sometimes seems to chase the Sun across the sky but sometimes the Sun seems to chase the Moon. Also intriguing after more systematic observation is the paradox that the Moon seems to move like the Sun, from east to west over several hours, but in the opposite direction, from west to east, over several days.
These are examples of the increasing complexity typical of scientific endeavors, in which more detailed observations prompt deeper pondering and questioning. This section explains these observations in the context of developing an explanatory model for the Moon’s changing phases. We then consider how the Moon appears to us here on Earth compared to how the Moon would appear if we could look down upon the Moon, Earth, and Sun from above the solar system. The section also discusses the causes of solar and lunar eclipses and concludes with exploring Internet resources about the Moon to share with a friend or family member.
Question 5.23 Why does the Moon seem to have different shapes at different times?
This section begins with a review of relevant central ideas, next presents a report on a child’s insights about the Sun and the Moon, and then proceeds with developing an explanation for the Moon’s changing phases as seen from here on Earth.
A. Reviewing central ideas about the relationship between the Sun and the Moon
What central ideas developed so far describe the relationship between the Sun and the Moon?
- The Sun is always on the same side as the lit side of the Moon.
- The shape of the lit portion of the Moon that we see from Earth changes in a regular pattern:
Where we live in the northern hemisphere, the shape of the lit portion of the Moon grows from a crescent lit on the right (waxing crescent), to half lit on the right (first quarter), to more than half lit on the right (waxing gibbous), to fully lit; then decreases to more than half lit on the left (waning gibbous), half lit on the left (third quarter), to a crescent lit on the left (waning crescent), to no moon visible (new moon). In the southern hemisphere, the shape of the lit portion of the Moon changes in a similar pattern but from lit on the left (waxing phases) to lit on the right (waning phases).
- The angle formed by pointing one arm at the Moon and one arm at the Sun changes in a regular pattern related to the changing shape of the lit portion of the Moon that we see:
As the lit side of the Moon seems to get bigger, the angle gets bigger until the Moon is fully lit and the angle is a straight angle; as the