What makes the moon change shape?
Pro Research Analysisby 
Searched over 200M research papers
Consensus Meter
Understanding the Moon's Changing Shape
The moon's changing shape, often observed as its phases, is a fascinating phenomenon influenced by various factors, including its orientation, gravitational interactions, and internal structure. This article synthesizes research findings to explain the primary reasons behind the moon's changing shape.
Impact Craters and Moon's Orientation
Large impact events have significantly influenced the moon's orientation in space. Research indicates that massive impacts, such as those forming the Imbrium and Orientale basins, have reoriented the moon by altering its moments of inertia. These changes in mass distribution cause the moon to adjust its orientation over time due to tidal friction, eventually stabilizing in a new position. This reorientation process is relatively quick, occurring within a few thousand years.
Fossil Figure and Rotational Deformation
The moon's unusual shape can also be attributed to a "fossil figure," which preserves the effects of past rotational and tidal deformations. This fossil figure results from the moon's elastic lithosphere maintaining a record of these deformations despite changes in its rotational and orbital states. This preserved shape helps explain the moon's current form, which is not solely due to its present-day gravitational interactions.
Tidal-Rotational Shape and Polar Wander
The moon's large-scale topography and gravity are influenced by tidal heating and rotational forces. Studies show that the moon's degree-2 topography is consistent with early tidal heating, which shaped its crust. Additionally, a frozen tidal-rotational bulge formed later, contributing to the moon's current shape. Internal density contrasts have also caused the moon's polar axis to reorient by approximately 36 degrees, a phenomenon known as polar wander. This reorientation links the geological features of the moon's near and far sides.
Thermal-Orbital Evolution
The early thermal and orbital evolution of the moon played a crucial role in its shape. A model considering a plagioclase lid over a magma ocean suggests that tidal dissipation and heat transport influenced the moon's orbital eccentricities. These large eccentricities impacted the moon's shape and the geochronology of lunar samples. However, the shape resulting from these early conditions could not be maintained against elastic deformation as the orbit evolved.
Observable Moon Phases
The observable phases of the moon, such as waxing and waning, are primarily due to its position relative to the Earth and the Sun. Educational studies have shown that guided inquiry-based instruction significantly improves middle school students' understanding of these lunar phases. Students learn to draw scientific sequences of moon phases and understand the cause of these changes, shifting from nonscientific to scientific conceptions.
Conclusion
The moon's changing shape is a complex interplay of impact events, fossil figures, tidal forces, and thermal-orbital evolution. These factors collectively influence its orientation, topography, and observable phases. Understanding these processes provides valuable insights into lunar geology and the moon's dynamic history.
Sources and full results
Most relevant research papers on this topic