Moon Appearance and Illusions
Overview
This lecture explores why the Moon appears to change size and color in the sky, focusing on the "moon illusion" and the effects of atmospheric and visual perception.
Visual Differences in the Full Moon
- The full moon can appear bigger or smaller at different times.
- The color of the full moon can change, sometimes appearing white, yellow, orange, or red (e.g., "Blood Moon").
- The moon may look brighter or dimmer due to atmospheric conditions.
The Moon's Apparent Size
- The moon appears larger near the horizon and smaller when overhead.
- This perceived change is not due to the Moon's actual distance from Earth at those times.
- The moon’s orbit is an ellipse, so its distance from Earth does change, but only minimally affects its size ("supermoon").
- The dramatic apparent size change near the horizon is an optical illusion called the "moon illusion."
The Moon Illusion Explained
- The brain judges object size by comparing it to known objects nearby.
- When the moon is near the horizon, nearby objects like trees or buildings trick the brain into perceiving it as larger.
- When the moon is overhead, there are no familiar objects for size reference, so the brain sees it as smaller.
- Photographs of the moon at different positions in the sky show the moon is actually the same size.
Other Objects and the Illusion
- The sun and even constellations appear larger near the horizon for the same reason as the moon.
- This effect is due to how human brains process visual information, not the actual size of celestial bodies.
Key Terms & Definitions
- Moon illusion — An optical illusion causing the moon to appear larger near the horizon than overhead.
- Supermoon — A full moon that appears slightly larger because it is at its closest point to Earth in its elliptical orbit.
- Blood Moon — A term for a red-colored moon, often during a lunar eclipse.
Action Items / Next Steps
- Review notes on moon phases for connection with today’s discussion.
- Observe the moon during different positions in the sky and note perceived size differences.
- No assigned homework.