Overview
This lecture explains why stars twinkle, the effect of atmospheric refraction on the apparent positions of stars and the Sun, and why our day length is slightly increased.
Why Do Stars Twinkle?
- Stars twinkle because Earth's atmosphere refracts their light, causing a flickering or twinkling effect.
- If viewed from space, stars do not twinkle because there is no atmosphere to refract their light.
- Changes in air temperature and density in the atmosphere cause continuous, random refraction of starlight.
- The twinkling effect occurs as the amount of light entering our eyes from a star fluctuates rapidly.
Demonstration of Atmospheric Refraction
- Hot air above a fire refracts light, making nearby and distant bulbs appear to flicker or twinkle.
- The effect is stronger for distant light sources, similar to how stars twinkle.
Apparent Position of Stars
- The Earth's atmosphere has layers of differing densities, causing the light from stars to refract multiple times.
- Due to refraction, stars appear higher in the sky than their actual positions.
- The apparent position of stars shifts due to constantly moving air layers.
Why Don't Planets Twinkle?
- Planets appear as extended sources of light and are much closer to Earth than stars.
- Variations in the light from planets average out, so they do not appear to twinkle to the unaided eye.
Early Sunrise and Delayed Sunset
- Atmospheric refraction allows us to see the Sun about two minutes before it actually rises above the horizon.
- After sunset, we continue to see the Sun for about two minutes, even though it has dipped below the horizon.
- The total increase in the length of the day due to atmospheric refraction is about four minutes.
Key Terms & Definitions
- Atmospheric Refraction — Bending of light as it passes through Earth's atmosphere with varying density.
- Apparent Position — The position where an object (like a star or sun) appears to be due to the bending of its light.
- Horizon — The line where Earth's surface and sky appear to meet.
Action Items / Next Steps
- Review the concepts of atmospheric refraction and its effects.
- Prepare for questions on why stars twinkle, the apparent positions of celestial bodies, and early sunrise/delayed sunset.
- Read about other optical illusions caused by atmospheric phenomena.