Using a laser and some boiling water,
you can show why planets do not appear to twinkle in the night sky.
Watch Video:
Teachable Topics:
- Refraction of Light
- Observational Astronomy
- Atmosphere
Theory:
When observing stars on a clear night, you can see them twinkle. As explained in the Twinkling Stars demo, this is due to the Earth's atmosphere; specifically the changing temperatures and current in the air bend the starlight. But when observing the planets in night sky, they do not appear to twinkle.
This is due to the fact that planets are much closer to the Earth than stars. Due to their relatively close distances, planets have a finite size when observing them from the Earth. Stars on the other hand, are so far away from the that we can only observe them as very small points of light. As such, a beam of light coming from a star appears much smaller than a beam of light from a nearby planet. The star's smaller light beam is more noticeably bent in the atmosphere, causing twinkling, whereas the light beam from a planet does not appear to move at all. This is why stars twinkle and planets do not.
Apparatus:
- Two lasers
- Laser beam expander
- Hot plate and a beaker
- Water
- Screen
Procedure:
- Fill the beaker with water and place it on the hot plate.
- Outfit one of the lasers with the beam expander. This laser's beam will represent an observed planet. The laser without the expander will represent an observed star.
- Set up both lasers side by side and place the screen so that the laser beams will hit it. Place the hot plate between the lasers and the screen so that the laser beams will pass right over the top of the beaker.
- Turn the lasers and the hot plate on.
- Turn of the lights and observe the position of the laser beams on the screen as the water starts to boil. The expanded beam should not move or change in brightness while the unexpanded beam should begin to twinkle like a star.
