So, Why do Stars Twinkle? Sorry to disappoint any children who might be reading this, but actually it's not the stars twinkling at all. The light from the star has travelled for many millions of years through space - nice and steadily, all the way - and then it meets the Earth's atmosphere, which is where all the twinkling takes place. Here the light is reflected, bent, shimmered and shaken by turbulent particles in the atmosphere until it makes it to your eye. From our perspective, the light from a star will appear in one location, then milliseconds later, it will be distorted to a different spot.
Stars would not appear to twinkle if we viewed them from outer space, or from a planet or the moon that didn't have an atmosphere.
Astronomers measure the amount of twinkling and call it scintillation, although it is commonly referred to as seeing and it can also be observed when looking at the moon - although the moon doesn't twinkle, you can see the effects of the turbulent atmosphere by the way it wobbles.
Stars appear as a single point in the sky due to the great distance between us and them. This single point is highly affected by atmospheric turbulence. Planets, on the other hand, are much closer and appear as disks. Although we can't resolve them as disks with our eyes, the difference in distance means the twinkling averages out and we see a more stable light in the sky.
How to Combat Poor Seeing
The most and most effective method for combating poor seeing is to eliminate the atmosphere completely. This is why space-based telescopes such as Hubble are so important.
Ground-based telescopes, however, cannot be moved into space, there are some techniques that they can use to help eliminate the turbulence in the atmosphere. Adaptive optics use lasers and powerful computers to physically distort the mirror of the telescope many times a second to compensate for the turbulence in the atmosphere.
For most of us however it isn't practical to move into space and we can't afford expensive adaptive optics, so there is not much we can do to eliminate poor seeing. What we can do however is to reduce the effects of atmospheric turbulence by following these simple guidelines.
- Leave your scope outside to cool to the ambient temperature, getting rid of air currents in the tube.
- Observe on grass rather than concrete. Concrete absorbs more heat from the Sun and radiates it out to the air above it for longer.
- Air currents tend to stay low to the ground, so it can be a good idea to raise up your scope on a platform.
- If you build an observatory, make it using thin materials like wood that can cool quickly.
- The geography of your observing site affects how air behaves. Being near the sea gives you calmer air than near a range of hills, where air is forced up, causing turbulence.
Another technique that can be used is called "lucky imaging" which dates back to pre-war naked-eye observations of moments of good seeing, followed by observations of the planets on cine film after World War II. The technique relies on the fact that every so often the effects of the atmosphere will be negligible, and by recording large numbers of images as quickly as possible, a 'lucky' excellent image can be picked out. This is most often accomplished by using a webcam or planetary camera.