Scintillation or Atmospheric Boil
When you look at the stars on a clear night, some of them twinkle more than others. If you avoid planets which do not twinkle (much), the stars directly overhead usually twinkle less than those which are near the horizon. Thus the twinkle, sometimes called astronomical scintillation, is caused by motions in the atmosphere. For the observational astronomer, even the heating in and around the telescope can be a problem and care should be taken to minimize it. During the day, atmospheric boil or shimmer, causes distant objects to appear with fluctuating distortions. Scintillation is caused by air parcels of different densities moving through the line of sight, thereby causing varying refraction of light coming from distant objects. You can see this phenomenon when looking over the top of a car, parked in the sun, toward an object at a distance; in winter, looking over a heated radiator out through the window will show the shimmering of objects.
In the classroom, hold a candle a foot or so from your eye and place a piece of metal or cardboard so that you do not see the candle flame; then look at any object a few feet away so that your line of sight passes through the flame gases, about one or two inches above the tip of the flame. If there is a draft in the room, shield the candle with a small can from which the top and bottom have been removed. The upper rim of the can should be slightly above the flame tip.
The phenomenon becomes particularly striking, if you look at a square grid made on a sheet of cardboard on which you draw six heavy straight parallel lines up and down, one inch apart, and six lines across. Place the grid about six feet from the observer.
For repeated demonstrations of scintillation, it may be convenient to make a simple stand for the candle with a shield that can be adjusted to various lengths of the candle. Stick a 1/4" or 3/8" dowel rod, 8 or 10" long, into a board of wood, 3/4 x 4 x 5" with an appropriate hole, and glue it fast. A small can without bottom and top, about 3" long and 2 to 3" diameter, is fastened to one of the fingertip ends of a clothes pin with cement or wire. The pin is clipped on the post and a metal jar lid is fastened with nails or cement to the base, directly under the can, and a short piece of candle is fastened in the middle of the lid. The top rim of the candle is adjusted to about 1/2" above the tip of the candle flame. The target grid is drawn on a piece of white cardboard, 6 x 7", and nailed or glued to an appropriate piece of wood (approximately 1/2 x 2 x 6") so that it stands upright.
Material: One candle
One piece of wood, 3/4 x 4 x 5"
One piece of wood, 1/2 x 2 x 6"
Sheet of white cardboard, 6 x 7"
One 10" length of 1/4" dowel rod (a long pencil can also be used)
One can 3" high, 2 to 3" diameter
One clothes pin
One metal jar-lid