So, why do stars twinkle? In actuality, they do not.
Although there are some stars that do change their brightness over time, called variable stars, they do not twinkle. Our atmosphere is made up of mostly nitrogen and oxygen, which are relatively clear gases, but at varying thicknesses are capable of refracting, or bending, light much as a magnifying lens or water in a glass does. Oxygen can also absorb light at certain visible colors. However, the atmosphere also contains dust, which can absorb and disperse in varying wavelengths of the visible spectrum. Then there is water vapor, which is also present, that can disperse white-light as well as refract it, bending it into the varying colors of the rainbow.
Even though stars are huge, the thing that causes star-light to scintillate in the night sky begins with the fact that they are so far away from Earth, all the light appears to come from a single point. As the starlight passes through the Earth’s atmosphere, which has varying thermal and turbulent thickness levels, it is bent, or refracted, in many changing directions from microsecond to microsecond, which blurs and distorts the stars appearance to an observer on Earth, even through telescopes. Planets, on the other hand, are relatively close to Earth and appear as small disks, which is why they rarely twinkle as stars appear to. For astronomers, the degree of twinkle is known as seeing and ranges from poor seeing, where a star’s light appears to jump around a lot, to excellent seeing, where the image remains relatively steady. Observing from satellites, such as the Hubble Telescope, scientists do not have to contend with these effects since they are well above the atmosphere.
Earth’s atmosphere also challenges observing in other ways. Though flame-red sunsets may be beautiful, astronomers resent the atmospheric refraction and distortion effects which compromise their views of the sun. During a sunset or sunrise, when the relatively round sun is just above the horizon, the thickness of the atmosphere through which the sunlight passes varies greatly, refracting the sun’s disk, distorting its image, squashing and flattening its appearance top to bottom. This is also the case for the moon and the planets, though the effect with the Sun is more obvious because of its brightness.
Dust in the atmosphere also effects observations of celestial objects. The varying colors of the visible spectrum are scattered differently when passing through more or less atmosphere. Short wavelength blue light is scattered very easily, even when directly overhead, which provides us with our blue sky while making the sun appear slightly yellower. However, during sunset, when the sun is near the horizon, the light passes through a very thick atmosphere where only the long wavelength red light can traverse, making the sun look red and the sky appear orange. This reddening effect is also apparent during a lunar eclipse. When the Moon passes into the Earth’s shadow, the sunlight passes through the Earth’s atmosphere, where only the red light makes it through and is refracted toward the lunar eclipsed disk, giving it a coppery or blood red hue.
July a great month for observing planets
With a small telescope or a pair of binoculars, Mercury will be visible on the western horizon setting around 9 p.m. (about 75 minutes after sunset.) Brilliant Venus is also in the west, in Leo, and can be viewed until it sets around 10 p.m. On July 15, it will be located just below the crescent moon. High in the sky, in Libra is the giant planet Jupiter with its own four large moons, which can be found just below the gibbous moon July 20. Moving to the east, the ringed planet Saturn rises just above the horizon at about 8 p.m. in the constellation Sagittarius, just above the top of the teapot. And later in the evening, reddish Mars will rise about 9 p.m. in Capricorn and will be in good observing position before midnight.