As a child, I learned that the way to distinguish a star from a planet in the night sky was to determine if it twinkles. That is the fundamental visual difference in these two types of celestial bodies. The reason behind this visual phenomenon has to do with the object's angular size, or its apparent size in our sky resulting from its actual size and distance. A star is a very large object but is at a tremendously large distance from us. By contrast, a planet is smaller, but very, very much closer to us. The result is a star being a point source of light while a planet appears as a disk (albeit a very small disk). Think of it as a star emitting a stream of photons in single file compared to a planet emitting a wide bundle of photons. Upon entering the Earth's turbulent atmosphere, the light from both the star and planet will be refracted (bent) to the same degree, but the skinny photon stream from the star will be affected more by the atmosphere than the thick photon stream from the planet. As the refractive index of the atmosphere varies from the turbulence in its different layers, the star will twinkle while the planet will shine with a steady light. It's like comparing a six-inch wide creek with a 100-foot wide river. Throw a rock in each and the creek will be affected to a larger degree than the river.
We can see this visual difference for a couple more weeks in the night sky. Look to the southwest just as the sky darkens enough to reveal the bright star Antares and the planet Mars. They are currently 5 or so degrees apart, with Antares being below Mars. The ruddy surface of Mars and the M-type spectral characteristic of Antares give them a very similar color, which is reddish-orange. As explained above, Antares twinkles because it has an apparent size in our sky of .0471 arc seconds (1.12 billion km in diameter at 550 light years away--virtually a point) compared to the apparent size of Mars of 6.1 arc seconds (6,800 km in diameter at 227,680,000 km away.)
The photograph was taken on September 29th, 2014, so the moon is in the scene also.
