Before you view a planet you must first find one. For many people this is a non-trivial problem. You can approach it as the ancient people did and watch every night for objects which slowly move amongst the stars. Five planets – Mercury, Venus, Mars, Jupiter and Saturn – were discovered this way. There is another clue and that involves the twinkle of the stars. The stars are so distant that they can be considered, for all practical purposes, point sources of light. As their light comes down through the atmosphere, small variations in density, temperature and upper atmosphere winds act like weak lenses, focusing and defocusing the light from moment to moment. Thus, the brightness of stars appears to change. Their position in the sky will also appear to change rapidly, causing the star to appear as a small fuzzy disk in the telescope. This is the atmospheric seeing limit discussed earlier.

Planets, on the other hand, cannot be considered point sources of light, for they have a measurable, if small, disk image. The light from one side of the planet does not travel the same path through the atmosphere that light from the other side travels. Each atmospheric disturbance thus affects only a small part of the light coming from the planet. The effects of many separate tiny disturbances tend to cancel each other out when looking at a planet with the naked eye.