Tibet tours for Visibility As comets approach the Sun, they produce more luorescent gas and sunlight-relecting dust. The closer a comet gets to the Sun, the larger and brighter the comet becomes in space though not necessarily to those observing from Earth. If the brightness increases sufficiently, a comet may become visible to the naked eye. Active comets can be seen with the naked eye in a clear, dark sky when their astronomical magnitude attains to approximately +. to +. Comet brightness is dependent on many factors, such as the comet’s size, its productivity, and its proximity to Earth. It is very important to differentiate between apparent magnitude, the brightness as it appears to a human observer on Earth at a given point in time, and absolute magnitude, which is a rough measure of the intrinsic brightness of a celestial body. Tibet tours 2016.