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A fireball is a meteor that appears to the human eye to have a brightness equal to or greater than that of Venus when it is visible in the sky. To achieve this level of brightness, the meteor is usually relatively large, and occasionally falls to the Earth's surface without being destroyed as it passes through the atmosphere. Fireballs are actually a fairly common occurrence, but they often occur during daylight or over unpopulated areas. They can appear to be different colors, to be followed by trails, and also generate sounds on rare occasions. If a fireball explodes with a bright flash, it is known as a bolide.
A very brightly lit meteor is often referred to as a fireball. According to experts in astronomy, the precise definition is that the meteor must appear to be at least as bright as Venus when seen in the evening or morning sky. According to the brightness scale commonly used, Venus is a -4 magnitude, with measurements further along the negative scale appearing brighter; for example a full moon has a magnitude of -12.6. For the fireball to be visible in daylight, it needs to be at least -6 magnitude — brighter if it appears close to the sun.
In general, to be bright enough to be categorized as a fireball, a meteor needs to be relatively large. Some are actually large enough to safely pass through the Earth's atmosphere without burning up and fall to the planet's surface, but this is a fairly rare occurrence. It is estimated that thousands of fireballs occur in the Earth's atmosphere daily. Most fireballs are never noticed and reported because they occur over unpopulated areas or they appear during daylight hours and aren't bright enough to be seen.
Fireballs can vary widely in appearance and duration. They can appear to be different colors, ranging from red to bright blues and purples; these hues are thought to result from the extreme heating of minerals and other substances of which the meteors are composed. Fireballs often appear to have a trail which can be made of smoke and visible during the day or they exhibit a glowing "train" of excited and ionized air molecules typically seen at night. The fireball and its trail can be visible for a variety of durations ranging from a few seconds to more than a minute. Anyone wishes to report a sighting should note the general appearance, color, location, and duration of the phenomenon, and then relay that information to the appropriate scientific organization for the region.