Of all the phenomena that occur in our galaxy, comets are one of the most amazing. Being the best known, it can be said that when we talk about these small bodies, Halley's comet comes to mind. Although it graces us with its presence only about once every 75 years, Halley's Comet continues to amaze us today. Despite centuries of observation, there is still much to learn about comets. Comets are made up of three main parts: the tail, the coma and the nucleus. The tail, made up of a narrow column of dust and gas, is the largest part of the comet and can extend up to 100 million kilometers across the inner solar system. It is only visible when the comet is 3 AU from the Sun. The coma is a cloud of gas that can have a diameter of 100,000 kilometers, and is created by the heat of the Sun causing the comet's nucleus to boil. The core is often called a gigantic "iceberg" or "dirty snowball" and is a block of ice with an irregular mass and a diameter of about 10 kilometers. The Oort cloud, a collection of trillions of icy bodies, is a spherical shell that completely surrounds the Solar System and is believed to be the origin of the long-period comets that now orbit the Sun. Short-period comets come from Kuiper Belt, a doughnut-shaped ring that extends just beyond Neptune's orbit from about 30 to 55 AU. Astronomers believe that Comet Halley, considered a short-period comet, also comes from the Kuiper Belt. Halley's Comet has a nucleus with dimensions of 10 x 5 x 5 miles and travels at a speed of 157,838 miles per hour, making it one of the fastest comets in the night sky. Although viewers on Earth may only see Halley once every 75 years or so, the remnants of the famous comet are evident more frequently. Earth passes through Halley's dusty debris once in May and again in October during the Eta-Aquarid and Orionid meteor showers, respectively. Halley's Comet has been observed once every 75.3 years for the past 2,200 years, and the first recorded sighting dates back to the year 240 BC by the Chinese. Babylonian tablets show visits by Halley recorded in 164 BC and again in 87 BC.