Star Points for April, 1997; by Curtis Roelle CLASSIC COMET REMAINS VISIBLE Comet Hale-Bopp remains visible in the western sky as evenings continue warming in April. Fortunately, the brightest comet in 21 years has favored observers in the Northern hemisphere. During the first half of April the bright head of the comet will be the first "star" visible in the Northwest 20-25 degrees above the horizon (about one-forth of the way between the horizon and straight up) as evening twilight ends. As the sky darkens the tail becomes conspicuous and appears to get longer as the sky gets darker. The comet is best in a dark sky -- free from bright light sources such as the Moon and artificial outdoor lighting. The light of the waxing moon starts to interfere after the second weekend of April but retreats from the early evening sky during the last week of the month. During April's latter half the distance above the horizon rapidly declines as the comet rounds the Sun. Notice how the orientation of the tail changes during the month. In mid-March the tail extended toward the North nearly parallel to the horizon. The appearance was remarkably similar to a view portrayed in a 1759 painting of Comet Halley by Samuel Scott. Night after night the tail appears to rotate counterclockwise such that by mid-April it will be nearly vertical. By early May the tail will be extending more towards the South. (See the illustration by John Levenson in Star Points for March 7.) The name comet comes from the greek "kometes", meaning "long- haired". This undoubtedly describes the long tails they often sport. Deep down inside comets are believed to contain a solid body or "nucleus" consisting of some combination of rocky and icy materials. Some astronomers describe them as dirty snowballs, while others think of them more as icy mudballs. Most comets spend the majority of their time far from the Sun beyond the orbit of Pluto, where they slowly orbit the Sun at speeds comparable to that of a slow-moving single-engine airplane. As a comet's orbit brings it into the inner region of the solar system their speed increases. The radiation pressure of the "solar wind" causes the icy material to sublimate, i.e. change from a solid to gas without passing through the liquid state. As the process continues the comet develops an envelope or cloud of material around the nucleus called the "coma". As the comet comes closer to the Sun and the solar wind pressure continues, material from the coma is driven back to form a tail. The tail can extend for 100 million miles or more. A comet's tail always points away from the Sun. This means that as the comet approaches the Sun, the tail streams out behind it. But as the comet heads away from the Sun, the tail extends out in front. In other words, the comet's direction of travel cannot be deduced simply by the orientation of the tail. In my opinion Hale-Bopp is in a class with the famous comets of the 19th century. Many observers I have talked with reported seeing things they have never seen in other comets during their lifetime. Combing through some comet books I noticed similarities between Hale-Bopp and sketches of Donati's Comet (1858), the Comet of 1861, and Coggia's Comet (1874). In addition Hale-Bopp is so bright that I've been able to observe it with a telescope during daylight with the Sun above the horizon. The next program at the Bear Branch Nature Center's Planetarium will be on Saturday, April 26. Show times are at 7:30 and 8:30 p.m.. Reservations are required and may be obtained by calling Bear Branch at 410-848-2517 for reservations. The Planetarium Web page is at http://www.erols.com/roelle/was/bearbranch.html. The program will be repeated at the same times on Friday, May 2.