From within the Moon's penumbra, a partial solar eclipse can be seen. Like any other opaque objects illuminated by a light source, the Moon and the Earth cast shadows into space as they block the sunlight that hits them. Each shadow has 3 different areas: the umbra, the penumbra, and the antumbra.
Group IV of the Periodic Table of the Elements contains carbon (C), silicon (Si) and several heavy metals. Carbon, of course, is the building block of life as we know it. So is it possible that a planet exists in some other solar system where silicon substitutes for carbon? Several science fiction stories feature silicon-based life-forms--sentient crystals, gruesome golden grains of sand and even a creature whose spoor or scat was bricks of silica left behind. The novellas are good reading, but there are a few problems with the chemistry.
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CRYSTALLINE CREATURES? Silicon can grow into a number of lifelike structures, but its chemistry makes it unlikely that it could be the basis for alien life-forms.</span>
Indeed, carbon and silicon share many characteristics. Each has a so-called valence of four--meaning that individual atoms make four bonds with other elements in forming chemical compounds. Each element bonds to oxygen. Each forms long chains, called polymers, in which it alternates with oxygen. In the simplest case, carbon yields a polymer called poly-acetal, a plastic used in synthetic fibers and equipment. Silicon yields polymeric silicones, which we use to waterproof cloth or lubricate metal and plastic parts.
Sea otters are a keystone species. They play an important role in the health and stability of near shore marine ecosystems. They eat sea urchins and other invertebrates that eat vast quantities of giant kelp. ... Providing habitat is one of many services kelp offers to the marine ecosystem.