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Do we have Eclipse science facts? Totally! Plus a free student handout from McGraw Hill’s AccessScience

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Eclipse (continued) Related phenomena are transits, such as those of Mercury and Venus, which occur when these planets cross the face of the Sun as seen from Earth. They are much too small to hide the solar surface. Transits of the Earth will be seen from spacecraft. Occultations of stars by the Moon are commonly seen from Earth, and are studied to monitor the shape and path of the Moon; a solar eclipse is a special case of such an occultation. Occultations of stars by planets, dwarf planets, asteroids, and Kuiper-belt objects are now increasingly studied; the rings of Uranus were discovered from observations of such an occulta- tion, and the atmospheres of Titan and Pluto are best known from occultation studies. Space missions, most notably NASA's Kepler mission, are discovering exoplanets by their transits across the faces of their parent stars. Solar Eclipses A solar eclipse can be understood as an occultation of the Sun by the Moon or, equivalently, the Moon's shadow crossing the Earth's surface. The darkest part of the shadow, from which the Sun is entirely hidden, is the umbra (Fig. 2). The outer part of the shadow, from which part of the Sun can be seen, is the penumbra. Solar eclipses can be central, in which the Moon passes entirely onto the solar disk as seen from Earth, or partial, in which one part of the Sun always remains visible. Central eclipses can be total, in which case the Moon entirely covers the solar photosphere, making the corona visible for the period of totality, or annular, in which case the Moon's angular diam- eter is smaller than that of the Sun because of the positions of the Earth and Moon in their elliptical orbits. At an annular eclipse, a bright annulus of photospheric sunlight remains visible; it is normally thousands of times brighter than the corona, leaving the sky too blue for the corona to be seen. The plane of the Moon's orbit is inclined by 5° to the plane of the Earth's orbit (the ecliptic), so the Moon's shadow commonly passes above or below the Earth each month at new moon. But two to five times each year, the Moon's shadow reaches the Earth, and a partial, annular, or total eclipse occurs. The Moon is approximately 400 times smaller than the Sun but is also approximately 400 times closer, so its angular diameter in the sky is about the same as the Sun's. Thus the Moon fits approxi- mately exactly over the photosphere, making the phenomenon of a total eclipse especially beautiful. Phenomena The partial phases of a total eclipse visible from the path of totality last over an hour. In the minute or two before totality, shadow bands—low-contrast bands of light and dark caused by irregularities in the Earth's upper atmosphere—may be seen to race across the landscape. As the Moon barely covers the Sun, photospheric light shines through valleys on the edge of the Moon, making dots of light—Baily's beads—that are very bright in contrast to the background. The last Baily's bead gleams so brightly that it appears as a jewel on a ring, with the band made of the corona; this appearance is known as the diamond-ring effect (Fig. 3). It usually lasts for 5–10 s, and in the clearest skies for as long as 40 s. + ward ' s science Fig. 2 Circumstances of solar and lunar eclipses (not to scale). Fig. 3 Diamond-ring effect at the beginning of totality during the solar eclipse of July 11, 2010, observed from Easter Island in the middle of the Pacific Ocean. (Credit: Jay M. Pasachoff/Science Faction, Muzhou Lu, and Craig Malamut, Williams College Eclipse Expedition)

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