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Eclipse (continued) + ward ' s science Penumbral eclipses can last up to 5 h. However, these events are of only academic interest since they are quite faint and dif- ficult to see. Only the deepest penumbral eclipses (penumbral eclipse magnitude greater than 0.7) are visible to the unaided eye as a subtle shading along one edge of the Moon. Partial eclipses are far easier to see and appear as an obvi- ous bite or missing section along one edge of an otherwise full moon. They can last as long as 31/3 hours but are usually shorter. Partial eclipses are relatively colorless although the edge of the Earth's umbra may exhibit a reddish tinge when viewed through a telescope. The umbral eclipse magnitude of a partial eclipse is always greater than 0.0 and less than 1.0. Total lunar eclipses have a maximum duration of nearly 21/3 hours. Their appearance is dramatically different from both penumbral and partial eclipses. When the Moon is completely immersed within the Earth's umbral shadow, it is illuminated only by indirect sunlight that has been refracted through and filtered by the Earth's atmosphere. In the process, most of the blue and green colors present in sunlight are removed. The remaining light is deep red or orange and is greatly dimmed. This gives total lunar eclipses their characteristically blood-red color, which frightened many early civilizations. If the Earth had no atmosphere, the Moon would be completely black during a total eclipse The actual appearance of the Moon during totality can vary from one eclipse to the next. The colors range from bright orange and yellow, through various shades of red, to dark brown or gray. These visual differences are directly related to the amount of volcanic material suspended in Earth's strato- sphere. Most dust and ash from volcanic eruptions rises into the troposphere where it falls back to Earth within a few days. But the sulfur dioxide gas also produced from volcanic activ- ity combines with water vapor in the stratosphere, where it forms a high-altitude haze of sulfuric acid droplets. This haze plays a major role in determining the color and darkness of the Moon during totality. When a large amount of sulfuric acid haze is present, it has a strong filtering effect and produces a correspondingly darker eclipse. For example, the 1991 eruption of Mount Pinatubo in the Philippines pumped large amounts of sulfurous gas and ash more than 35,000 m (115,000 ft) high into the stratosphere where it was transported globally via the jet stream. Red sunsets were seen around the world, and the total lunar eclipse of December 1992 was so dark it rendered the Moon nearly invisible during totality (Fig. 9). Frequency The geometry and recurrence of lunar eclipses have much in common with solar eclipses. Lunar eclipses occur during eclipse seasons lasting about 39 days with a cycle of two seasons every 346.6 days. Lunar eclipses with similar characteristics repeat ev- ery 6585.32 days (about 18 years 111/3 days), which is the same saros cycle described above for solar eclipses. Every lunar eclipse is either preceded or followed by a solar eclipse within a time span of 15 days. Of course, the lunar eclipse will occur at full moon, while the solar eclipse will occur 2 weeks earlier or later at new moon. The link between lunar and solar eclipses means that there are a maximum of seven eclipses within a calendar year. They can occur in combinations of five and two, or four and three, with either solar or lunar eclipses in the majority. In an average century, there are 241 lunar eclipses consisting of 87 penumbral, 84 partial, and 70 total eclipses. Every year has a minimum of two and a maximum of five lunar eclipses. Future lunar eclipses The twenty-first century will have 228 eclipses of the Moon, consisting of 86 penumbral, 57 partial, and 85 total eclipses. During this epoch, 78 years will have 2 lunar eclipses each, 16 years will have 3 eclipses, and 4 years will have 4 eclipses. No year contains more than 4 eclipses. The last time 5 lunar eclipses occurred during one calendar year was 1879, and the next will be 2132. During the 25-year period 2011 through 2035, there will be 23 total eclipses of the Moon (Table 2). Deep eclipses have large magnitudes and long durations (for example, June 15, 2011; July 27, 2018; and June 26, 2029). Fig. 9 Total lunar eclipse of December 9, 1992, which was so dark that it rendered the Moon nearly invisible during totality. (Credit: F. Espenak)