Description:
Jul. 27, 2018 - Total Lunar Eclipse (Blood Moon)
A lunar eclipse (also known as a blood moon) occurs when the Moon passes directly behind the Earth into its umbra (shadow). This can occur only when the sun, Earth and moon are aligned (in "syzygy") exactly, or very closely so, with the Earth in the middle. Hence, a lunar eclipse can occur only the night of a full moon. The type and length of an eclipse depend upon the Moon's location relative to its orbital nodes. Unlike a solar eclipse, which can be viewed only from a certain relatively small area of the world, a lunar eclipse may be viewed from anywhere on the night side of the Earth. A lunar eclipse lasts for a few hours, whereas a total solar eclipse lasts for only a few minutes at any given place, due to the smaller size of the Moon's shadow. Also unlike solar eclipses, lunar eclipses are safe to view without any eye protection or special precautions, as they are dimmer than the full moon. The shadow of the Earth can be divided into two distinctive parts: the umbra and penumbra. Within the umbra, there is no direct solar radiation. However, as a result of the Sun's large angular size, solar illumination is only partially blocked in the outer portion of the Earth's shadow, which is given the name penumbra.
A penumbral eclipse occurs when the moon passes through the Earth's penumbra. The penumbra causes a subtle darkening of the moon's surface. A special type of penumbral eclipse is a total penumbral eclipse, during which the Moon lies exclusively within the Earth's penumbra. Total penumbral eclipses are rare, and when these occur, that portion of the moon which is closest to the umbra can appear somewhat darker than the rest of the moon. A partial lunar eclipse occurs when only a portion of the moon enters the umbra. When the moon travels completely into the Earth's umbra, one observes a total lunar eclipse. The moon's speed through the shadow is about one kilometer per second (2,300 mph), and totality may last up to nearly 107 minutes. Nevertheless, the total time between the moon's first and last contact with the shadow is much longer, and could last up to four hours. The relative distance of the moon from the Earth at the time of an eclipse can affect the eclipse's duration. In particular, when the moon is near its apogee, the farthest point from the Earth in its orbit, its orbital speed is the slowest. The diameter of the umbra does not decrease appreciably within the changes in the orbital distance of the moon. Thus, a totally eclipsed moon occurring near apogee will lengthen the duration of totality. A central lunar eclipse is a total lunar eclipse during which the moon passes through the centre of the Earth's shadow. These are relatively rare. Earth's shadow is red at the edges for the same reason a sunset is red: When sunlight is scattered by passing through Earth's atmosphere, the other colors of the spectrum are removed. In a solar eclipse, the moon casts its shadow on Earth. The darkest part of the moon's shadow - the UMBRA - is no more than 166 miles wide (267 kilometers) on the surface of the Earth. When sunlight entering the Earth's atmosphere strikes the particles that are smaller than the light's wavelength, it gets scattered. Not all colors in the light spectrum, however, get equally scattered. Colors with shorter wavelengths, for example those towards the violet spectrum, are scattered more strongly than those with longer wavelengths like red and orange, which pass through the atmosphere. This light then gets bent or refracted back to the surface of the Moon and gives it the reddish-orange glow that total lunar eclipses are famous for. Because the orbit of the moon is tilted relative to the orbit of Earth, eclipses can't happen at every new and full moon. The shadow cones pass above or below the moon and Earth most of the time. Only at two points during the year do the sun, Earth and moon line up properly to allow for eclipses. A lunar eclipse is visible to anyone who can see the moon, which usually means half of the Earth at a time. In a total solar eclipse, the moon's shadow passes directly over only a narrow strip on the Earth's surface. Observers in a larger area can see the sun partially eclipsed.
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