The longest total lunar eclipse of the 21st century takes place today Friday, July 27. Blood Moon 2018: The time of greatest eclipse will be 4:21 p.m. EDT (2021 GMT) on July 27, according to EarthSky.org. The total eclipse will last from 3:30 p.m. to 5:13 p.m. EDT (1930 to 2113 GMT). There will also be sometime before and after when the moon is in the lighter part of Earth’s shadow, which is called the penumbra. Including that penumbral time, the eclipse will last for 3 hours and 55 minutes.
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Blood Moon 2018:
The total phase of the “blood moon” eclipse of July 27 will last 1 hour and 43 minutes, during which Earth’s natural satellite will turn a spectacular red or ruddy-brown color. From start to finish, the entire celestial event will last nearly 4 hours. The eclipse won’t be visible to viewers in North America, except via webcasts. But observers in much of Africa, the Middle East, southern Asia and the Indian Ocean region will get an eyeful, given the cooperative weather, according to lunar scientist Noah Petro, of NASA’s Goddard Space Flight Center in Greenbelt, Maryland.
[In Photos: The Rare Super Blue Blood Moon Eclipse of 2018] July’s total lunar eclipse occurs on the same day the planet Mars reaches its opposition when it will shine at its best in the night sky. This month, Mars will be at its closest to Earth since 2003. After opposition, when Mars will be brightest, it will reach that closest point on July 31. You can learn more about that event in our dedicated guide here: Mars at Opposition 2018: How to See It and What to Expect
What is a lunar eclipse?
A lunar eclipse occurs when the Moon passes directly behind Earth and into its shadow. This can occur only when the Sun, Earth, and the Moon are aligned (in syzygy) exactly or very closely so, with the planet in between. Hence, a lunar eclipse can occur only on the night of a full moon. The type and length of an eclipse depend on the Moon’s proximity to either node of its orbit.
During a total lunar eclipse, Earth completely blocks direct sunlight from reaching the Moon. The only light reflected from the lunar surface has been refracted by Earth’s atmosphere. This light appears reddish for the same reason that a sunset or sunrise does: the Rayleigh scattering of bluer light. Due to this reddish color, a totally eclipsed Moon is sometimes called a blood moon.
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 Earth. A total lunar eclipse lasts a few hours, whereas a total solar eclipse lasts only a few minutes as viewed from 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.
Types of the lunar eclipse
Earth’s shadow can be divided into two distinctive parts: the umbra and penumbra. Earth totally occludes direct solar radiation within the umbra, the central region of the shadow. However, since the Sun’s diameter appears about one-quarter of Earth’s in the lunar sky, the planet only partially blocks direct sunlight within the penumbra, the outer portion of the shadow.
A penumbral lunar eclipse occurs when the Moon passes through Earth’s penumbra. The penumbra causes a subtle dimming of the lunar surface. A special type of penumbral eclipse is a total penumbral lunar eclipse, during which the Moon lies exclusively within Earth’s penumbra. Total penumbral eclipses are rare, and when these occur, the portion of the Moon closest to the umbra may appear slightly darker than the rest of the lunar disk.
A partial lunar eclipse occurs when only a portion of the Moon enters Earth’s umbra, while a total lunar eclipse occurs when the entire Moon enters the planet’s umbra. The Moon’s average orbital speed is about 2,300 mph (1.03 km/s), or a little more than its diameter per hour, so totality may last up to nearly 107 minutes. Nevertheless, the total time between the first and the last contacts of the Moon’s limb with Earth’s shadow is much longer and could last up to four hours.
The relative distance of the Moon from Earth at the time of an eclipse can affect the eclipse’s duration. In particular, when the Moon is near apogee, the farthest point from Earth in its orbit, its orbital speed is the slowest. The diameter of Earth’s umbra does not decrease appreciably within the changes in the Moon’s orbital distance. Thus, the concurrence of a totally eclipsed Moon near apogee will lengthen the duration of totality.
A selenelion or selenehelion occurs when both the Sun and an eclipsed Moon can be observed at the same time. This can occur only just before sunset or just after sunrise when both bodies will appear just above the horizon at nearly opposite points in the sky. This arrangement has led to the phenomenon being also called a horizontal eclipse.
Typically, a number of high ridges undergoing sunrise or sunset can view it. Although the Moon is in Earth’s umbra, both the Sun and an eclipsed Moon can be simultaneously seen because atmospheric refraction causes each body to appear higher in the sky than their true geometric positions.