By Mitzi Adams, NASA Marshall solar scientist
Last August, citizens and visitors to the United States of America had a rare opportunity to see a total solar eclipse, because the path of totality ranged from Oregon to South Carolina, essentially bisecting the country. But alas, the total lunar eclipse happening on Friday, July 27, will totally miss the United States. Being able to observe the Moon totally immersed in Earth’s shadow depends mostly on whether it is dark at the time the eclipse happens, so about half the Earth would be in the right place to see the eclipse, weather permitting of course. This time, residents of Europe, Africa, Asia, Australia, and parts of South America will be so lucky. In contrast, totality for a solar eclipse is very narrow and only a very small portion of Earth is in the shadow of the Moon. For the August 2017 eclipse, only those within an approximately 100 km (63 miles) wide path saw the Sun totally eclipsed.
So what happens when there is a lunar eclipse? Unlike the solar variety, Earth blocks the Sun for a lunar eclipse. For the lunar eclipse to happen, the Moon’s phase must be “full”, which means that the orbiting Moon is opposite the Sun, with Earth in between. When the Sun sets in the west, the Moon rises in the east — and this event happens once a “moonth” (or month). But a lunar eclipse does not happen every month. Why is that?
Well, now we get into more tricky territory. Let’s try a thought experiment. Draw a line between the centers of the Sun, Earth, and Moon. This line is part of a plane that describes how Earth orbits the Sun, called the plane of the ecliptic. The Moon orbits Earth, only its orbit is tilted with respect to the plane of the ecliptic, sometimes the Moon is above the plane, sometimes it is below the plane. Only when the Moon’s orbit lines up with the ecliptic plane do we have a chance for an eclipse. If the phase of the Moon is “full” when this happens, we have a lunar eclipse. If the phase of the Moon is “new,” we have a solar eclipse. Sometimes the orbital planes do not line up exactly, in those cases, we would have partial eclipses.
The July 27 eclipse is somewhat special because the length of totality will be the longest of this century at one hour, 43 minutes. Why? Several reasons. The Moon will be at apogee, or at the farthest distance from Earth (406,000 km or 252,000 mi) possible for our Moon. Objects in orbit around Earth move slower the farther away they are, which means it will take longer for the Moon to traverse the width of Earth’s shadow. In addition, the Moon will be almost exactly on that line that connects Sun, Earth, Moon, also increasing the length of time the Moon will spend in the umbral (darkest) part of Earth’s shadow. Finally, Earth reached its greatest distance from the Sun (aphelion) quite recently (July 6), meaning that Earth’s shadow on July 27 will be close to the largest it can be, adding even more distance (and time) to the Moon’s shadowy traverse.
The partial phase of the eclipse will begin at 18:24 UT, with totality beginning at 19:30 UT (see the NASA time zone page for help with conversion to your local time and official U.S. time). Totality will be over at 21:13 UT and the partial phase ends at 22:19 UT. Viewing a lunar eclipse does not require a telescope or even special glasses; however, while waiting for totality to begin, which is marked by a reddish-brown color to the Moon, a telescope could be used to view two planets that are in the evening sky. Mars will be visible, and should be pretty bright since there is currently a dust storm covering the entire planet. So the telescope will not see any surface detail here, but the redness of the planet will contrast well with the reddish hue of a totally eclipsed Moon. Saturn will be visible to the west of Mars — and even binoculars will resolve the rings, but a telescope could provide more detail. For all observers, find the full Moon in the night sky, Mars will be close to and below (south of) the Moon, a bright reddish “star-like” object. For detailed information about this eclipse, click here.