Have you ever noticed that when there's a solar or lunar eclipse, an eclipse of the other variety comes two weeks before or after? Sometimes, we'll even get three eclipses in less than a month.
Just such a situation is happening right now. We already have had a partial eclipse of the sun this month, on Friday the 13th, visible only from parts of Antarctica, Tasmania, Australia and a very narrow slice of New Zealand. Another partial eclipse of the sun will take place on Aug. 11; it will be visible to those living in northern Europe as well as a large portion of central and eastern Asia. Even parts of far northern and eastern Canada will get a brief glimpse of it at sunrise.
And right in the middle, between the two solar eclipses, there will be a total eclipse of the moon on July 27. [Blood Moon 2018: Longest Total Lunar Eclipse of Century Occurs July 27]
That's because on that day, the moon will cross the ecliptic, the line that marks the sun's path around the sky. If the moon crosses the ecliptic when full, we'll get an eclipse of the moon; if the moon crosses the ecliptic when new, we'll get an eclipse of the sun. Incidentally, this is the reason the sun's path around the sky is called the "ecliptic" — for "eclipse"!
And whenever a full or new moon crosses the ecliptic on one side of the lunar orbit, it will also cross that line on the opposite side of the lunar orbit, either two weeks earlier or two weeks later. We call this period an "eclipse season."
Three for the price of two
We typically get two eclipses during an eclipse season. But as was just noted, during this current eclipse season, not two, but three eclipses will take place within a single synodic month of 29.53 days — the time it takes to go from one new moon to the next.
The upcoming total eclipse of the moon on July 27 is going to be especially noteworthy, as the satellite will pass just to the north of the middle of Earth's shadow. In fact, the moon will reach the descending node of its orbit — the point where it crosses the ecliptic going from north to south — just 138 minutes after it arrives at full phase, resulting in a nearly central total eclipse.
Because of this, the two new moons that flank the July 27 full moon will skirt just close enough to the moon's opposite (ascending) node to allow the moon to marginally eclipse the sun both times. Hence, we get three eclipses instead of the usual two.
A small, slow moon + a big shadow = a long eclipse
Another artifact of the moon's near-central passage through the Earth's shadow is that totality will last an unusually long time. Actually, the moon's path through the shadow is just one factor in making for a long-lasting total phase; the moon's distance also has an effect. About 14 hours before it's full, the moon will be at its farthest point from Earth (apogee), a distance of 252,415 miles (406,223 kilometers), and the moon moves slowest in its orbit when it's farthest from the Earth.
This also results in a full moon that is much smaller than average; in fact, this will be the smallest full moon of 2018. This eclipse will occur three weeks after the Earth has arrived at aphelion, the farthest point in the planet's orbit from the sun, when the Earth's umbral shadow appears largest in angular size.
So, summing it all up, we will have a small moon, moving more slowly than usual, passing almost straight through the middle of a larger-than-normal shadow of Earth. This will result in an inordinately long eclipse whose total phase will last 1 hour and 43 minutes, just 4 minutes shy of the longest possible totality. According to "The Five Millennium Canon of Lunar Eclipses: (-1999to +3000)" by Fred Espenak and Jean Meeus, this is the longest total lunar eclipse Earth will see until June 9, 2123.
The average duration of a total lunar eclipse is about 50 minutes.
North America shut out
Now for some bad news, if you live in North or Central America: None of this eclipse will be visible in these locations — not even the partial stages — because the entire event will take place during the midday and afternoon hours, when it will be daytime and the moon will be below the horizon.
If, however, you happen to be in Europe, Africa or Asia, you'll be able to see the event. The moon will appear directly overhead from a point only a few hundred miles off the east coast of Madagascar in the Indian Ocean.
Much of Japan and eastern Australia will see the moon set while it's in total eclipse. The central and eastern portions of South America will see the moon rise either during or after totality.
Don't forget Mars
Along with the totally eclipsed moon, something else in the sky will vie for attention: Mars. On the day of the eclipse, Mars will be at opposition to the sun. Shining with a dazzling yellow-orange glow, this planet will be situated about a half-dozen degrees below the moon. Some longtime eclipse enthusiasts might recall a similar pairing of a totally eclipsed moon with Mars at an exceptionally bright opposition on Aug. 6, 1971. That eclipse, like this upcoming one, was visible only from the Old World.
The next total eclipse of the moon will occur Sunday, Jan. 20, 2019.
As if to try to compensate North Americans who will miss out on this summer's show, the next lunar eclipse will strongly favor the Western Hemisphere. From most of the United States and Canada, the eclipse will be visible from start to finish, with totality occurring during convenient "prime-time" evening hours. In addition, the totally eclipsed moon will appear very high in the winter sky.
It will also occur during the three-day Martin Luther King Jr. holiday weekend, ensuring a large viewing audience nationally. This will be the first time that a total lunar eclipse has occurred during a holiday weekend since May 1975. In short, it likely will be a very popular and much-anticipated celestial event.
Hopefully, the weather in your area will cooperate. Mark your calendars.
Editor's note: If you snap an amazing photo of Friday's total lunar eclipse and would like to share it with Space.com for a story or photo gallery, send comments and images to firstname.lastname@example.org.
Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for Natural History magazine, the Farmer's Almanac and other publications, and he is also an on-camera meteorologist for FiOS1 News in New York's lower Hudson Valley. Follow us @Spacedotcom, Facebook and Google+. Original article on Space.com.