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OUT OF THIS WORLD | What's Up In Space - the biggest news coming down to Earth from space

See the century’s longest Total Lunar Eclipse from ANYWHERE

Scott Sutherland
Meteorologist/Science Writer

Monday, July 23, 2018, 2:28 PM - Since Canada and the United States won't see Friday's Total Lunar Eclipse in person, here's how to watch this century's longest total lunar eclipse from anywhere in the world!

July 27, 2018 is a busy night for astronomers and skywatchers.

First, there's the July Full Moon - known as the 'Full Thunder Moon', due to the thunder storms often encountered during this month of the year, or 'Full Buck Moon', as this is the time of year when male deer begin to regrow their antlers, so it'll be a great time to look up and see the Moon in all its glory.

Accompanying the Full Moon across the sky, is a very bright planet Mars, which will be up all night long, as Earth, Mars and the Sun line up perfectly in an alignment known as Opposition.

Big, bright Mars, along with the Mini Thunder Moon, on July 27. Still-bright Saturn will be visible off to the southwest. Credit: Stellarium/Scott Sutherland

This Full Thunder Moon is remarkable in two different ways:

1) It's the farthest and smallest Full Moon of 2018, and
2) it's producing the longest total lunar eclipse that many on Earth will ever see.

If you happen to be in the right place in the world, or you are glued to your computer monitor watching the event as it is live-streamed (see below), for one hour and 43 minutes on Friday, the Moon is going to change colour to match Mars, as it passes straight through the deepest, darkest part of Earth's shadow.

The path of the Moon through Earth's shadow on July 27, 2018. Credit: NASA/Scott Sutherland

The timing of the eclipse is such that it will be occurring on Friday night for Europe, Africa, Asia and Australia, but during Friday afternoon for those of us in North America.

The visibility of the July 27, 2018 Total Lunar Eclipse. Credit: Fred Espenak/NASA

As the map above shows, the best places to watch from are in the Middle East, western Asia, and the eastern half of Africa. It will completely skip the United States, and the only part of Canada that will see any part of it, in person, is the eastern half of the island of Newfoundland, just as the Moon rises that night.

It's doubtful anyone who watches the Moon-rise from there will actually notice the eclipse, however, because only the diffuse penumbra will be covering the Moon at that time, and it is very difficult to notice that part of the eclipse.

Don't fret, though. In this digital age, there always seem to be options for us, so that we won't miss out on anything going on in the astronomy world.


The truly remarkable part of this eclipse, though: it's going to be the longest total lunar eclipse this century!

From start to finish, the time of the total lunar eclipse, where the Moon will appear completely red, will last for 103 minutes. That's 1 hour and 43 minutes of totality. There was a longer one just 18 years ago, on July 19, 2000, which lasted for 106 minutes. Going forward, there isn't another total lunar eclipse that's longer than this one for the rest of this century, and in fact, until the year 2123!

Why is this eclipse so long? There's a few reasons behind it, so let's take a close look at how they all mesh together to produce this.

The most important factor here is the shape of the Moon's orbit, since that controls the relative distance, the apparent size of the Moon, and the speed at which the Moon crosses the Earth's shadow. Also important is the tilt of the Moon's orbit, since that controls the exact path the Moon takes as it crosses the umbra.

The Moon's Orbit

As the Moon orbits around Earth, it doesn't trace a perfect circle. Instead, its path is slightly elliptical, with the Earth ever so slightly off-centre. Thus, at times the Moon is closer to Earth, and other times it's farther away.

Due to this, each month, the Moon passes through its closest point to Earth, which is known as 'perigee'. At its farthest, it's at 'apogee'. Perigee can range from 356,400 km to 370,400 km away, while apogee can be anywhere from 404,000 km to 406,700 km away (the Moon's average distance away is roughly 384,400 km).

The ellipse of the Moon's orbit around Earth, for July 2018. Lunar perigee for July is on the 13th, during the New Moon, at a distance of 357,431 km. Lunar apogee for July is on the 27th, during the Full Moon, at a distance of 406,222 km. Credit: NASA Goddard Scientific Visualization Studio/Scott Sutherland

Perigees and apogees vary in distance, month to month, as well, though. This is because the planet the Moon is orbiting around - Earth - is on a constant looping path around the Sun.

As we all know, when each Full Moon occurs, the Moon is on the exact opposite side of Earth from the Sun.

Starting at any particular Full Moon, it takes a little over 27 days for the Moon to complete one full orbit around the Earth, so that its back in the same exact position it was during that Full Moon. This 27.3-day period is known as a 'sidereal' orbit.

As shown in the animation below, however, when the Moon completes that sidereal orbit, it is not a Full Moon. It is only mostly full. This is because, as the Moon was travelling for 27 days around the Earth, Earth was advancing 27 days further along its own orbit, and thus changing its orientation with respect to the Sun.

So, while Moon does end up back in the same place it was, the last time it was full, that position in its orbit doesn't put it back on the exact opposite side of Earth from the Sun again.

To end up on the exact opposite side of Earth from the Sun again, and thus give us the next Full Moon, the Moon needs a little over two days of further travel along its orbit. The full 29.5-day period between successive Full Moons is called a 'synodic' orbit.

The Moon's orbit around Earth, showing the difference between a sidereal orbit and a synodic orbit, revealing how the Moon's position in its orbit advances for each Full Moon. Watch the face of the Moon, top left, to see how it appears larger or smaller, depending on where the Moon is along its elliptical orbit. Credit: NASA Goddard Scientific Visualization Studio/Scott Sutherland

This difference between a sidereal orbit and a synodic orbit means that, whenever we see a Full Moon, the Moon is at a different position along its orbit than it was the previous time. And, since different positions along the ellipse put the Moon closer or farther from Earth, the Full Moons go through a cycle - getting progressively closer (appearing larger and brighter) until we have our Perigee Full Moon, and then getting farther away (appearing smaller and dimmer) until we see the Apogee Full Moon. Watch the top left corner of the above animation to see this happen.

Back on January 2, 2018, we had 2018's Perigee Full Moon. While the lunar apogee that immediately followed, on January 15, was the farthest away the Moon will reach all year (406,459 km), that corresponded with the New Moon - when the Moon is on the same side of Earth as the Sun.

The July 27 lunar apogee - the second farthest lunar apogee of the year - happens on the same night as the Full Moon. That makes it this year's Apogee Full Moon.

July's Apogee Thunder Moon. On the left, the Moon reaches apogee, at a distance of 406,228 km, at 06 UTC, while it is still 99.9 per cent full. On the right, the Moon reaches 100 per cent full, roughly 16 hours later, but when the Moon is slightly closer to Earth. In the background, the January 1 Perigee Supermoon looms for comparison. Credit: NASA GSVS/Scott Sutherland

With the Full Moon being so distant, that means that it will appear smaller than usual, and it will present a smaller face as it passes through Earth's shadow. Thus, it will take less time for it to be completely covered by Earth's shadow, compared to a closer (and thus larger) Full Moon, and it will spend more time crossing to exit out the other side.

There's also a 'quirk' to travelling on an elliptical orbit around a massive body like Earth. When you're passing through apogee - the farthest point of the ellipse - you will be moving at the slowest speed of your orbit (your fastest speed is at perigee).

Thus, with this Full Moon occurring at apogee, it is a very slow moving Full Moon. Accordingly, when the Moon is passing through Earth's shadow, it will do so at a very leisurely pace, and thus take longer to pass through the umbra.

The Path of Totality

The exact path of the Moon through the umbra also comes into play, because the umbra is a circle. So, any path of totality closer to the centre of that circle is going to translate into the Moon spending more time as a total lunar eclipse.

If the Moon follows a path of totality that passes through the umbra closer to the top or the bottom of that circle, it will be a shorter total lunar eclipse. For good example of this, compare the path the July 27 eclipse takes through the umbra (above), to the path of the total lunar eclipse of May 26, 2021 (below).

The path of totality of the May 26, 2021 Total Lunar Eclipse. Due to the path being very close to the northern end of the umbra, this total eclipse will only last for 14 minutes. Credit: Fred Espenak/NASA

Apogee vs Aphelion

There is one other tiny factor that can matter, by the way, but not so much in this particular case.

The shape of Earth's umbra and penumbra, and the phases of a total lunar eclipse, based on its position inside those two parts of the shadow. The diagram is not to scale. Credit: NASA Goddard Scientific Visualization Studio

As the diagram above shows, Earth's umbra forms a cone of shadow, pointing off into space, directly away from the Sun. Because of that shape, the farther away you are from Earth, the more narrow the umbra is. So, when the Moon is at its apogee, it is passing through a smaller part of the umbral cone than it does at any other point in its orbit.

Now, that would make an apogee lunar eclipse shorter than any other kind, but this time, there's actually something compensating for this factor. On July 6, Earth passed through its aphelion - the planet's farthest distance from the Sun of the year. So, since the source of light that produces the umbra is farther away, the umbra stretches out longer, and is therefore wider than it normally would be, at every point along its length. And it just so happens that the effects of the Earth at aphelion and the Moon at apogee roughly cancel each other out.

So, it really comes down to the combination of three things:

the Full Moon is at its farthest distance from Earth, and thus presents the smallest apparent size as it crosses Earth's shadow,
the Moon is travelling slower than at any other time in its orbit, due to being at apogee, and
the Moon is crossing Earth's shadow along a path of totality very near the centre of the umbra.

While there are absolutely going to be farther Full Moons than this one over the next century, which will appear smaller and be travelling slower at that time, and there are most certainly more lunar eclipses, and even those that will traverse the umbra closer to the centre, none will have the combination of all these factors, together, to produce a longer total lunar eclipse.

Sources: Royal Astronomical Society of Canada | NASA Lunar Eclipses | NASA Goddard Scientific Visualization Studio


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