Lunar eclipse is coming

By Thomas Webber
Posted 7/18/18

Let’s begin today’s effort not by looking forward, but by looking backward. I invite you to join me in reminiscing about an adventure that began 57 years ago when a young John F. Kennedy …

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Lunar eclipse is coming

Posted

Let’s begin today’s effort not by looking forward, but by looking backward. I invite you to join me in reminiscing about an adventure that began 57 years ago when a young John F. Kennedy addressed a joint session of Congress:

“I believe that this nation should commit itself to achieving the goal, before this decade is out, of landing a man on the moon and returning him safely to the earth.”

Fueled by the Cold War and the successes of the Soviet Space Program, the United States accepted this challenge. It would not be an easy task, but nothing worthwhile ever is. Icarus himself did not even dare to dream such a dream.

President Kennedy reinforced our American resolve a year later when he told a crowd at Rice University, “We choose to go to the moon. We choose to go to the moon in this decade and do the other things, not because they are easy, but because they are hard, because that goal will serve to organize and measure the best of our energies and skills…”

That measure was soon realized. On July 20, 1969, American astronauts Neil Armstrong and Edwin “Buzz” Aldrin walked on the surface of the moon. We had broken free from the bonds of gravity and touched another world.

Let us not let July pass without taking pause to reflect on this greatest of accomplishments.

More on the moon:

The interaction of the Sun, Earth and Moon yield phenomena that we can observe directly, such as tides and eclipses. The study of tides is complex, to be sure, involving the positions of all three gravitating bodies.

Eclipses occur when these three celestial orbs are oriented in a straight line.

A solar eclipse happens when the Moon is between the Sun and Earth, blocking part or all of the disk of the Sun. You might remember that Northeast Florida was able to experience a solar eclipse last August.

When Earth is between the Moon and Sun, conditions are right for a lunar eclipse, when the full Moon passes through Earth’s shadow in space.

There will be a total lunar eclipse on July 27. And while eclipses are not rare, this one is particularly noteworthy, for it will be the longest lunar eclipse of the 21st century, lasting for 1 hour and 43 minutes.

This is just 4 minutes shy of the longest a lunar eclipse can last. In the words of the immortal Maxwell Smart, “Missed it by that much!”

So why aren’t all eclipses the same? To answer this, we must consider both optics and celestial mechanics.

A shadow has two main parts: The lighter region called the penumbra which surrounds the dark inner region called the umbra. During any total lunar eclipse, the Moon is passing through Earth’s umbral shadow.

As for the mechanics involved, remember that orbits are not circles, but ellipses. This means that the distances between orbiting bodies is not constant.

Earth is at its farthest point from the Sun, called the aphelion, in July. This results in Earth’s umbra to be longer and wider.

Meanwhile, the date of the full Moon and the time the Moon is farthest from Earth, the apogee, fall on – yep, you guessed it – July 27. This means that the Moon will appear smaller in the sky and that it will be moving slower in its orbit.

Finally, the orbital path of the Moon is through the center of the umbra.

Putting it all together, we have a slower-moving, smaller full Moon passing straight through a larger umbral shadow being cast by Earth. Viola – the makings of the longest lunar eclipse of the century.

Since this total lunar eclipse will be preceded and followed by a partial eclipse, from start to finish this eclipse will last 3 hours and 55 minutes. The accompanying Fact Box has the precise times.

Now, you’re saying, “Tom, you truly are ‘out of this world.’ What can we do to witness this for ourselves?”

Sadly, not too much from the United States. Although this eclipse provides us the opportunity to learn some physics and astronomy, it will be visible primarily in Europe, Africa, Asia and Australia. However, many sites will be streaming the eclipse live.

So, a twofer today on the Moon. It’s not something I usually do – I guess its just a “phase” I’m going through!

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