It is just a little over one month until Americans will witness the celestial event that is nearly a century in the making.

Some may be aware that instances of total solar eclipses are not all that rare, but what makes the great American eclipse's rarity increase is because of all the factors contributing to the August 2017 event.

As many already know, a total solar eclipse happens when the moon directly passes between the Earth and sun and casts a shadow that darkens some areas in our planet. Judging from the moon's orbit alone, a location on Earth could potentially experience a solar eclipse, however, there are more factors to consider before this celestial event happens.

Why Total Solar Eclipses Are 'Rare'

As mentioned above, the instance of the moon blocking the sun's light is, in itself, not rare and could potentially lead to a total solar eclipse. Since the sun is about 400 times larger and farther than the moon, the two heavenly bodies could align perfectly at any time — except, they don't because it's not that simple.

Here are the reasons why total solar eclipses are not often witnessed:

Uneven Orbital Shape
Just like the Earth, the moon follows an elliptical orbit, not a circle. This means that the moon's distance from Earth is not uniform and some instances wherein the moon passes between the Earth and the sun would not produce the same effect as a total solar eclipse.

When the moon is at its aphelion — when the moon is farther away from Earth — it does not fully cover the sun. The sun's ring of fire can still be seen and we instead experience an annular eclipse. An annular eclipse looks like the photo below.

Earth's Tilt And Moon's Inclined Orbit
The moon's distance is already established so the next thing to consider is our satellite's inclined orbit relative to our planet's equator.

Since the moon's orbit is inclined to follow the equator, it means that majority of the instances when the moon eclipses the sun is not actually seen from Earth. In case there was still a question why this is not seen, it is because the moon's shadow ends up projected either below or above the Earth. No shadow, no eclipse.

Water World
Let's consider the possibility that both distance and incline are aligned perfectly, which actually happens two to five times a year. Why don't people see the eclipse?

Well, it's because only a third of the Earth is made up of land, which means majority of the eclipses are witnessed by sea creatures. They may not prepare greatly for the event, but it doesn't change the fact that 70 percent of eclipses that occur cannot be seen from land.

For those on the waters during this time however, witnessing an eclipse is still subject to the condition of the sky. If there is cloud cover, then it won't matter because the clouds will obscure the view.

Total Solar Eclipses Are Getting Rarer

In 1695, Edmond Halley hypothesized that the planet's rotation is slowing down and the days are getting longer, based on calculations of ancient and more "current" total solar eclipses. This hypothesis was proven correct using the equipment on the moon setup by Apollo astronauts.

According to explanation, the Earth's rotation is slowing down due to the force created by the tides and this, in turn, causes the moon to move away to preserve balance in the Earth-Moon system. This basically means that future generations may only get to witness annular eclipses.

If it's any consolation, however, the moon is moving away at 3.8 centimeters (1.5 inches) per year so it will be a few million years before humanity completely waves goodbye to total solar eclipses. For now, we can all enjoy the great phenomenon that only touches down on the same Earth spot every 375 years.

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