Saturday, July 08, 2017

Searching for Meaning in the Shadow of the Moon

In our solar system, there are 8 planets.

Of these 8 planets, four of them are gas giants with no solid surfaces so no place to stand to make observations.

Of the four remaining planets, two of them have moons.

Of those two, only one has a moon of appreciable size.

You live on that planet.

In the early days of our planet, the moon was three times closer than it is today. So it looked three times larger. But the moon always appeared larger than the sun.

The moon is moving away from the Earth at a rate of about an inch per year. There will come a time when the moon will always look smaller than the sun.

It has now receded to a distance so it subtends an angle of about half a degree in the sky. The sun also subtends an angle of about half a degree.

You live in a rare time when the sun and the moon are almost the exact same angular size in the sky.

The moon isn't always the same distance from the Earth. Sometimes it looks slightly larger and other times it looks slightly smaller. Only when its at its closest, perigee, does it appear the same angular diameter as the sun, every 27.55454988 days

The path of its orbit isn't parallel with the path of the Earth's orbit around the sun so these two paths cross about twice per month or twice every 27.212221 days.

It aligns on the same side of the Earth as the sun on the day of the "New Moon," every 29.530588853 days.

About twice per year, all of these numbers line up so the moon casts a shadow on the Earth.

This is a solar eclipse.

Much of the time, the inner part of the shadow, the umbra, is cast into space and only the outer part of the shadow, the penumbra, falls on Earth. This is a partial solar eclipse. These are fairly common.

At other times, the umbra is cast on the Earth but, because we live on a planet that's covered more than 70% by oceans, it often falls on the ocean or in a remote, hard to reach place.

This year, the shadow of the moon will cross the entire continental United States.

The last time this happened was in 1918 and it won't happen again until 2045.

It's not a common event for any given location. Most "eclipse chasers" have to travel around the world to see a total eclipse of the sun.

For a good share of people living in the continental United States, it won't be such a journey.

Yes, there is a lot of hype going on about the so-called "Great American Eclipse."

We live in a world where there is so much nonsense that's celebrated that it's often difficult to get excited about events like this. People are talking about watching it on the Internet or ignoring it all together.

However, this isn't just a matter of the rarity of the event. It's also a celebration of humanity's ability to make predictions about this kind of thing. Astronomy is, arguably, the best predictive science known to man.

This eclipse is a celebration of that science.

In antiquity, people were able to predict eclipses with a fair degree of accuracy, often getting the date within a day or two or the path of the eclipse to within a few hundred miles.

Today, we can predict where the path will be within a matter of meters and predict the beginning, middle and end of totality for a given location within fractions of a second.

That's amazing.

That's worth celebrating.


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