The recent total eclipse that we experienced in Oregon and all across the United States was one of those things that come along perhaps only once in a lifetime. It definitely took me out of my ordinary, business-as-usual, lifestyle.
Amazing as the experience was, I think most people don’t realize just how unique eclipses here on Earth really are.
I’m amused every time I hear someone say that something was “more unique” or “the most unique.” The word ‘unique’ means that there is only one of whatever is being described. There are no degrees of uniqueness. It’s not possible for something to be more unique than something else. A unique experience is one that cannot be duplicated.
Having said that, and knowing that a total eclipse of the sun happens somewhere on the surface of the Earth once every eighteen months, there is a sense that a total eclipse of the sun, as seen from the Earth, is really unique.
The fact that we can see total eclipses at all is due to a freak coincidence that is so unlikely that it probably does not occur anywhere else in the entire Milky Way galaxy.
The Milky Way contains somewhere between 300 billion and 400 billion stars, and just about every one of them has at least one planet orbiting it. An eclipse happens when something, such as a moon or another planet, comes between a planet and its star.
A partial eclipse occurs when something comes between a star and an observer on a planet, when something comes between the two that does not completely block the light from the star. This slide shows what the sun looks like when Venus comes between the Earth and the Sun.
This is called a transit of Venus, because Venus appears to move across the face of the sun. You wouldn’t even know this was happening if you weren’t looking for it because the dimming caused by the transit is so slight.
Another example of an eclipse, but from the surface of Mars rather than Earth is the eclipse of the Sun by Mars’ moon Phobos. As is the case with the transit of Venus, Phobos is too small to completely eclipse the sun, so there is a ring of brightness around it.
I mentioned that the total eclipses that we see on Earth are due to a freak coincidence. That coincidence is the fact that the sun is 400 times larger in diameter than the moon, and the moon is 400 times closer to the Earth than is the sun. That means that during a total eclipse, the moon EXACTLY covers the disk of the sun, known as the chromosphere. This allows us to see the sun’s corona, which at any other time is too dim to see because of the overpowering brightness of the chromosphere.
What does it take for a total eclipse to be observed from the surface of a planet?
- The planet must have a moon that orbits in the same plane as the line of sight between the planet and its star.
- The ratio of the distance between the planet and the moon, and the planet and its star, must be the same as the ratio of the diameter of the moon to the diameter of the star.
- The planet must be habitable, with a solid surface, a breathable atmosphere, and be in the star’s “Goldilocks” zone where the temperature is such that liquid water can exist on the surface, not too hot and not to cold.
- Intelligent beings, capable of experiencing a sense of wonder, must exist to appreciate the eclipse.
- All the above conditions must occur at the same time.
What are the odds that these conditions will be met? The odds are VERY long. In fact, out of the hundreds of billions of stars in the Milky Way, it is most likely that there is only one planet in one star system, out of all those hundreds of billions, that satisfies ALL those conditions. It’s the planet you are on right now. On the scale of the Milky Way galaxy, a total eclipse of a star is indeed a UNIQUE experience. Next time you get a chance to see one, go for it.