The Last Eclipse
Unless you plan to be around until September 2090, chances are if you want to see a total solar eclipse, you’ll have to travel away from the UK. If you’ve ever seen a total eclipse, you’ll know the wonder and majesty of such an event; the way everything goes quiet as totality hits, darkness sweeps across the landscape and the corona, the vibrant and energetic atmosphere of the sun becomes visible.
There are three types of solar eclipse that you can see from the Earth - total, partial and annular. Total eclipses happen when the Moon completely covers the disc of the sun, partial, where it only covers a portion of the sun and annular where the orbit of the Moon brings it closer to the Earth, meaning that an outer ring (or annulus) of the sun is visible. Each of these types of eclipse are stunning in their own way, but it is only from Earth that a total eclipse is possible.
A total solar eclipse is actually a misnomer - an eclipse is when one astronomical body enters the shadow of another or has another body pass between it and the viewer, so a lunar eclipse, where the Moon is in Earth’s shadow is correctly named. However, a total solar eclipse is technically an occultation, whereas a partial solar eclipse is actually a transit.
We get total eclipses because of a chance orientation and spacing of ourselves, our parent star and our only natural satellite. The Moon, unlike all other natural satellites in our solar system, is more inclined to the ecliptic (the path the other planets follow around the sun) than to the equatorial plane of its parent planet. This is why the Moon appears to follow much the same path through the sky as the other planets. Secondary to this is the fact that the Moon is the right distance away from Earth to appear the same size in the sky as the sun. This, however, was not always the case.
Thanks to accurate observational measurements and a series of Laser Ranging Retroreflectors left on the surface of the Moon by the Apollo astronauts, we know the Moon is slowly moving away from the Earth thanks to gravitational tidal forces at the rate of 3.8 cm per year. This in turn slows the rotation of the Earth down by 23 microseconds per year, necessitating the addition of leap seconds every few years. It has been calculated that in about 50 billion years, the Earth-Moon system would become tidally locked, with both the Earth and the Moon rotating and orbiting at the same rate, meaning that the Moon would appear to be staying still in the sky over the same place. It also means that the Moon would appear too small in the sky to ever block the entire disk of the sun. There would be no more total solar eclipses, only partial and annular eclipses. The orbit of the Moon and the rotation of the Earth would both be in the order of 47 days long (currently, the Moon orbits every 29.5 days).
To put this into perspective, when the Moon was first formed, some 4.527 ± 0.010 billion years ago, the average day on Earth was around 6 hours long. By the time of the extinction of the dinosaurs and the rise of mammals to evolutionary dominance around 65 million years ago, the average day length had stretched to 21 hours.
Whilst a day lasting 1128 hours might not sound too appealing to most of us, it will be some 50 billion years into the future, ten times the age of the Earth so far and it is very doubtful humanity, or anything even remotely resembling humanity will be around then. What should be more concerning in the more immediate (and by immediate, I mean 2.3 billion years, but astronomers like to think big) future is the life cycle of the sun which, by this time, would have started to heat up and enter a red giant phase, due to depletion of hydrogen in its core and burnt off most of the oceans and atmosphere on the planet.
What all this means, ultimately, is that there will be one last perfect total solar eclipse in the future; one last chance to see totality before the recession of the Moon due to tidal forces takes it outside the near perfect match of apparent size in the sky and the only eclipses visible from thereon in will be partial and annular. Our distant descendants will never again see the awe inherent in a total solar eclipse and be the poorer for it. However, we still have a good few hundred million years before that happens, so my advice would be to go and seek out at least one or two in your lifetime.
Finding information on the dates of eclipses is easy, they are predictable far into the future thanks to the near clockwork precision of the solar system and there are somewhere between two and five every eclipse season (yes, even eclipses have seasons). NASA keeps a good record of when these will occur at http://eclipse.gsfc.nasa.gov/eclipse.html
So go and book a holiday that coincides with the date and location of a future eclipse, take a pair of eclipse glasses with you (or welders goggles rated 14 or higher), as I’m sure you don’t need me to tell you that looking directly at the sun is a bad idea, even a second or two before and after totality can be damaging, find out if there will be an ‘eclipse part’ or eclipse observing session organised by some astronomy society nearby and bask in the truly cosmic wonder that will unfold before your eyes, all thanks to the chance alignment and orientation of our sun and Moon before we lose it forever.
