How fortunate the moon and sun fit together so well during a total or annular eclipse. Despite their vast difference in size, the moon covers the sun in eclipse several times a year from somewhere on Earth. This neat trick is possible because the moon, though 400 times smaller than the sun, is also 400 times closer. An exquisite balance.
You may have read elsewhere that Earth’s the only place from which to watch a total solar eclipse. That’s not quite true.
As you might expect, since Mercury and Venus have no moons, there’s nothing to create an eclipse. Mars’ moons Phobos and Deimos pass in front of the sun, but they’re too small for anything but partial and annular eclipses. As compensation, eclipses on Mars are frequent, sometimes happening every day during certain seasons, because the moons revolve rapidly around the planet and pass in front of the sun numerous times.
Seen from distant Jupiter and Saturn, the sun is considerably smaller and easily eclipsable by many of those planets’ moons. Unfortunately neither has a solid surface on which to set up a comfy chair for viewing, but the view from a zeppelin would be spectacular. Ditto for Uranus and Neptune.
Eclipses might be visible from the moons of all these planets, especially those of Jupiter which orbit in the same plane as the sun. Standing on Jupiter’s moon Io with solar filter in hand, we might watch an eclipse of the sun by Europa or Callisto. The jolly girth of Jupiter itself would also eclipse the sun regularly as seen by an observer on any of the inner bright moons orbiting the planet. Jupiter’s big enough to hide the sun for many hours if not days seen from Callisto.
From Saturn’s moons, we’d regularly see the ring plane eclipse the sun. Because the rings are composed of millions of chunks of nearly pure water ice a half-inch up to 30 feet across, the sun during eclipse would alternately be hidden and then pop back in view. Imagine the sparkle and shimmer of the sight in binoculars!
Let’s return to Earth. Millions of years ago, total eclipses were more frequent and annulars never happened. Back then the moon was closer to the Earth, so it appeared larger in the sky and covered the sun with ease. Yes, the trilobites with their calcite crystal eyes got to see more total eclipses than we do.
Since its formation some 4 billion years ago, the moon’s been pulling away from the Earth. At the same time, Earth’s rotation has been gradually slowing. The moon gravity tugs on Earth’s oceans creating the two tidal bulges we experience as the daily rising and falling of the tides.
Since the Earth rotates much faster than the moon travels along its orbit, the tidal bulge pulls the moon slightly forward in its orbit, causing it to accelerate. The accelerating moon responds by backing away slightly from Earth. Although it only amounts to 3.8 cm or 1.5 inches per year, those inches add up over geologic time.
Meanwhile the energy that goes into accelerating the moon has the opposite effect on Earth. Our planet’s rotation slows a tad in response, causing the day to lengthen about half a second a year. To square everything up, members of the International Earth Rotation Service add a leap second every few years to keep Earth’s changing rotation rate in line with more reliable atomic clocks.
Four billion years ago, Earth’s day was only about 6 hours long and a huge moon lit up the night. When the first dinosaurs and frogs appeared 245 million years ago, the day had lengthened to 21 hours, the moon had shrunk and eclipses were less common.
We now live in an era when eclipses – other than partials – can be annular or total. Yet the moon keeps slip-sliding away. In the distant future, there will come day when it’ll be too far away to cover the sun even if an eclipse were to happen at lunar perigee, when the moon is closest to Earth.
That cutoff point is 13,297 miles (21,400 km) beyond where the moon is right now. If you convert this distance to inches and divide by the 1.5 inch per year departure rate, you come up with 561,665,280 years left until the last total eclipse of the sun. Add in 2012 and we arrive at the figure in the headline. After that, we’ll have nothing but annulars!
Of course, I don’t mean for you to take this as an exact date. Look at it as an approximation based on current tides, the arrangement of the continents and rate of lunar recession. What is interesting is that even something as seemingly eternal as total eclipses have their day in the sun before evolving into something else.
Billions of years hence, the length of a day will increase to about 47 days. Should the Earth survive the sun’s expansion into a red giant at that distant time, the moon will have stopped moving outward and instead hang directly over the same spot on the Earth. What a world awaits our distant descendants. Oh … don’t forget to enjoy today’s solar eclipse.