This beautiful image of a total solar eclipse was taken by NASA astronomer Fred Espenak.

The Earth-Moon system is unique in the solar system, because only for this system at the present time, does the angular size of the Moon match the angular size of the sun as seen from the surface of the Earth. This means that sometime during its orbit, the Moon can exactly cover the Sun, causing an observer to be thrown into an eery night time in the middle of the day!

But, the orbit of the Moon is not stable. Because of tidal friction, the orbit of the Moon is steadily growing larger, so that the angular size of the Moon from the Earth is growing smaller. When we get to the point that the Moon only covers 98 percent of the Sun's disk, enough of the Sun will still be visible at totality, that you will not experience night time during a total eclipse.

The Sun has a diameter of 870,000 miles. At the present time, the Sun's angular diameter varies from 32.7 minutes of arc when the Earth is at its farthest point in its orbit (aphelion), and 31.6 arcminutes when it is at its closest (perihelion). The Moon on the other hand has a diameter of 3476 kilometers, and varies in distance between 356,000 (perigee) and 406,000 kilometers (apogee). This means its angular size changes from 33.5 to 29.43 arc minutes. So, there is plenty of opportunity for the angular sizes of the Moon and Sun to be equal for a total eclipse.

But, the Moon's linear distance from Earth is currently increasing at a rate of 3.82 ± 0.07 centimetres (1.504 ± 0.028 in) per year, so that when the Moon drifts about 20,200 kilometers further out from the Earth, the Moon will be so far away even at perigee, that its disk will be smaller than the Sun's disk even at perihelion. At 3.8 centimeters per year, it will take about 500 million years for the last total eclipse to occur. A complicating factor is that the size of the Sun itself will grow slightly during this time, which will act to make the time of 'no more total eclipses' a bit earlier than 500 million years hence. From then on, all we will see are annular eclipses like the one below:A perfect 'Ring of Fire' from 2012. Image credit and copyright: Kevin Baird.

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