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Totality- Our Perfect Eclipse

Total solar eclipses

We are all familiar with the characteristic depiction of a solar eclipse, when the Moon passes directly between the Sun and the Earth. However, the significance of solar eclipses extends far beyond their aesthetic appeal. Major scientific discoveries, cultural practices, and even the behaviour of wild animals are derived from total solar eclipses that we have the privilege of experiencing (See image 1).

A solar eclipse occurs when the Earth, Moon, and Sun all appear to lie on a straight line. They are collinear. Total solar eclipses occur when the Moon completely obscures the Sun's photosphere, enabling prominences and coronal filaments to be seen along the limb. This phenomenon is unique to the Earth, Sun, and Moon system and to understand why we must explore the mathematics underlying these ‘orbital gymnastics’.

We wish to compare the ‘apparent’ size of the Sun and Moon, a quantity proportional to the ratio of their size and distance from Earth. The Moon has a radius of around 3,400 km, and is approximately 384,000 km from Earth. The Sun has a much larger radius of 1.4 million km, and is located at a distance of 150 million km. By dividing the Sun's radius by the Moon's radius and dividing the Earth-Sun distance by the Earth-Moon distance, we can determine that the Sun is 400 times larger than the Moon and 400 times further away. This unique relationship allows for total solar eclipses, where totality indicates **the complete blocking of sunlight from the Sun’s disk by the Moon. In partial eclipses, only part of the Sun is obscured.

One might wonder why we don’t have total solar eclipses every month, and the reason is that the plane of the Moon’s orbit around Earth is tilted at 5 degrees relative to Earth’s orbital plane. This hugely decreases the likelihood of such perfect alignment. Of the hundreds of moons orbiting planets in our Solar System, only our Moon totally eclipses the Sun. For example, none of Jupiter’s 95 moons have the correct size and orbital separation that completely block out the Sun from any point on Jupiter’s surface!

Surely this serendipitous interplay of Earth, Sun, and Moon cannot be a coincidence? (See image 2)

It is at this point where divine intervention is typically invoked. There are a few problems with doing this. The Moon's eccentric orbit around Earth means that it will be closer during some total solar eclipses than others, resulting in annular eclipses when the Moon is furthest from Earth. Additionally, the Moon is receding from the Earth at a rate of 4 cm/year, which means that total solar eclipses will only be observable for another 250 million years. (See image 3)

For those of you who wish to make the most of this brief window of opportunity, this website shows the dates and locations of upcoming total solar eclipses.

By Joseph Brennan


Guillermo Gonzalez, Wonderful eclipses, Astronomy & Geophysics, Volume 40, Issue 3, June 1999, Pages 3.18–3.20,

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