Celestial Rendez-Vous – An Equinoctial Total Eclipse of the Sun

A photograph showing the Sun's diamond ring during a total eclipse.

Polar Equinoctial Eclipse 2015

On 20th March 2015, the Moon will pass in front of the Sun and exactly block out most of its light.  It will be the first total solar eclipse of the 21st century that is visible from the northernmost regions of Europe…

Full solar eclipses are comparatively infrequent astronomical events because the Moon‘s orbit is tilted and not quite in the same plane relative to the Earth’s orbit, making it even rarer when the Sun, the Moon and the Earth align.  That this can happen at all is due to a remarkable coincidence.

Although the Sun is much bigger than the Moon – about 400 times bigger across its diameter, it also happens to be much further away from our sight by almost exactly the same amount, meaning the Sun and the Moon appear to have the same size in the sky.  We can describe this phenomenon by saying that the Sun and the Moon have the same angular size.

Angular Sizes

Angular size is a very useful measurement in astronomy, since angular sizes are by far easier to measure than actual sizes, as they merely refer to how large an object appears in the sky.  Both the Sun and the Moon have an angular size of about half a degree.

A diagram explaining the angular sizes of objects at varying distances.
Any object will have an angular size of 10°, provided you look at it from a distance that is 5.7 times its actual size. Credit: The Open University

If you can measure the distance to an object, as well as its angular size, then you can work out the object’s actual size.  By the same token, if you know an object’s actual size, you can find out its distance.

A photo-diagram showing how to estimate the angular sizes of celestial objects using one's hand.
This neat little trick shows how you can use your hand to make rough estimates of angular sizes.  At arm’s length, your little finger is about 1 degree across, your fist is about 10 degrees across, the distance between your thumb and little finger is about 25 degrees, etc. Credit: NASA/CXC/M.Weiss

As an example, the Sun is about 150 million kilometres from the Earth, and the Moon is 384,500 kilometres away.  The Moon’s diameter is 3,476 kilometres.

So, you ask…

How can we work out the actual diameter of the Sun?

We determine the ratio of the Moon’s distance to the Moon’s diameter:

\frac{distance_{Moon}}{diameter_{Moon}} = \frac{384500 km}{3476 km} = 110.6

For an object of this angular size:

distance = 110.6 \times diameter

Thus, rearranging the equation:

diameter = \frac {distance}{110.6}

Since the Sun and the Moon have the same angular size,

diameter_{Sun} = \frac{distance_{Sun}}{110.6}

So, the Sun’s approximate diameter is

\frac{150 \times 10^6 \mathrm{km}}{110.6} = 1.36  \times 10^6 \mathrm{km}

This means the Sun’s diameter as about 1.36  million kilometres.


Total Solar Eclipses

A photograph showing the Sun's corona and diamond ring during the total eclipse in Tokyo in 2012.
Despite the clouds, this photograph, taken in Tokyo at the height of the total eclipse of 2012, shows the thin extended outer atmosphere of the Sun – the solar corona.

A total solar eclipse occurs when the Moon passes directly between the Earth and the Sun, thereby obscuring the bright solar surface.  At the same time, you can see the atmosphere of the sun, called the corona during a full eclipse.

The corona is super-hot, 100 times hotter than the already impressive 5,538 °C temperature of the Sun itself.

The orbital motion of the Moon, combined to the rotation of the Earth, causes the umbral shadow – the shadow cast on the Earth from within which the Sun is completely blocked –  to travel across the Earth, typically at well over the speed of sound, c_s = 343.59 m s^{-1}, making total eclipses rather short-lived.

Total eclipses can last for up to 7 minutes at any given location.  On average, they are spaced in time by a little over a year, although each one is only visible from a narrow path on the Earth’s surface.

The speed of the Moon as it moves across the Sun is approximately 2,250 km (1,398 miles) per hour.

A diagram explaining how solar eclipses develop step-by-step.Since August 1999, there has not been a total solar eclipse.  The one happening this month will only be a partial eclipse, if observed from mainland U.K.  Should you miss this one, it will be 2026 before you can catch a view of the next partial eclipse of the Sun.  However, if you are willing to travel to the Southern Hemisphere, the next eclipses to happen will take place on 9th March 2016, 2nd July 2019 and 14th December 2020.


Rendez-Vous at Sunrise

This time around, the Faroe Islands will be the best place to see it.  From the United Kingdom, the partial solar eclipse will begin at 8.45 am.  The maximum eclipse, when the Moon nears the middle of the Sun, will occur at 9.31 am.  The celestial spectacle will wind down at 10.41 am.

A map showing the progress of the total eclipse of the Sun of 2015 at different locations, with associated magnitudes.The further North you go, the greater the shadow over the Sun and the fuller the eclipse.  Up to 94% of the Sun’s light will be blocked by the Moon in Scotland. 

Further south, like in London, the Sun will probably only be 85% obscured by the Moon.

All you need to enjoy watching the eclipse is a nice clear horizon and view of the sky.  Remember, NEVER look at the Sun for more than an instant with the naked eye.

Let’s hope that come Friday, the partial eclipse of the Sun won’t simply be due to the very overcast skies of Scotland.


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