Solar Eclipse 20 March 2015

A total solar eclipse is due to occur on Friday 20 March 2015 over much of the Northern Hemisphere – are you ready for it?

Total solar eclipses* are very rare astronomical events so the opportunity to view one shouldn’t be passed up too quickly – the next major solar eclipse in the UK is due to happen in 2090!  This eclipse is also special because the path of the total eclipse will end at the North Pole on the spring equinox.  That is not due to happen again for thousands of years.  The shadow of this total eclipse (i.e. the umbra) will cross the Faroe Islands and Svalbard.

The UK will experience a partial eclipse ranging from about 86% coverage in the south to about 98% in northern Scotland.  If the weather is clear we should observe the Sun’s light be reduced by the same amount as if a very thick cloud had covered the Sun.

Solar Eclipse 2015 poster.003

*Don’t get your eclipses muddled up: solar eclipses are when the Moon comes between the Sun and Earth; lunar eclipses are when the Earth comes between the Sun and Moon.  During a lunar eclipse the Moon appears red/orange because only some of the long wavelength end of the visible light spectrum is refracted around the Earth enough to reach the Moon.

Be Safe When Observing the Eclipse

Viewing the Sun without proper eye protection is always dangerous, even during an eclipse!  The fact that the retina has no pain receptors and any damage caused is not immediately noticeable only adds to the risk.  Even with solar filters the Sun should not be observed for too long, always check the instructions supplied with solar filters.  For example our filters, which reduce light intensity by approx. 100,000 times, have a maximum viewing time of 3 minutes.  The reason for this time limit is that there is still a lot of potentially harmful infrared radiation that gets through the filters.

A Great Astronomy App

Last week I scouted out a location for viewing the eclipse with the help of a great astronomy app called Skyview made by Terminal Eleven.  The app is available on iOS and Android – details on their website.


Eclipse Geometry and Factors

Solar eclipses are possible because the Sun is 400 times larger than the Moon and also 400 times further from the Earth than the Moon is.  But there are several factors that affect whether the Moon will actually move exactly between the Earth and Sun and where the shadow will fall on the Earth’s surface.

  1. The Earth’s orbit around the Sun.
  2. The Earth’s rotation.
  3. The tilt of Earth’s axis of rotation.
  4. The Moon’s orbit around the Earth.
  5. The inclination of the plane of the Moon’s orbit to the plane of the Earth’s orbit.

Why are Solar Eclipses Possible- poster.003

Scientific Observations During Eclipses

Solar eclipses are wonderful events to witness and yet they are also events of significant scientific interest.  Total solar eclipses enable important scientific observations to take place. Below are two observations that are enabled by total eclipses.

1. The Sun’s corona. 


The aura of plasma that surrounds the Sun and extends millions of kilometres is the corona.  It is not possible to observe the corona ordinarily as the glare from the main body of the Sun is too intense.  Coronagraphs have been designed which block out the main part of the Sun to allow for observation of the corona.  However, the Moon during a total eclipse is a vastly superior coronagraph!  Total eclipses allow for observation of the lowest parts of the corona, this is something not currently achievable by artificial coronagraphs.  The corona is of great interest to scientists because it is much, much, hotter than the surface of the Sun and the method by which it is heated is still the subject of debate.

See more amazing photography of solar eclipses by Miloslav here.

2. Gravitational lensing.
Gravitational Lensing Diagram.001
Objects with mass curve the space-time continuum, and the space-time continuum defines the passage of light.  Therefore, the presence of objects with mass can alter the passage of light compared to the absence of such objects.  Gravitational lensing is only visible near objects with very large masses and of objects very far behind the massive object.  The Sun is a suitable candidate for causing gravitational lensing of distant stars behind it.  However, the Sun emits a lot of light which obscures the lower intensity light from distant stars.  But during a total eclipse virtually all of its light is obscured.  That gives a small window of opportunity for astronomers in the umbra (i.e. in the Moon’s total shadow) to observe distant stars to see if their apparent position shifted due to the presence of the Sun.  The position during the eclipse is compared with the position as measured approximately six months earlier / later which is when the Sun is not have been between Earth and the star(s) in question.

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