NEWS THIS MONTH - 20th MARCH 2015

Solar Eclipse

On the morning of the 20th March there will be a Solar Eclipse that can be seen throughout the British Isles. The eclipse will not be total from anywhere in Britain but can still be seen as a partial eclipse. A Solar eclipse occurs when the Moon passes in front of the Sun. If the Moon does not competely cover the the Sun then a 'Partial Eclipse' will be seen. If the whole of the Sun is obscurred by the Moon then a 'Total Eclipse' will be seen. Anyone standing directly in the path of the Total Eclipse will first see a partial eclipse as the Moon begins to pass in front of the Sun. The amount of the Sun obscured by the Moon will increase until it is totally covered for as little as less then a minute or up to maximum of about five minutes. Then the Sun will reappear and the partial cover will decrease until the Sun is completely revealed again. To either side of the path of totality only a partial eclipse will be seen in which the amont of the Sun covered by the Moon will decrease with the distance from the path of totality.

The path of 'Totality' (blue track) on the morning of 20th March 2015

The chart above shows the path of 'Totality' (the dark blue track) passing from west to east between the north of Britain and south of Iceland. The light blue lines show where the Sun will be 80%, 60%, 40% 20% covered by the Moon as a Partial Eclipse and the edge of the partial eclipse. Green lines show the time that totality passes over and when the partial eclipse is at maximum cover.

The Faroe Islands to the north of Scotland will be on the path of Totality and will see the Sun completely covered by the Moon for just over 2 minutes. In the Shetland Islands 97% of the Sun will be covered by the Moon, this is the maximum cover to be seen from anywhere in the British Isles. Other places in the country will see less coverage as shown below:

Lerwick (Shetland) 97%
Edinburgh 94%
Newcastle 91%
York 90%
Birmingham 88%
London / Newbury 84%

From London and the Newbury area the eclipse will begin at 08:25 when the Moon will be seen on the edge of the Sun. Maximum eclipse will occur at 09:31 and we in Newbury will see (clouds permitting) 84% of the Sun obscured by the Moon. The Moon will finally clear the edge of the Sun at 10:41 when the eclipse will be over.

. Chart showing the amount of the Sun that will be covered around the country

 

OBSERVING THE SOLAR ECLIPSE

Do not look directly at the Sun and do not use binoculars or an unfiltered telescope to view the eclipse

'EYE DAMAGE OR BLINDNESS WILL RESULT'.

The Solar eclipse must not be observed by looking directly at the Sun even if the Sun is covered by light cloud. Binoculars and Telescopes must not be used to look directly at the Sun or blindness will result. Binoculars or a telescope can be used if fitted with a correct safe type of approved SOLAR filter. Alternatively binoculars or a telescope can be used to project the image of the Sun on to a piece of card where it can be examined safely (see the image below). It is also safe to use approved Solar Observation glasses that can be obtained from the British Astronomical Society at: http://www.britastro.org/

The Partial eclipse of 2008 as seen from Thatcham using a 100mm telescope to project the image

The timetable for the eclipse to be seen from Newbury is:

08:25 First sign of the Moon in front of the Sun

09:31 Maximum coverage of the Sun (84%)

10:41 Last sign of the Moon in front of the Sun

So shortly after 08:25 on the morning of 20th March the edge of the Moon will appear on the side of the Sun. At 09:31 maximum amount of the Moon will be visible in front of the Sun. At 10:41 the Moon will finally clear the Sun.

Weather permitting the eclipse, at maximum cover from Newbury, should appear something like the computer generated images below.

The Eclipse at 08:30
The Eclipse at 09:30
The Eclipse at 10:30

MORE ABOUT ECLIPSES

Eclipses occur when the shadow of one body is cast on to another body. There are two basic types of eclipse, these are called: Solar Eclipse and Lunar Eclipse. Both are caused by shadows being cast by the light of the Sun on to the Moon or Earth.

SOLAR ECLIPSES

Solar eclipses occur when the Sun, Earth and Moon are aligned in their orbits with the Moon in the middle. When this happens, the shadow of the moon is cast on to the surface of Earth. There are two distinct parts to the Moon's shadow called the Umbra and Penumbra. The Umbra is the dark area at the centre of the shadow where the whole of the Sun is blocked out (known as totality) and the Penumbra which is a much larger area surrounding the Umbra where the Sun is only partially covered by the Moon.

A diagram showing how the Umbra and Penumbra are produced

The Umbra occurs because the Sun has a much greater diameter than the Moon and the shadow forms a conical shape tapering with an angle subtending the edges of the moon and the Sun. Depending on the relative positions of the Sun, Earth and the Moon the size of the shadow can vary in size. If the eclipse occurs when Earth is at its furthest from the Sun, because of its elliptical orbit, the Sun appears smaller. If the Moon is at its closest therefore appearing larger, the shadow on the surface Earth surface will be at its largest. This shadow may be more than 200km across.

When the Earth is at about half its maximum distance from the Sun and the moon half its maximum from Earth the Umbra will be at its minimum of only a few kilometres in diameter. This is because the Moon appears to be only slightly larger than the Sun. Totality may last only a few seconds but there is likely to be a magnificent show of Bailey's Beads and prominences.

BAILY'S BEADS were first recorded by Francis Baily, the famous astronomer who was born in Newbury. They are beads of bright sunlight that appear on the edge of the Moon when it is in front of the Sun during a Solar Eclipse. When the edge of the Sun is aligned with the edge of the Moon during a Solar Eclipse the sunlight can shine through the valleys between the mountains on the edge of the Moon. This causes the very bright spots on the edge of the eclipse as shown below.

Baily's Beads around an eclipse

By an incredible natural coincidence the Sun and Moon appear to be the same size when viewed from Earth. The Sun is about 400 times the size of the Moon but the Sun is about 400 times further away so they look the same size. However the orbit of Earth around the Sun and the orbit of the Moon around Earth are not circular but are eliptical so the Sun / Earth / Moon distances can vary. When Earth is at its closest to the Sun and the Moon at its furthest from Earth, the moon appears too small to completely cover the Sun. During an eclipse the outer edge of the Sun will still be visible even when the Moon is directly in line. This is called an Annular Eclipse. When Earth is at its furthest from the Sun and the Moon at its nearest to Earth, the moon appears larger than the Sun and the total eclipse will last longer sometimes more than 5 minutes.

A Total Eclipse
An Annular Eclipse

The outer and fainter part of the shadow formed around the Umbra is called the Penumbra. This occurs because there is an area where the Sun has not been completely obscured by the Moon. The penumbra will be thousands of kilometres in diameter and from anywhere within it, a partial eclipse will be seen. On the line of totality the penumbra arrives at first contact when the edge of the Moon first touches the edge of the Sun. The observer remains in the penumbra until the Sun is completely obscured as the umbra passes over that position.

When the umbra has passed the observer the edge of the Sun reappears and the trailing part of the penumbra passes over the observer until the whole of the Sun is again revealed.

Observing from a location above or below the path of totality, the umbra will not pass over the observer therefore only a partial eclipse will be seen. The closer the observer is to the umbra, the more of the Sun will appear to be covered. It will get progressively darker nearer the centre of the penumbra but the Sun will never be completely covered by the Moon.

DIFFERENT TYPES OF SOLAR ECLIPSE

Because Earth's orbit is elliptical and to a much smaller degree so is the Moon's, no two solar eclipses are the same. The diameter of the shadows can vary in size and can track across any part of the surface of Earth. There are three distinct classes of solar eclipse.

TOTAL SOLAR ECLIPSE

Total eclipses occur when an umbra is produced in the shadow. The umbra may be from a few kilometres wide to about 200 kilometres depending on the relative position of Earth and the Moon in their orbits. An observer standing on the path of the umbra will see the Sun completely obscured by the moon. Totality may last from a few seconds with the smallest umbra or up to five minutes or more for the largest. See the left image above.

PARTIAL SOLAR ECLIPSE

A partial eclipse can be seen from anywhere within the path of the penumbra. The further an observer is from the centre of the shadow less of the Sun will be covered by the Moon. As the shadow passes over the observer the deeper he will be into the shadow. The observer will see the Moon advance across the surface of the Sun. On the line of totality the appearance will start as a partial eclipse gradually the Moon will cover the Sun until it is completely obscured at totality. Totality will end as the umbra passes and a decreasing partial eclipse will continue. Anywhere within the path of the penumbra but not on the path of totality will see the Moon shadow cross the Sun but will not completely cover it. Depending on how far into the penumbra the observer is the Sun will appear as a thin crescent near the centre and a thick crescent nearer the edge.

ANNULAR SOLAR ECLIPSE

An Annular Eclipse occurs when the Earth is at its closest to the Sun and the Moon at its furthest from Earth (although this is not quite so significant). Under these circumstances the Moon appears too small to completely cover the Sun. At maximum cover, on the path of the centre of the shadow, a bright ring of light from the edge of the Sun will still be visible around the outside of the black Moon. See the right image above.

LUNAR ECLIPSES

Lunar eclipses occur when the moon passes through the shadow cast by the earth. This type of eclipse is more common than the solar eclipse simply because the shadow of Earth is larger than that of the Moon. As the light from the Moon is just reflected sunlight there is no danger in observing the Lunar Eclipse. Also there is no rush as with the total Solar Eclipse, the Lunar Eclipse totality will last for more than an hour.

As Earth orbits the Sun and our Moon orbits Earth, there are occasions when all three are aligned. A lunar eclipse will occur when Earth is between the Sun and Moon and the Moon passes through the Earth's shadow (see the diagram below).

Lunar eclipses always occur at night. This is because the sunlight has to be shining on the opposite side of Earth (where it is daytime) to produce the shadow on the side of the Moon facing the night side of Earth. The Moon will also be full because it is in direct line with Earth but further out so looking from the dark side of Earth the light side of the Moon will be seen.

As the Moon moves into the Earth's shadow, the shadow will be seen to start moving across the face of the moon from the left side as viewed from Earth. The passage of the Penumbra will be faint and almost inperceivable. Although the edge of Umbra shadow is not as sharp as in a solar eclipse it is quite noticable and will creep across the Moon until the whole surface is covered. Not all Lunar Eclipses are 'total'.

The surface of the Moon at totality is never completely dark. It takes on a reddish glow that can vary between orange to deep red. This is caused by sunlight passing through the atmosphere around the edge of Earth and being refracted on to the surface of the Moon.

The Moon during the total eclipse of 2008 imaged by Lee Mcdonald

If there has been a recent volcanic eruption and there may be a lot of smoke and ash in the atmosphere so the Moon will glow a deeper red. From the surface of the Moon, the solar eclipse must be a glorious sight. The bright disc of the Sun will gradually be covered by the dark silhouette of Earth. Then at totality the Sun will be replaced by a very thin ring of red light. This is sunlight shining through the thin layer of the atmosphere around the edge of the dark disk of the night side of Earth.

There will be a Total Lunar Eclipse in the early morning of 28th September which will be visible from the UK.

The times for the eclipse to be seen from Newbury are:

01:07 The Moon enters the inner shadow (Umbra)
02:11 The Moon starts ‘Totality'
03:23 The Moon starts to move out of Totality
04:27 The Moon leaves the inner shadow (Umbra)

More about the Lunar eclipse in September Magazine.

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