Total Solar Eclipses
A total solar eclipse is one of the most beautiful events of the natural world. It is an extraordinary quirk of geometry that the apparent sizes of the Moon and Sun are nearly equal in our sky and when the Moon passes directly between Earth and the Sun the result is a spectacular piece of natural theatre for those lucky enough to be within the narrow band of the Earth's surface beneath the shadow. The following notes offer some details of what to watch for:
First Contact: The moment when the first small bite appears in the edge of the Sun as the Moon starts to pass over it.
Second Contact: As the Moon completely covers the Sun, the razor thin solar crescent breaks up into a chain of beads which gradually wink out. These are known as Baily's beads and are a familiar feature of total eclipses. When the last of the beads disappears totality has started.
Totality: Just as the eclipse becomes total, for a second or two you will see a bright red streak along the limb of the Moon. This is an upper layer in the solar atmosphere called the chromosphere. It is quite bright though not as bright as the photosphere, the region of the sun's atmosphere that we normally see.
The most noticeable feature during totality is the solar corona which is the outer atmosphere of the Sun that is only visible to the naked eye during a total eclipse. It consists of pearly-white streamers radiating outwards.
During totality one of the most stunning features are the prominences which are flame like appendages to the chromosphere which may be seen at any location round the eclipsed Sun, perhaps being larger and more spectacular at times when the Sun is very active. It is also interesting to note that as the Moon moves across the Sun during totality, prominences in the area where second contact took place start to be covered up whereas others in the vicinity of where third contact is about to occur become larger as the Moon uncovers them.
Among the most dramatic features of an eclipse are the colours during totality. The disk of the Moon is completely black, surrounded by the pearly-white of the corona with occasional flame red prominences. The sky is a deep purple-blue and around the horizon the sky is usually an orange colour reminiscent of sunset. This strange mixture of colours can make everyday objects look positively surreal.
A glow around the horizon is due to sunlight outside the shadow of totality being reflected inwards. The actual colour can vary from reddish-orange to yellow.
During totality the sky is not particularly dark so only the brightest stars may be seen however it is worth having just a quick glance around the sky to see which stars and planets are visible. There is always the possibility that totality will reveal a comet close to the Sun which had not been observed before due to its proximity to the Sun.
Third Contact: The first sign that totality is drawing to an end is when the chromosphere starts to emerge from behind the rim of the Moon at the location where the photosphere will reappear at third contact. This occurs just a second or two before third contact and gives a good warning that the Sun is about to reappear. If you are using optical equipment to observe totality this warns you to now look away before the light of the photosphere reappears. The ‘diamond ring' is one of the most dramatic features of the whole eclipse. As the first point of the photosphere reappears, we see something that resembles a giant diamond ring in the sky with the small portion of the photosphere being the diamond and the rapidly fading corona being the rest of the ring.
At second contact our eyes are used to the bright light and can perceive the last bead as a small point. However our eyes become dark adapted during totality and so the reappearance of the Sun can be dazzling.
Fourth Contact: The moment when the Sun is completely restored.
Eye Safety
The Sun radiates visible light but its photosphere also emits intense infrared and ultra violet radiation. Just as this radiation causes sunburn it will also cause damage to your eyes which need only be exposed to direct sunlight for a few seconds to sustain permanent damage.
The only way to view the uneclipsed or partially eclipsed Sun is through a special filter. Explorers Tours will provide a solar filter for each person for safely viewing the partial phases of the eclipse.
During totality the Moon completely masks the photosphere making it completely safe to look at the Sun with the naked eye, through your telescope or with binoculars.
The Partial Phases It sometimes surprises people that the eclipse can be quite advanced before it becomes noticeable that something is happening to the quality of the light. The fall in light level is very gradual at first but seems to accelerate particularly in the last ten minutes before totality. The greatest drop in light intensity is in the last four seconds and is very dramatic.
As the eclipse progresses it is interesting to note how the colour of the sky seems to change and the intensity of brightly coloured objects seem to soften. Just before the eclipse becomes total you may see the shadow of totality advancing like a wall across the landscape rapidly towards you.
Annular Eclipses
Since both the orbit of the Earth around the Sun and the orbit of the Moon around the Earth are not exactly circular (they are elliptical) the apparent size of the Moon and Sun in the sky vary over the course of the year and lunar cycle. If the Moon is at its furthest point from Earth and the Sun is at its closest to the Earth then the Moon's disk will not cover the whole of the Sun's disk and we have an annular eclipse where the outer edge of the Sun is visible surrounding the Moon like a bright ‘ring of fire'. Whilst much of the Sun's light is obscured during an annular eclipse the sky does not go very dark and special filters or eclipse glasses are required to view all stages of the eclipse. Eclipse glasses are supplied to all clients of Explorers tours.
An annular eclipse is an unusual and interesting variation on total eclipses and worth travelling to see. Features worth looking out for include the projected images of the sun caused by the passing of light through narrow gaps in trees or other small apertures and the colours and tones of light on the ground and in the clouds. Unlike total eclipses, some thin cloud often adds to the effect.
The next annular eclipse will be visible from the South Western United States in May 2012 for which Explorers will be based at Bryce Canyon. The rich colouration in the sandstone rocks of the canyon will surely be highlighted during the eclipse which occurs close to sunset.
Planetary Transits
Planetary transits are far rarer than eclipses of the Sun by the Moon. The transit or passage of a planet across the disk of the Sun may be thought of as a special kind of eclipse. From Earth, only transits of the inner planets Mercury and Venus can be viewed. On the average, there are 13 transits of Mercury each century. In comparison, transits of Venus usually occur in pairs with eight years separating the two events. However, more than a century elapses between each transit pair so after the transit of Venus in 2012 there won't be another till the year 2117. When planetary transits occur they are visible from much of the globe however only the final few minutes of the 2012 transit of Venus will be visible from the UK and just after dawn when clouds are very likely to interfere with the view. Those keen to witness this event in full will need to head for East Asia, Eastern Australia and the Pacific. Explorers plan to view from Hawaii.
Northern Lights
The Northern Lights (also known as the aurora borealis) are a natural display of coloured lights in the night sky which are often visible from northern latitudes.
A stream of charged particles called the solar wind is continuously emitted from the Sun and the when these particles interact with the Earth's upper atmosphere light is emitted from the ionization of oxygen and nitrogen atoms. The different constituent elements produce a range of colours with oxygen giving a greenish glow and nitrogen giving a blue or red glow.
Auroral displays can never be guaranteed but the likelihood of seeing a good display can be maximized by viewing from locations in the high latitudes such as Iceland, Greenland, Northern Canada and Alaska. The level of solar activity is another important factor as the strength of the solar wind is determined by the Sun's activity which runs on a cycle of approximately 11years. The cycle is due to peak around 2013 and should produce increased auroral activity from early 2011.
Meteor Showers
If you go out on a clear, dark night and spend ten minutes looking up at the stars you're likely to see a meteor streak across your view. A meteor is the bright trail of a small particle entering the Earth's atmosphere and burning up as it falls towards Earth. At certain times of year the Earth passes through regions which are particularly rich in these particles, perhaps the remains of the tail of a comet and at these times the number of meteors can increase dramatically producing a meteor shower. Observations made over the centuries have helped astronomers map the boundaries of the particle rich regions which in turn helps them predict the likelihood of the meteor showers.
Whilst most meteors appear randomly in the sky and are referred to as sporadic, those which are part of shower appear to radiate from a specific point in the sky and that is what gives them their names e.g. The Draconid meteor shower appear to radiate from the constellation of Draco the Dragon. Just occasionally the number of meteors can exceed a thousand per hour and are sometimes referred to as meteor storms.
As with all faint objects the best views will occur when viewing from a dark location away from streetlights and during the period around New Moon.
Current forecasting suggests that the Draconid meteors will put on a good show in October 2011 and Explorers are looking at sites in the Middle East from which to view the event.