The Stages of an Eclipse
The moment when the first small bite appears in the edge of the sun.
The moment when the sun is completely covered.
The period of time (usually a few minutes) following second contact when the sun can be observed with the naked eye.
The moment when the sun reappears from behind the Moon.
The moment when the sun is completely restored.
The bright solar surface that we normally see and which is completely covered during the total eclipse.
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 the colour of the sky seems to change, and the intensity of brightly coloured objects seem to soften. If there are trees or bushes around, the little gaps between the leaves act as pin-hole cameras to throw many overlapping images of the crescent sun on the ground. This becomes more noticeable as the crescent sun narrows.
Just before the eclipse becomes total you may see the shadow of totality advancing like a wall across the landscape rapidly towards you. Except in special circumstances, the shadow of totality moves from west to east. The visibility of the shadow will depend on the clarity of the atmosphere and your location, whether you are on a mountain overlooking a plain for example. Also, the speed of the shadow varies. In the early morning or late evening the shadow may be travelling at a speed of perhaps 12,000 miles per hour, whereas if you are observing the eclipse near noon on the equator the shadow may be travelling at 1,500 mph.
Baily Beads are a familiar feature of total eclipses. As the moon completely covers the sun, the razor-thin solar crescent breaks up into a chain of beads which gradually wink out. When the last one disappears, totality has started.
Just as the eclipse becomes total, at the point where the last Baily Bead has disappeared 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 most noticeable feature during totality is the solar corona, 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.
Prominences are flamelike 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 disc 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 as dark as nightime, 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.
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. It is in fact the same configuration as the last Baily bead at second contact but the difference lies in the adaptation of our eye. 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.