[9] Some of the stages visible during a total eclipse of the Sun.eclipse. In the strictest sense this isn’t an eclipse at all, it’s a transit of the Sun
by the Moon, but the term eclipse is still used.
Finally, if the Moon’s apparent disc happens to appear larger than the
Sun in the sky, if you’re in the right place on the Earth’s surface at the right
time, you will witness that most sought after of astronomical events, a total
eclipse of the Sun. Here, the tip of the dark shadow cone would lie below the
surface of the Earth. As the tip of the cone is truncated, on the Earth’s surface
the shadow has a tangible size and if you’re located so that it passes over
you, the total eclipse you get to see has duration.
The size and shape of the shadow, as well as how fast it appears to move
across the Earth’s surface, is determined by the relative apparent sizes of
the Sun and the Moon along with the geometry of the eclipse relative to the
Earth. Shadows cast to the edge of the planet give rise to more distorted and
faster moving ellipses.
The largest umbral shadow has a width of around 150 miles and the
speed at which the shadow races across the Earth’s surface varies from
up to 1,100 mph at the equator to 5,000 mph at the poles. The shadow
moves across the surface in a west to east direction. The longest duration
that a total eclipse of the Sun can last is around 7.5 minutes. You need to be
standing in the path of the track in order to experience the full majesty of
a total solar eclipse. Observers either side of the track experience a partial
eclipse of ever decreasing magnitude the further from the main track they
are located. A typical eclipse shadow is 100 miles wide and travels 10,000
miles across the surface of the Earth, covering less than one percent of the
Earth’s surface as it goes.
As stated, the main types of solar eclipse are partial, annular and total,
but the fact that the Earth’s curving surface presents a projection screen of
varying distance for the shadow gives rise to another type of solar eclipse,
known as a hybrid. The term refers to the situation where the dark shadow
cone tip can’t quite reach the Earth’s surface at the track start but as our
planet’s surface curves upward towards it, it can. So at the start of the track,
the eclipse would appear annular but further along the track it becomes
total. Finally, as the Earth’s surface curves away again, the eclipse can return
to an annular type.
Of course, it’s impossible to view all of the stages because of the speed
that’s required to chase the shadow, but at least, with forward planning, youappears partially eclipsed. Outside the penumbral shadow, the Sun’s disc
would be totally clear from obstruction and the Sun would appear with
complete full intensity.
The Moon is approximately 1/400th the size of the Sun and, by a
remarkable coincidence, the Sun is approximately 400x further away than
the Moon. This is the reason why both bodies have very similar apparent
sizes in the sky. The Moon’s umbral shadow extends out to a distance similar
to the distance between the Moon and the Earth. For the situation where
the Moon and Sun have the same apparent diameter in the sky, the umbral
shadow cone tip just reaches the Earth’s surface. If you were to stand in
the correct location to experience the shadow tip pass over you, the Sun
would appear to undergo an almost instantaneous total eclipse under these
strange conditions.
For the situation where the apparent Moon’s disc looks smaller than
the Sun, the dark shadow cone tip isn’t able to quite reach to the Earth’s
surface. If you positioned yourself accurately, using suitable protective filters,
at the time of maximum "eclipse" you’d see the Moon’s disc concentrically
positioned within the Sun’s disc – the event described earlier as an annular[10] Dappled sunlight through trees takes on the shape of the crescent Sun during
the partial phase of an annular eclipse.910Astronomer Book