A solar eclipse is actually the shadow of the Moon, which falls on the Earth. Thus the larger the shadow, the longer appears to the observer the eclipse.

When is the shadow of the moon particularly large?

First the Earth must be as far as possible away from the Sun or from our viewpoint, as small as possible. The Earth is in the aphelion furthest from the Sun. In this time, e.g. in July, occurs the longest solar eclipse. One calls the point, at which it is nearest the Earth, perihelia. Between these two points the size of the Sun seems to vary around 1.7%.

But not only the Sun, also the moon is not always equally far from the Earth and appears to the viewer in various sizes. The point of its largest distance is called perigee, that of the smallest- apogee. Its size seems to change between these two points around 7%.
For the duration of the eclipse this undoubtedly plays an important role. The closer the Moon is to the Earth, the larger becomes its shadow on the Earth's surface.

The most favorable for solar eclipses is thus a distance from the Earth to the Sun which has a simultaneous proximity to the distance from the Earth to the Moon.

One can test this very simply, by projecting the shadow of any article with a source of light to a wall. Playing with the distance between the wall, the article and the source of light, one will find out that the shadow becomes larger, if one brings the article closer to the wall or the source of light far away from it.

The shadow of the Moon " moves " over the earth. Since the Earth rotates in the same direction, it also moves with the shadow. The faster the Earth moves along with the shadow, the longer it takes for the shadow of the moon to cover that particular area. The movement of the surface of the Earth is fastest at the equator. For instance, one point on the equator moves with 400m/s, while the Moon turns with a peripheral speed of approx. 1km/s. Its shadow moves itself thus with an accordingly high speed over the Earth. This speed is lowest at noon, since then the Sun is highest in the sky (seen from the Earth). Thus the shadow of the Moon needs at noon longest, in order to cover one point.

This means that the most favorable conditions for long observation times are at the equator at midday.

If all these factors coincide, the max. diameter of the shadow is 264 km, whereby its speed amounts to 0.58 km /s. Then the longest possible solar eclipse is with a duration of 7 min 34 s. This is however very rare and even one with a duration of 6 is already very unusual.
. Almost the maximum time of the eclipse will be achieved on 16. 07. 2186 with 7 min 29s.

The eclipse on 11.08.1999 does not achieve it by far. The max. diameter of its shadow amounts to 112 km at a minimum rate of 0,679 km/s. The max. duration of the total darkness is " only " 2 min 22 s.