More about eclipses
When does an eclipse occur?
Solar and lunar eclipses are not independent. For both kinds of eclipse the Sun, Earth and Moon have to be aligned. If the lunar orbit plane did coincide with the ecliptic plane, there would be a lunar eclipse at each Full Moon and a solar eclipse at each New Moon. However, due to the inclination (5 degrees 8' 43") of the Moon's orbit compared to that of the Earth, for an eclipse to occur the Moon has to pass through the plane of the Earth's orbit close to Full Moon, or close to New Moon. This happens about twice per year. Due to perturbations in the Moon's orbit the eclipse cycle is not exactly six months.
When these conditions are met, there can be a lunar eclipse and a solar eclipse within 15 days of each other. So, before the solar eclipse of 11 August 1999, there was a lunar eclipse of 28 July 1999.
A periodicity in the cycle of eclipses was noticed by ancient Greek astronomers. Its duration is 6585.32 days (or 18 years and 10.3 days), after which time the position of the Sun and the Moon, as viewed from the Earth, recur. This is known as the 'Saros cycle'. Within a given Saros cycle the eclipses succeed each other in almost identical manner, except that the observation position on Earth shifts by 120 degrees (0.32 days) from one eclipse to the next.
On average there are 42 solar eclipses (14 partial solar eclipses, 28 central solar eclipses) and 42 lunar eclipses (including 14 total lunar eclipses) per Saros cycle.
Geometry and timing of eclipses
For a total eclipse, the region of totality is a narrow band (up to 300 km wide) around the line of centrality. Outside the zone of totality (or annular eclipse) there exists a much more extended area (more than 7000 km) where the eclipse is partial. The shape of this area depends on the respective positions of the Earth, Moon and Sun. The amount of the Sun covered is greatest closest to the totality zone, and decreases symmetrically away from it.
The Moon's shadow sweeps across the Earth at high speed (more than 2500 km/h) from West to East. This determines the duration of the partial eclipse that can last as much as three hours, and of the total eclipse (from seconds to a few minutes) depending on the position of the observer.
The maximum duration of a total eclipse in the most favourable conditions corresponds to 7min 30s in equatorial regions and 6min 10s at the latitude of Paris.
What actually happens during a total eclipse?
A total eclipse is one of the most impressive natural phenomena. Before totality, the first phase of partial eclipsing of the solar disk takes some 40 minutes when the luminosity progressively drops to a few percent. The cooling of the local atmosphere is already noticeable.
The moment of totality comes very suddenly, at the precise instant when the solar surface is totally occulted, and the ambient luminosity drops down to one part in 10 000 of normal solar light in a few seconds. The last solar photospheric rays shine through the lunar valleys at the lunar limb allowing a light from the solar photosphere just before second contact and after third contact, giving the phenomena of fast flashing Baily's Beads for a few seconds. In the middle of the day, suddenly it is deep dawn, with stars to be seen.
We can distinguish four key moments during a solar eclipse:
- the 1st contact when the apparent disks of the Sun and Moon touch for the first time (starting the partial eclipse for some 40 minutes),
- the 2nd contact at the start of totality,
- the 3rd contact at the end of totality (after a few seconds or minutes), with another partial eclipse lasting some more 40 minutes and ending with
- the 4th contact when the apparent disks separate.
The last photospheric light together with the pink ring of the chromosphere, and white inner corona is known as the "Diamond ring". During the few minutes of totality, the dark lunar disk appears with a ring of pink pearls (the solar prominences). During the totality, one sees at last a large diffuse aura (the solar corona) with streak structures streaming at low solar latitudes and fine radial structures (or plumes) near the poles.