A solar eclipse occurs when the Moon passes between Earth and the Sun, thereby totally or partly obscuring the image of the Sun for a viewer on Earth. A total solar eclipse occurs when the Moon’s apparent diameter is larger than the Sun’s, blocking all direct sunlight, turning day into darkness. Totality occurs in a narrow path across Earth’s surface, with partial eclipses widespread over regions thousands of kilometres wide.
Solar eclipses are rare events. Although total eclipses occur on average once every 18 months somewhere on Earth, they are often visible only from a small area and most people go through life without ever seeing one. A partial eclipse, such as occurred over much of Europe and Asia on August 11, 1999, is far more common but still an unusual event requiring special equipment or planning to observe it properly.
During a total eclipse of the Sun, the Moon’s shadow sweeps across Earth at up to 1700 km/h (1000 mph). Its track typically covers between 100 and 200 km (60–120 mi) wide and may be up to 12000 km (7500 mi) long. The width of totality generally decreases with increasing distance from any given point in space along which it passes; for example, at its greatest extent during the August 11th 1999 event totality was about 270 km (170 mi) wide along its central path but only about 160 km (100 mi) wide near its edges 25% farther from that path.
As seen from within the cone-shaped or cylindrical shadow cast by the body blocking sunlight from reaching an observer (“umbral shadow”), any source of light appears dimmer than usual because there is less available light reaching each square metre due to interception by that body; if there were no atmosphere filtering out some colours while scattering others in different directions due to Rayleigh scattering then objects would appear black except for any glowing edges resulting from refraction effects producing “Baily’s beads”. The colouration effect seen during an eclipse results primarily because blue light is scattered more than other colours by atmospheric molecules; this includes both Rayleigh scattering as well as Raleigh- Jeans scattering caused by larger particulates such as water droplets and ice crystals present in clouds high above Earth’s surface where most of our atmosphere resides rather than near ground level where we live.
If one looks directly at sunlight not obscured by either Eclipse situation then permanent damage can result including blindness due to retinal burns (“eclipse blindness” or “solar retinopathy”). This can happen even when no more than 90%
of sunlight is blocked because unprotected eyes will receive intense visible plus infrared radiation causing severe eye pain while damaging tissue even if one does not stare directly at sunspots or prominences for long periods; looking directly at an annular eclipse also carries this same risk regardless how low percentage coverage exists because large amounts of infrared radiation still reach retina despite reduced brightness levels.. It is therefore essential that proper eye protection be used whenever viewing any type of Solar Eclipse situation whether total, annular ,or partial .