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An eclipse occurs when a body (e.g. planet, or satellite) gets between a source of light (e.g. the Sun) and another body. Examples are:

  • Lunar eclipses - where the Earth obscures the Sun, from the Moon's point of view. The Moon moves through the shadow cast by the Earth. This can only happen at full moon.
  • Solar eclipses - where the Moon obscures the Sun, from the Earth's point of view. The Moon casts a shadow that touches the surface of the Earth. This can only happen at new moon.

Total eclipses occur where the light source is totally blocked off by the eclipsing body. An eclipses is partial at places where only part of the luminary is covered (solar eclipses), or when only part of a body is eclipsed by the shadow (lunar eclipses).

A lunar eclipse is visible when the Moon is above the horizon when the eclipse happens: so it is visible from half the world. In contrast, solar eclipses are only total in the region where the shadow of the Moon touches the Earth: this area is comparatively small (of the order of 100 km across), so at any specific place it is rare to observe a total solar eclipse. The Sun appears partially covered by the Moon from a much larger region of the world.

An eclipse can only occur when the Sun, Earth and Moon are in a line. Because the plane of the orbit of the Moon is tilted with respect to the plane of the orbit of the Earth (the ecliptic), eclipses occur only when the three bodies are near the intersection of these planes: the so-called nodes. The Sun passes either node once a year, and eclipses occur in a period of about 2 draconic months around these times.

In total there can be from 2 to 7 eclipses in a calendar year.