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Annular solar eclipses explained

An annular solar eclipse happens when the Moon moves in front of the Sun, but it is so far away from Earth that it appears smaller in the sky and cannot cover the Sun completely.

Annular solar eclipse
All annular solar eclipses have a “ring of fire”.
©iStockphoto.com/peisen zhao

The Sun's outer edges are then still visible and form a “ring of fire” around the Moon.

Like total solar eclipses, annular solar eclipses are quite rare. They can only be observed during New Moon, when

  • the Sun is near one of the nodes of the lunar orbit, so Earth, Sun and Moon form a straight line,
  • the Moon is at apogee - the far side of the Moon's orbit around the Earth,
  • and the observer is located in the path of the Moon's antumbra.

The Moon's orbit & solar eclipses

The Earth revolves around the Sun, and the Moon circles the Earth. During New Moon, the Moon passes roughly between Earth and Sun. However, in most cases the three celestial bodies do not form a straight line, so the Sun is not eclipsed.

The reason why total solar eclipses do not happen every New Moon is that the lunar orbital plane - the imaginary flat surface whose outer rim is formed by the Moon's path around Earth - runs at an angle of approximately 5 degrees to the Earth's orbital plane around the Sun (ecliptic). The points where the two orbital planes meet are called lunar nodes. Only if the Sun appears near one of the two lunar nodes during New Moon so Earth, Moon and Sun form a straight line can a solar eclipse be observed from Earth.

Perigee and apogee

The Moon's orbit around Earth is not round but forms an ellipse. This is why the distance from Earth to the Moon varies constantly. The Moon is at perigee when it is closest to Earth. The point of greatest distance is called apogee.

Because of the varying distance, the Moon appears larger when it is at perigee. Although it is much smaller than the Sun, the Moon's apparent size then roughly matches the Sun's, as seen from Earth. For that reason, it can only cover all of the Sun's disk during a solar eclipse when it is at perigee. On the other hand, when the Moon is at apogee, its apparent size is so small that the Sun's outer edges are still visible and an annular solar eclipse can be observed.

The Moon's shadow & solar eclipses

Like any other object's shadow, the Moon's shadow consists of three different areas: the innermost and darkest part (umbra), the lighter, outer part (penumbra), and an area beyond the umbra that is only visible from Earth when the Moon is at apogee (antumbra). To observe an annular solar eclipse, the observer must be located in the antumbra.

Topics: Astronomy, Eclipses, Moon, Earth, Sun

In this Article


All about solar eclipses

  1. Types of solar and lunar eclipses
  2. Total solar eclipses
  3. Partial solar eclipses
  4. Annular solar eclipses
  5. Solar eclipses in history
  6. Eye safety during solar eclipses
  7. Make a pinhole projector

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