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What is an Annular Solar Eclipse?

An annular solar eclipse happens when the Moon covers the Sun's center, leaving the Sun's visible outer edges to form a “ring of fire” or annulus around the Moon.

Annular solar eclipse
Annular solar eclipses have a “ring of fire”.
An annular solar eclipse occurs when the Moon moves in front of the Sun but does not cover the it completely.
©iStockphoto.com/peisen zhao

In an annular eclipse of the Sun, the Moon casts its antumbra - the outer part of the Moon's umbra - on the Earth.

Like total solar eclipses, annular solar eclipses can also be seen as partial eclipses from locations outside the Moon's antumbra but inside its penumbra.

Science of Annular Solar Eclipses

Annular solar eclipses take place only when:

  • The Moon is a new Moon.
  • The Moon is at or near a lunar node.
  • The Earth, Moon and Sun are perfectly aligned in a straight line.
  • The Moon is at its apogee.

Not Every New Moon Night

Even though a new Moon is necessary for an annular solar eclipse to take place, we don't see an eclipse every new Moon night. This is because the plane of the Moon's orbital path around the Earth is inclined at an angle of 5° to the Earth's orbital plane (ecliptic) around the Sun. The points where the two orbital planes meet are called lunar nodes. Solar eclipses occur only when a new Moon takes place near a lunar node.

Lunar Apogee & Annular Solar Eclipses

The Moon's path around the Earth is elliptical, with one side of the orbit closer to Earth than the other. The side closest to the Earth is called the perigee and the side farthest from the Earth is known as the apogee.

The Earth's orbit around the Sun is also elliptical, with the Sun closer to one end (perihelion) of the orbit than the other (aphelion).

The Earth's and the Moon's elliptical orbits mean that the Earth’s distance from the Sun and the Moon's distance from the Earth vary throughout the year. It also means that from Earth, the apparent size of the Sun and Moon changes over the year.

When the Moon is farthest from the Earth, its apparent size is much smaller than the Sun's apparent size. Because of this, annular eclipses of the Sun can only occur when the Moon is at apogee – it is the only time when the entire disc of the Moon can block the central part the Sun's disc, while leaving the outer parts of the Sun visible.

Phases of an Annular Solar Eclipse

Annular solar eclipses can last for a couple of hours from start to end. Annularity, the phase where the “ring of fire” is visible, can range from a few seconds up to 12 and a half minutes.

There are 5 distinct stages of an annular solar eclipse:

  • Partial eclipse begins (first contact): The Moon's shadow starts becoming visible over the Sun's disc. The sun looks as if a bite has been taken from it. For a few seconds before the full eclipse begins, Baily's beads, which look like beads of light, are seen towards the Moon's trailing edge.
  • Full eclipse begins (second contact): The “ring of fire” appears.
  • Maximum eclipse or annularity: The Moon completely blocks the center of the Sun's disc. Only the Sun's outer atmosphere or photosphere is seen in the form of a bright ring of rays around the dark disc of the Moon.
  • Full eclipse ends (third contact): The Moon starts moving away from the disc of the Sun. Once again, Baily's bead are visible along the Moon's leading edge.
  • Partial eclipse ends (fourth contact): The Moon stops overlapping the Sun's disc. The eclipse ends at this stage.

Viewing an Annular Solar Eclipse

To see an annular solar eclipse, you must be in the path of the Moon's antumbra. The maximum zone of visibility for an annular solar eclipse is about 230 miles (270 kms). If you're at the center of this zone, you can see a perfect ring of fire. If you're at the edge of this zone, you may see a broken ring of fire and Baily's beads throughout the duration of the eclipse.

Safely View an Eclipse

Never look at the Sun, eclipsed or otherwise, without protective eyewear. The Sun’s UV radiation can burn the retinas in the eyes leading to permanent damage or even blindness.

A safe way to see an annular solar eclipse is to wear protective eclipse glasses or to project an image of the eclipsed Sun using a pinhole projector.

Topics: Astronomy, Eclipses, Moon, Earth, Sun

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In this Article

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All about solar eclipses

  1. Types of Solar and Lunar Eclipses
  2. What are solar eclipses?
  3. Total solar eclipses
  4. Partial solar eclipses
  5. Annular solar eclipses
  6. 10 things: Solar Eclipse, 20 March, 2015
  7. Solar Eclipses in History
  8. Solar eclipse myths and superstitions
  9. Eye safety during solar eclipses
  10. Make a pinhole projector

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