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Total solar eclipses explained

Total solar eclipses occur when the Moon comes between the Sun and the Earth and casts the darkest part of its shadow (the umbra) on Earth. A total solar eclipse can be amazing to see if you are at a place where this shadow falls.

Solar Eclipse
The Sun's corona during a total solar eclipse.
©iStockphoto.com/Simon Podgorsek

In a total solar eclipse, the Moon covers the Sun's entire disk, while in partial and annular solar eclipses, the Moon blocks only part of the Sun.

Next total solar eclipse: March 20, 2015

The science of total solar eclipses

Total solar eclipses are rare. They only happen when:

  • There is a New Moon.
  • The Moon is near a lunar node.
  • The Earth, Moon and Sun are aligned in a straight line.
  • The Moon is at perigee.

Why don't we see a total solar eclipse every new Moon?

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 2 orbital planes meet are called lunar nodes. Solar eclipses occur only when a new Moon takes place near a lunar node.

Moon Phases

Lunar perigee and total solar eclipse

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.

Lunar perigee and 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).

Earth's orbit & solstices

The Earth and the Moon's elliptical orbits mean that the Earth’s distance from the Sun and the Moon's distance from the Earth varies throughout the year. It also means that from Earth, the Sun's and Moon's apparent sizes change during the year.

When the Moon is closest to the Earth, its apparent size roughly matches the Sun's apparent size. Because of this, total eclipses of the Sun can only occur when the Moon is at perigee – it is the only time when the disc of the Moon looks big enough to cover the entire disc of the Sun.

What happens if the Moon is at its apogee?

Total solar eclipse phases

A total solar eclipse can last for a couple of hours. Totality can range from a few seconds to 7 and a half minutes. The longest total solar eclipse of the 21st century took place on July 22, 2009. Totality in this eclipse lasted for 6 mins 39 secs.

There are 5 distinct stages in a total 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.
  • Full eclipse begins (second contact): Almost the entire disc of the Sun is covered by the Moon. Observers in the path of the Moon's umbra may be able to see Baily's beads and the diamond ring effect.
  • Maximum eclipse or totality: The Moon completely covers the disc of the Sun. Only the Sun's corona is visible. This is the most dramatic stage of a total solar eclipse. At this time, the Sky goes dark, temperatures fall drastically and birds and animals often go quiet.
  • Full eclipse ends (third contact): The Moon's shadow starts moving away and the Sun reappears.
  • Partial eclipse ends (fourth contact): The Moon stops overlapping the Sun's disc. The eclipse ends at this stage.

March 20, 2015 total solar eclipse: Find out when these stages happen

Where can I view a total solar eclipse?

To observe a total solar eclipse, one must be at the right place at the right time. Only those located in the path of the umbra can see a total solar eclipse. Because of this, even though solar eclipses occur about 2-5 times a year, not many people get to see them. At any place on Earth, a total solar eclipse can be seen on average once every 360 years.

Fun fact: The Moon's umbra is only a few hundred kilometers wide and travels eastward at about 1,056 mph (1,700 km/h).

When is the next total solar eclipse at your location?

How to safely view a total solar eclipse

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

The only way to safely see a total solar eclipse is to wear protective eclipse glasses or to project an image of the eclipsed Sun using a pinhole projector.

What can you do to protect your eyes?

How to make a pinhole projector to view a solar eclipse?

What to look for during a total solar eclipse?

Total solar eclipses are true wonders of nature – there are certain phenomena that can only be seen during a total solar eclipse:

  • Baily's beads: Seen about 10-15 seconds before and after totality, Baily's beads are little bead-like blobs of light at the edge of the Moon. These happen because the gaps in the mountains and valleys on the Moon's surface allow sunlight to pass through in some places but not others.
  • Diamond ring: As the Moon moves to cover the entire disc of the Sun, Baily's beads disappear, leaving one last bead a few seconds before totality. At this point in the eclipse, the Sun's corona forms a ring around the Moon. The ring around the Moon and the leftover Baily's bead gives the appearance of a diamond ring.
  • The Sun's chromosphere: The Sun's atmosphere has 3 layers: the photosphere, the chromosphere and the corona. The chromosphere, which gives out a reddish glow can only be seen for a few seconds right after the diamond ring disappears during a total eclipse of the Sun.
  • The Sun's corona: Like the chromosphere, the Sun's corona is only visible during a total solar eclipse. It can be seen as a faint ring of rays surrounding the dark Moon during totality.
  • Shadow bands: About one minute before and after totality, moving wavy lines of alternating light and dark can be observed on plain-colored surfaces. These shadow bands are the result of the light emitted from a thin solar crescent being refracted by the Earth's atmophere.

Topics: Astronomy, Eclipses, Moon, Earth, Sun

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. Solar eclipses in history
  7. Solar eclipse myths and superstitions
  8. Eye safety during solar eclipses
  9. Make a pinhole projector

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