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What Is a Total Solar Eclipse?

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. The darkest point of the eclipse is almost as dark as night.

Total Solar Eclipse

The Sun's corona during a Total Solar Eclipse.

As the Moon moves to cover the entire disc of the Sun, the Sun's corona forms a ring of light like a diamond ring.

©bigstockphoto.com/JohanSwanepoel

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

Not Total Everywhere

Eclipses are named after their darkest phase. If a solar eclipse is total at any point on Earth, it is called a total solar eclipse, even though it's seen as a partial eclipse in most parts of the world.

5 Phases

There are 5 stages in a total solar sclipse:

  1. 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.
  2. Total 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.
  3. 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.
  4. Total eclipse ends (third contact): The Moon's shadow starts moving away and the Sun reappears.
  5. Partial eclipse ends (fourth contact): The Moon stops overlapping the Sun's disc. The eclipse ends at this stage.

Protect Your Eyes!

Never look directly at the Sun, eclipsed or otherwise, without proper protective eyewear. The Sun’s UV radiation can burn the retinas in your eyes, and cause permanent damage or even blindness.

Be Safe: DIY Pinhole Projector

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

Special Sights at Totality

Illustration of totality of a Total Solar Eclipse
Phenomena only visible at the totality.
The Sun's corona, Baily's beads and the diamond ring effect are only visible at the totality (darkest point) of a Total Solar Eclipse.
©bigstockphoto.com/JohanSwanepoel

Certain phenomena 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 seen 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 atmosphere.

The Science of Total Solar Eclipses

Illustration image
The Sun, Moon and Earth are aligned
The Moon's umbra (full shadow) and penumbra (half shadow) determines where an eclipse will be total or just partial.

Only those located in the path of the Moon's umbra can see a Total Solar Eclipse. The Moon's umbra is only a few hundred kilometers wide and travels eastward at about 1,056 mph (1,700 km/h). Because of this, even though solar eclipses happen 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.

A Total Solar Eclipse can last for several 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 when the totality lasted 6 mins and 39 secs.

Total Solar Eclipses are rare, and only happen (every one or two to three years) 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.
Illustration image

Total Solar Eclipse Infographic. Click image for full version.

Not Every New Moon Night

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.

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.

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 and the Moon's elliptical orbits mean that the Earth’s distance from the Sun and the Moon's distance from 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 if the Moon is at its apogee?

Topics: Astronomy, Eclipses, Moon, Earth, Sun

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Total Solar Eclipse

Aug 21, 2017 at 3:46 PM UTCSee more


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