What three conditions must be met if there is to be a total eclipse of the Sun

A solar eclipse is a spectacular sight and a rare astronomical event. Each one is only visible from a limited area.

Solar eclipses can have a maximum point that is either partial, annular, or total.

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An eclipse of the Sun happens when the New Moon moves between the Sun and Earth, blocking out the Sun's rays and casting a shadow on parts of Earth.

The Moon's shadow is not big enough to engulf the entire planet, so the shadow is always limited to a certain area (see map illustrations below). This area changes during the course of the eclipse because the Moon and Earth are in constant motion: Earth continuously rotates around its axis while it orbits the Sun, and the Moon orbits Earth. This is why solar eclipses seem to travel from one place to another.

Types of Solar Eclipses

There are 4 different types of solar eclipses. How much of the Sun's disk is eclipsed, the eclipse magnitude, depends on which part of the Moon's shadow falls on Earth.

1. Partial solar eclipses occur when the Moon only partially obscures the Sun's disk and casts only its penumbra on Earth.
2. Annular solar eclipses take place when the Moon's disk is not big enough to cover the entire disk of the Sun, and the Sun's outer edges remain visible to form a ring of fire in the sky. An annular eclipse of the Sun takes place when the Moon is near apogee, and the Moon's antumbra falls on Earth.
3. Total solar eclipses happen when the Moon completely covers the Sun, and it can only take place when the Moon is near perigee, the point of the Moon's orbit closest to Earth. You can only see a total solar eclipse if you're in the path where the Moon casts its darkest shadow, the umbra.
4. Hybrid Solar Eclipses, also known as annular-total eclipses, are the rarest type. They occur when the same eclipse changes from an annular to a total solar eclipse, and/or vice versa, along the eclipse's path.

Solar Eclipses Mainly Look Partial

Solar eclipses are only visible from within the area on Earth where the Moon's shadow falls, and the closer you are to the center of the shadow's path, the bigger the eclipse looks.

Solar eclipses are usually named for their darkest, or maximum, point. The exception is the hybrid eclipse.

The darkest point of solar eclipses is only visible from a small area. In most places and for most of the duration, total, annular, and hybrid eclipses look like a partial solar eclipse.

Only around New Moon

For a solar eclipse to take place, the Sun, the Moon, and Earth must be aligned in a perfect or near perfect straight line. A rough alignment of the three bodies happens every lunar month, at the New Moon.

So, why isn't there a solar eclipse every New Moon?

Lunar nodes are the locations where the Moon crosses the Earth's orbital plane.

The plane of the Moon's path around the Earth is inclined at an angle of approximately 5° to Earth's orbital plane around the Sun—the ecliptic. The points where the plane of the Moon's orbital path meets the ecliptic are called lunar nodes.

A perfect or near-perfect alignment of the Sun, the Moon, and the Earth can only take place when the New Moon is near a lunar node. This can only happen during periods that come a little less than 6 months apart, and last, on average, around 34.5 days. It is only during this time, also known as the eclipse season, that eclipses can occur.

When there is a Full Moon during the eclipse season, we see a lunar eclipse.

Different types of eclipses

Protect Your Eyes!

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

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

More details about upcoming Eclipses

Topics: Sun, Moon, Astronomy, Eclipses

Total solar eclipses happen when the New Moon comes between the Sun and Earth and casts the darkest part of its shadow, the umbra, on Earth.

A total eclipse is spectacular to see.

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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 a part of the Sun.

Not Total Everywhere

Only viewers located in the path of the Moon's full shadow, its umbra, can see a total solar eclipse. Those outside the path see a partial eclipse. By convention, therefore, 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.

Hybrid solar eclipses or annular-total eclipses are an exception to this rule. This is because they change from an annular to a total eclipse or from a total to an annular eclipse along their path.

Complete Solar Eclipse Has Five Phases

There are five stages in a total solar eclipse:

1. Partial eclipse begins (1st contact): The Moon becomes visible over the Sun's disk. It looks like the Moon has taken a bite out of the Sun.
2. Total eclipse begins (2nd contact): The Moon covers the entire disk of the Sun. Observers in the Moon's umbral path may be able to see the diamond ring effect and Baily's beads just before totality.
3. Totality and maximum eclipse: The Moon completely covers the disk 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 can fall, and birds and animals often go quiet. The midpoint of time of totality is known as the maximum point of the eclipse.
4. Total eclipse ends (3rd contact): The Moon starts moving away, and the Sun reappears. Those fortunate enough to be in the Moon's umbral can see Baily's beads and the diamond ring effect just after totality ends.
5. Partial eclipse ends (4th contact): The eclipse ends as the Moon leaves the Sun's disk.

Protect Your Eyes!

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

To safely watch a solar eclipse, wear protective eclipse glasses or project an image of the eclipsed Sun using a pinhole projector.

Only Safe during Full Eclipse

It is safe to view a fully eclipsed Sun, totality, with the naked eye. It is also safe to observe totality through cameras, telescopes, or binoculars without any special filters. Do not look at the Sun with naked eyes unless the Moon blocks the entire Sun. Even a small amount of direct sunlight can damage your eyes. Make sure you know how long totality lasts in your location.

Unique Sights around Totality

Phenomena only visible near the totality.

©bigstockphoto.com/JohanSwanepoel

Some sights are only visible during a total solar eclipse. They appear in this order as totality sets in (and in reverse order as totality ends):

1. Shadow bands: About a minute before totality, moving, wavy lines of alternating light and dark can be seen on the ground and along walls. These shadow bands are the result of Earth’s turbulent atmosphere refracting the last rays of sunlight.
2. Diamond ring: Seen about 10 to 15 seconds before and after totality, the solar corona (the outer atmosphere of the Sun) becomes visible. Together with the single jewel of light from the Sun, it creates the well-known diamond ring effect.
3. The Sun's corona: As the diamond ring fades, the outermost part of the Sun's atmosphere becomes more prominent. It is visible as a faint ring of rays surrounding the silhouetted Moon and is around 200–300 times hotter than the Sun’s surface—its temperature can reach over 1 million °C (1.8 million °F).
4. Baily's beads: About five seconds before totality, Baily's beads appear. They are little bead-like blobs of light at the edge of the Moon created by the sunlight passing through gaps in the mountains and valleys on the Moon's surface.
5. The Sun's chromosphere: The second most outer layer of the Sun’s atmosphere gives out a reddish glow. It is only visible for a few seconds right after totality.

These events then repeat in reverse order: First, the Sun's chromosphere reappears just before totality ends. The Baily's beads follow this as the Moon continues to move away. The diamond ring and corona then makes an appearance, following which shadow bands reappear before the crescent Sun is visible again.

The Science of Total Solar Eclipses

The Sun, Moon, and Earth are aligned

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On average, one total solar eclipse happens every 18 months, when:

• There's a New Moon,
• at the same time, the Moon is at (or very near) a lunar node, so the Earth, the Moon, and the Sun line up in a straight (or nearly straight) line,
• and the Moon is near perigee.

Eclipse Shadows: Umbra, Penumbra, and Antumbra

Not Every New Moon

Lunar nodes are the locations where the Moon crosses the Earth's orbital plane.

The Moon's orbital path around Earth is inclined at an angle of approximately 5° to the Earth's orbital plane around the Sun (ecliptic). Without this slant, we would be able to see two eclipses per lunar month—a solar eclipse at every New Moon and a lunar eclipse at every Full Moon. In reality, solar eclipses happen only 2—5 times a year.

For a solar eclipse to occur, the New Moon must be at or very close to one of the two points where the orbital planes meet. These locations are called lunar nodes.

If the Moon is not near a lunar node during New Moon, the Sun, Moon, and Earth do not align in a straight or almost straight line, and a solar eclipse cannot occur. Seen from Earth, the Moon passes just above or just below the Sun (see image).

Moon Phases

Near Lunar Perigee

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

Earth's orbit around the Sun is also elliptical, with the Sun closest at perihelion, and farthest away at aphelion.

Earth's and the Moon's elliptical orbits mean that 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 about 400 times closer to Earth than the Sun, the Moon's and the Sun's apparent sizes roughly match. Because of this, total eclipses of the Sun can only occur when the Moon is near perigee—it is the only time when the disk of the Moon looks big enough to cover the entire disk of the Sun.

What if the Moon is near apogee?

How Long Does Totality Last?

A total solar eclipse can last for several hours and totality can range from a few seconds to 7.5 minutes. The longest total solar eclipse of the 21st century took place on July 22, 2009, when totality lasted 6 minutes and 39 seconds!

More details about upcoming Eclipses

Topics: Astronomy, Eclipses, Moon, Sun