What Are Total Solar Eclipses?
Total solar eclipses occur when the New Moon comes between the Sun and Earth and casts the darkest part of its shadow, the umbra, on Earth. A full solar eclipse, known as totality, is almost as dark as night.
Not Total Everywhere
Eclipses are normally 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 solar eclipse in most areas.
However, there is an exception, the hybrid solar eclipse. This type of eclipse is also known as an annular-total eclipse because it changes from an annular to a total solar eclipse, and/or vice versa, along its path.
Complete Solar Eclipse Has 5 Phases
There are 5 stages in a total solar eclipse:
- Partial eclipse begins (1st contact): The Moon starts becoming visible over the Sun's disk. The Sun looks as if a bite has been taken from it.
- Total eclipse begins (2nd contact): The entire disk 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, just before totality.
- 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. Observers in the path of the Moon's umbra may be able to see Baily's beads and the diamond ring effect, just after totality ends.
- Total eclipse ends (3rd contact): The Moon starts moving away, and the Sun reappears.
- Partial eclipse ends (4th contact): The Moon stops overlapping the Sun's disk. The eclipse ends at this stage in this location.
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.
To watch a solar eclipse safely, 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. However, naked eye observations MUST NOT begin until Baily's beads have completely disappeared (see below). Also, naked eye observations MUST finish before Baily's beads reappear at the end of totality. Make sure you know how long totality will last at your location.
Unique Sights around Totality
Certain phenomena can only be seen during a total solar eclipse:
- Shadow bands: About 1 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.
- Diamond ring: Seen about 10 to 15 seconds before and after totality, the solar corona (the outer atmosphere of the sun) becomes visible; seen together with a single jewel of light from the sun, this creates a diamond ring effect.
- The Sun's corona: As the diamond ring fades, the Sun's corona becomes more prominent and is visible as a faint ring of rays surrounding the silhouetted Moon. The corona is the outermost layer of the Sun’s atmosphere, and it is around 200–300 times hotter than the Sun’s surface. The corona’s temperature can reach over 1 million °C (1.8 million °F).
- Baily's beads: About 5 seconds before totality, Baily's beads appear. They are little bead-like blobs of light at the edge of the Moon. They are created because gaps in the mountains and valleys on the Moon's surface allow sunlight to pass through in some places but not others.
- The Sun's chromosphere: A lower layer of the Sun’s atmosphere, the chromosphere, gives out a reddish glow which can only be seen for a few seconds after totality sets in.
- Baily's beads: The beads grow and merge into a crescent as the Moon continues to move away.
- Diamond ring and corona: As the diamond ring grows brighter, the corona fades.
- Shadow bands: The moving wavy lines reappear on the ground shortly before the crescent Sun becomes visible again and nature recovers.
The phenomena then repeat in reverse order:
The Science of Total Solar Eclipses
Only viewers located in the path of the Moon's full shadow, its umbra, can see a total solar eclipse. The Moon's umbra travels eastward at about 1,700 km/h (1,056 mph).
A total solar eclipse can last for several hours. 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 the totality lasted 6 minutes and 39 seconds.
On average, there's approximately one total solar eclipse every 18 months, when:
- It is New Moon.
- At the same time, the Moon is at (or very near) a lunar node, so the Earth, the Moon, and the Sun are aligned in a straight (or nearly straight) line.
- The Moon is near perigee.
Not Every New Moon
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 about 2—5 times a year.
For a solar eclipse to take place, 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 take place. Seen from Earth, the Moon passes just above or just below the Sun (see image).
Near Lunar Perigee
The Moon's path around Earth is elliptical, with one side of the orbit closer to Earth than the other. The side 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 closer to one end, the perihelion of the orbit than the other 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.