Guide to Solar Eclipses

Solar EclipseSolar eclipses stand out as visual treasures; a natural phenomenon to behold. The good news is that there are three solar eclipses visible in 2019. This guide is the perfect introduction to understanding how solar eclipses happen and the different types of eclipses that ensue.

A solar eclipse occurs when the Moon passes between Earth and the Sun and its shadow falls on the Earth’s surface. Anyone located within the path of the shadow as it falls upon Earth and sweeps across the planet’s face will view some type of solar eclipse.

The way solar eclipses line up is truly extraordinary: the Sun’s distance from the Earth is roughly 400 times the distance between the Earth and the Moon, and the Sun’s diameter is about 400 times the diameter of the Moon. These near perfect ratios result in the apparent size of the Sun and Moon’s disks measuring almost exactly the same – each covering approximately 0.5°of the sky – hence the Moon can entirely cover the Sun’s disk as seen from Earth.

The Moon’s orbit around the Earth is not an exact circle; it is elliptical, meaning the Moon lies closer to Earth on one side of its orbit compared to the other. The Earth’s orbit around the Sun is also elliptical. These variations in the distances between the bodies causes the apparent sizes of the Sun and Moon to be bigger or smaller throughout the year. The Moon is at perigee when it is on the side of its orbit closer to Earth. Apogee is when the Moon lies on the ‘far’ side of its orbit.

Scientists calculate the magnitude of a solar eclipse by determining the ratio of the apparent size of the Moon to the apparent size of the Sun. If the Moon is at perigee, the ratio will be greater than or equal to 1. The Moon appears slightly larger than the Sun in the sky due to its closer proximity to Earth. The magnitude of the solar eclipse is therefore also greater than or equal to 1. The outcome is a total solar eclipse.

When the Moon is at apogee it appears slightly smaller than the disk of the Sun, creating an annular eclipse. The eclipse will have a magnitude of less than 1. On average, the Moon is farther away from Earth rather than closer, therefore the chances of an annular eclipse are higher than the possibility of a total solar eclipse.

Types of Eclipses

The greatest factor that determines the type of eclipse an observer sees is which part of the Moon’s shadow the observer falls in. As with any object that is illuminated by another body, the Moon has three parts to its shadow: umbra, penumbra and antumbra.

Total Solar Eclipse

Total Solar EclipseThe Moon completely obscures the Sun in a total solar eclipse. They are rare events and several conditions must be met for one to occur:

  • There has to be a New Moon.
  • The Earth, Moon and Sun have to be aligned.
  • The Moon must be near perigee, the point of its closest approach to Earth in its orbit.
  • The Sun and Moon are near a lunar node.

The apparent path of the Sun across the celestial sphere is called the ecliptic. This is nothing more than a trace of Earth’s orbit around the Sun. The Moon’s orbit around Earth is tilted 5° to the ecliptic. Lunar nodes are the two points where the Moon’s orbit and the ecliptic intersect. The Moon crosses one node in its movement southward and the second node two weeks later as it moves northward.

The reason why solar eclipses do not occur at every New Moon is because the Moon and Sun are not always near the same node at every New Moon. In fact, the Moon and Sun being near the lunar node only occurs 2 – 5 times a year. If the two bodies are near the same node (at New Moon), a solar eclipse occurs, and if the Moon is near the other node (at Full Moon) a lunar eclipse occurs. These times are known as eclipse seasons.

An observer must be in Moon’s umbra to see a total solar eclipse. The umbra is the central and darkest part of an object’s shadow. It also has the narrowest area and the shortest reach. It completely obscures the light source (in this case, the Sun’s bright photosphere). Totality is relatively short lived because of the narrow path of the umbra sweeping the Earth, travelling eastward at 1700 kilometres per hour (1056 miles per hour) along the planet’s surface. Due to the umbra’s movement, totality can be as short as a couple of seconds, while the longest it can be is 7 minute and 32 seconds. On 30 June 1973, passengers aboard a Concorde were able to observe totality for an incredible 74 minutes by flying along the path of the Moon’s umbra.

The length of totality is decreasing with each passing millennium. The last time totality stretched over 7 minutes was in the 20th century on 20 June 1955, lasting 7 minutes and 8 seconds. The longest total solar eclipse of the 21st century was on 22 July 2009, lasting 6 minutes and 34 seconds. Looking at an 11 000 year period between 3000 BC and 8000 AD, the longest total solar eclipse will occur 16 July 2186, with a grand total of 7 minutes and 29 seconds.

Considering the very specific conditions and timing required for solar eclipses to occur in the first place, it is not hard to see why they are rare. On average, a total solar eclipse happens only once every 18 months on Earth. It takes 360 – 410 years for a total solar eclipse to re-occur in the same location.

The Stages of a Total Eclipse

Stages of a Solar EclipseThe first contact the Moon’s shadow has with Earth marks the beginning of the eclipse, and appears as a partial eclipse. Viewed from Earth, the New Moon’s silhouette becomes visible as it starts to obscure the Sun’s photosphere. The Sun looks like a chunk of it has been bitten off at this stage of the eclipse. The partial eclipse ‘grows’, covering an increasingly greater area of the Sun’s disk.

Totality begins when the photosphere is completely blocked by the Moon, leaving only the Sun’s corona visible. It is remarkable to see as the observer’s region is plummeted in darkness comparable to night, paving the way for unique response form nature. The sky is dark enough to see bright stars and planets during the day, the temperature may fall slightly, and birds and other diurnal animals may fall silent as if it were night.

The Moon begins to move away from the Sun as totality ends. The eclipse once again becomes partial until the photosphere is no longer blocked off by any part of its shadow.

Spectacular Sights

There are sky phenomena only visible before, during, and again after a total solar eclipse:

  • Shadow bands: These thin and elusive bands are ripples of alternate light and dark viewed on the ground and along walls about a minute before totality. They are caused by the Earth refracting the last rays of the Sun’s light before the star is blocked off by the Moon.
  • Bailey’s beads/ Diamond ring: The Moon’s topography includes mountains and valleys. Light from the Sun’s photosphere may peak out from valleys that mark the edge of the Moon’s profile, causing tiny beads of light to seep through along the dark disk of the Moon. These are called Bailey’s beads, and it is how the diamond ring effect is produced before some solar eclipses reach totality. The diamond ring effect is not visible before all total solar ellipses as it depends on the exact orientation and motion of the Moon. The effect is seen 15 – 5 seconds before totality.
  • Corona: The diamond ring – if any – will rapidly vanish and totality will begin. The faint corona becomes visible at this point and looks like wispy rays of light emanating from the around the Moon’s black disk.
  • Chromosphere The Sun’s chromosphere is a faint outer layer just above the photosphere. It is visible at totality as a thin layer of bright gas, often marked by eruptions on the solar surface called prominences. Prominences glow with an elusive pinkish colour because the temperature of the gas is so high. Such eruptions are about 3.5 time the diameter of the Earth.

It is highly important to note that it is only safe to view a solar eclipse with our naked eyes for the duration of totality as only 0.001% of the photosphere is visible. Protective eyewear must be worn for the rest of the eclipse. The Sun’s UV radiation is able to burn the eyes’ retinae causing permanent damage and even blindness. Always wear reliable eclipse glasses, or indirectly (but safely) view the eclipse using a pinhole projector.

Annular Solar Eclipse

Annular eclipses are similar to solar eclipses in that all the same conditions must be met save one. The Moon must still be new, and the Sun and Moon both have to be near the same lunar node. The difference here is that the Moon is at apogee, further away from the Earth in its orbit. The increased distance between the two bodies leads to the umbra unable to extend its reach to the surface of the Earth.

The antumbra is an extension of the umbra, beginning where the umbra ends. It is lighter than the umbra. The umbral shadow doesn’t reach the observer, but the antumbral shadow does. The result is an annular solar eclipse where the Sun appears as a bright ring (annulus) around the dark silhouette of the Moon.

Annular eclipses also begin as partial eclipses before reaching annularity (the full eclipse). Observers may be see a diamond ring effect. The full eclipse follows, and the ring of fire that characterizes annular eclipses appears. The eclipse ends as the Moon moves away from the apparent path of the Sun.

There is no safe time during an annular eclipse to be able to view the Sun without protective eyewear. Too much of the photosphere remains visible during annularity – enough to permanently damage vision.

Partial Solar Eclipse

Partial solar eclipse are seen depending on your time and location during a total or annular eclipse. You may be in a location where the eclipse is visible in its entirety – starting and ending as a partial eclipse; or you may completely fall outside the umbra and antumbra, but are situated within the penumbra.

The penumbra is the lightest and uttermost part of the Moon’s shadow. It covers the greatest area and has the longest reach. A more technical definition concludes that the penumbra obscures all or some of the light, because an observer in the umbra automatically falls under the penumbra too (the umbra is a subset of the penumbra). Being outside of the umbra and antumbra but still within the penumbra means you will only see a slight section of the Sun obscured by the Moon.

Partial eclipse are the most commonly seen solar eclipses. This makes sense as the penumbra covers a far greater area than any other part of the Moon’s shadow. They are also more common nearer the poles.

Hybrid Solar Eclipse

Hybrid eclipses are the rarest type of solar eclipses. We have seen that you need to be in the umbra when the Moon is at perigee and the antumbra at apogee to see total and annular eclipses respectively. The maximum point of the solar eclipse (the halfway point of the eclipse) is either a total eclipse with a magnitude equal to or greater than 1, or an annular eclipse with a magnitude less than 1 accordingly (the diameter of the umbra decreases as when the observer is further away from the Moon, creating an annular eclipse).

Hybrid eclipses are also named annular-total eclipses because the what is seen at the maximum point changes depending on the observer’s location. One observer will see the maximum eclipse as a total eclipse while a separate observer in a different location views the maximum point as an annular eclipse.

The effect is only possible because of the curvature of the Earth. The Moon is at its closest point to an observer when it is directly overhead (the zenith). The curvature of the Earth bends away from the Moon as an observer moves from the zenith towards the horizons – increasing the distance between the Moon and that observer. The umbra of the Moon only extends so far before it becomes the antumbra. This is how one observer at the zenith may see a total eclipse in one location while his friend views the eclipse as annular from a further location.

Other Occultations

A separate category of solar eclipses exists for occultations that do not involve the Moon. Two such examples include when the crew of Apollo 12 witnessed the Earth eclipsing the Sun, and when the Cassini space prober caught Saturn eclipsing our home star.

Predicting Eclipses

Predicting eclipses comes down to understanding the conditions of an eclipse and by seeing the patterns of these natural events.

Astronomers know that eclipses can only occur when:

  • The Sun is crossing a lunar node.
  • The Moon is near the same node (for a solar eclipse to occur at New Moon) or near the other node (for a lunar eclipse to take place at Full Moon). An eclipse season is marked by these two conditions being satisfied. Tracking the lunar nodes is essential to knowing when eclipse seasons will happen.

Solar eclipse seasons are 32 days long whereas lunar eclipse seasons are shorter at 22 days long. Any New Moon within 16 days of the Sun crossing a lunar node will result in a solar eclipse, and any Full Moon within 11 days of the Sun crossing a lunar node will produce a lunar eclipse.

The lines of the nodes rotate slowly in space, slipping 19.4° westward every year. This mean that the eclipse season beings 19 days earlier each year. This effect of precession results in it taking approximately 18 years, 11 days and 8 hours for the node to be in its same starting place (with regard to the background of the stars). The entire eclipse pattern then repeats. This is called the saros cycle, and it is the equivalent of 223 lunar months.

The eclipse pattern will repeat, but the location it is observed from on Earth will have changed slightly, as the saros cycle is one-third of a day longer than 1 years and 11 days and therefore the Earth will have moved 1/3 further east.

Conclusion

A solar eclipse is a unique and powerful event to witness. They are also the perfect reason to get adventurous with amateur astronomy as you can travel the globe hunting solar eclipses. See our next article for a complete guide on the best solar telescopes and filters.

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