SOLAR and LUNAR ECLIPSES : Part 1

INTRODUCTION

All observed eclipses are caused by shadows. The Sun emits light just like any luminous source, and therefore any solid object like a planet or moon, is illuminated on one half of its ball (or sphere) and is darkened on its other half. Furthermore, the side away from the Sun also produces a long cone-like shaft of darkness or shadow, which projects at exactly 180^o from the source of light. If the planet or moon intersects one of these extended shadows, this produces an eclipse. If the Sun is obscured by the Moon, for example, this creates the various types Solar Eclipses. Conversely, when ' the Moon passes behind the long projected shadow of the Earth, this makes the various types of Lunar Eclipses.

SOLAR ECLIPSES

One of the very first things that are truly remarkably about all solar eclipses is that the roughly ½^o sized Moon and Sun happens to be about the same apparent diameter from the Earth. This planet-moon combination does not happen anywhere else in our Solar System.

Solar Eclipses are indeed rare, spectacular and awe-inspiring events, which occur in many exotic but predictable places and locations throughout the world. They often produce varied, interesting and exciting phenomena, which can direct affect wildlife, the people that observe them, and the overall appearance of the surrounding landscape. Birds, for example, will think the night has come again, and go back to roost in their nests. At mid-eclipse, for a short time, day can become like the night or even the brightening or failing twilight, where the bright stars and planets appear in the sky, but this time during the daytime.

Many superstitions once surrounded solar eclipses, and seeing one often spread fear and terror amongst many people in ancient and even some modern cultures. For example, the Chinese, once viewed solar eclipses as an immense dragon who was consuming the Sun. They once believed by making enough noise during the eclipse then the terrible dragon would be frightened away. Of course, it always did! Other cultures just hid away and shut themselves inside their own houses just before the mid-eclipse, and do not go outside again until the event is finally over. After this, they will rejoice in praise, singing in unison that the life-giving sunlight has finally returned and allow life on Earth to continue.

The Types of Solar Eclipses

There are four types of solar eclipses;

Total, Annular, Partial and Hybrid.

Total Solar Eclipses can only occur at the exact time of New Moon. All solar eclipses happen to be more rare than lunar eclipses, mainly as the apparent size of the Sun is lesser than the size of the Earth’s shadow in which the Moon becomes immersed.

Duration for any total eclipses is dependent on where the Moon happens to lie in its orbit and, at the time of the eclipse, its true distance from the Earth. It is also, and to a lesser extent, is dependant on the place on Earth, and the true distance between the Earth and Sun.

Considering the apparent observed size of the Moon alone, these small differences can only happen because the lunar orbit is not actually circular but is an ellipse, which moderately varies the lunar distance from the Earth between the Moon’s apogee (furthest away) and perigee (closest to us) - a variation of about 43,800 kilometres.

[Moon mean distance is 399,100 km, and this means that the observed apparent size or diameter of the Moon from the Earth can vary by as much as 11%.]

(a.) Total Solar Eclipses

Near perigee, when the moon is closer to the Earth, the Moon’s apparent diameter is larger than the Sun, so the Sun is completely obscured. Here the observer on the Earth standing where the eclipse path intersects the location, means we can observe a total solar eclipse. When both perigee and the New Moon coincide, the maximum duration of eclipse totality may last as long as seven minutes and forty seconds. Most eclipses last about two minutes or so.

(b.) Annular Solar Eclipses

Near the time of apogee, the opposite is true. Here the apparent lunar diameter is slightly smaller than the apparent diameter of the Sun. Therefore, in the middle of this kind of solar eclipse, the Sun appears like an annulus ring of bright sunlight. This produces the Annular Eclipse, whose duration may last as long as thirteen-and-a-half minutes.

(c.) Partial Solar Eclipses

A third variety is the Partial Eclipse, which can occur for observers anywhere outside the path of totality, or when the sun disk is only partial obscured anywhere on the Earth. Most solar eclipses are only partial ones, and the reason for this because not only is the lunar orbit elliptical, but lunar orbit is also titled by about 5½^o to the plane that all the planets and the Sun follow - the ecliptic. This simply explains why at every successive New Moon or Full Moon, there is not some sort of observed eclipse. As said in introduction, the projected shadow can intersection with the position of the Earth or Moon, but it can also pass either above or below it. So, sometimes the Moon might passing just above or below the Earth’s orbit. This produces total solar eclipses in space - missing the Earth altogether. Sometimes this just produces partial eclipses place somewhere upon the Earth’s surface. I.e. Near the north or south poles, for example.

(d.) Hybrid Solar Eclipses

Hybrid eclipses are those between Annular and Total Solar Eclipses, and are much rarer. As the path of the eclipse across the Earth’s surface is on a projected sphere, this cause the size of the Moon to vary in size by half the diameter of the Earth or about 6,300 kilometres - being roughly 6300/363600 km. or 1.7%. This means if an eclipse is bordering between total and annular, that it is possible that some part of the eclipse, near mid-eclipse, can be briefly total, while at it beginning and end may appear annular.

Hybrid eclipses do show some of the typical phenomena of the total eclipse, such as Baily’s beads or even the chromosphere, but do not show well the inner or corona of the Sun. Their brevity make them unpopular with observers as the location of totality is within a small region of the Earth and totality does not last very long - especially in gaining images.

TOTAL SOLAR ECLIPSES

The Path of Totality

Any total solar eclipse is really a very unique circumstance of planetary alignments. At any instant, from space each total solar eclipse appears like a round spot projected on the surface of the Earth, surrounded by a larger circular grey area. The observed central spot is the place where the Sun is seen in total eclipse on the Earth, while the grey area is in partial eclipse on the Earth.

Now as the Sun, Moon and all the planets are in constant motion, therefore the location seeing totality is also on the move, making its path move across the surface of the Earth. This is known as the path of totality. Such a path is fairly narrow, averaging about 127 kilometres across or about 1^o of latitude, and this width moves along the path of the eclipse. This dark lunar shadow travels at velocities around 3,200 kilometres per hour - or four times the speed of sound. As the rounded Earth is spherical, then nearer the poles the maximum path width can extend to about 325 kilometres across. An entire eclipse length for the whole eclipse can cover perhaps between 8,000 to 10,000 kilometres across the Earth’s surface, though is some instances, nearer the poles this distance can be much shorter.

Frequency of Total Solar Eclipses

Occurrences of any total solar eclipse averages once every 2½ years or so. For some total solar eclipses to take place at the same location on land may take place on average once every 324 years in the northern hemisphere and 640 years in the southern hemisphere. This is because land mostly happens to lie in the northern hemisphere, while in the southern hemisphere, much of this area is just ocean. In truth, the average time for any observable solar eclipse to reoccur somewhere on the Earth is about 482 years. Sometimes two solar eclipses may occur within several years in the one location, then a third total solar eclipse will not again happen for more than one thousand years later!

Appearance of the Sun
During Total Solar Eclipses

During any total solar eclipse, we can actually see the outer atmosphere layers of the Sun. The Sun itself does not have a hard surface like the Earth, but is really a bubbling cauldron of hot gases around 6,000^oC, called the photosphere - the “surface” seen in all images of the Sun. Like the Earth, the Sun has a lower denser atmosphere called the chromosphere, which is visible just before and after totality during total solar eclipses. This hot region is about 4,000^oC appears against the lunar limb as bright pink colour and is about is between ten and twenty thousand kilometres in thickness and is mainly made of hydrogen.

Beyond the chromosphere is the corona, whose true maximum high is uncertain, but can be seen as far out as 1½ times to 5 times the diameter of the Sun. The corona is a very hot gas made mostly of hydrogen and helium, and exists as a tenuous gas that exceeds several million degrees. This can be seen either by the naked-eye during totality, or in using a specially designed telescope called the coronascope. The corona appears as pearly white and is about as bright as the Full Moon, and looks like many radiating rays of light point directly away from the Sun, and is distributed along various places around the solar limb, but always tends to avoid the Sun’s poles. Visually the corona may extend between half and twice the diameter of the Sun, but this does vary significantly from eclipse to eclipse.

Beginning and End of Total Solar Eclipses

Near the central line of the eclipse path will see only the two important Bailey’s Beads, which occur when the total solar eclipse starts and finishes. This produces the renown and spectacular Diamond Ring, when the sunlight does final disappears before totality, and as the eclipse ends. Both of these contact points will are exactly 180^o apart.

Bailey’s Beads

During both annular and total eclipses, some observers can make observations of so-called Bailey’s Beads. These are bright beads of light seen prior and after totality, and are formed by Sun shining through the many valleys and craters seen on the uneven Moon’s edge or limb. Timing these beads can give important information on the true solar diameter, which until recently times could not be measured directly. Astronomers believe that the Sun maybe gradually changing in physical size and this might produce changes in the solar energy emissions. This in turn may influence the Earth’s climate, and might be the reason for climate change.

Upcoming Solar Eclipses

The following tables gives a list of solar eclipses visible throughout the world. These tables give the following information;

• Year and Date of the Eclipse
• Eclipse Type, being; A = Annular, T = Total, P = Partial and H = Hybrid.
• Eclipse magnitude (Eclip. Mag.) is the ratio difference in areas between the Moon and Sun. Total eclipses have values >1.0, Annular eclipses >1.0. For partial eclipses, this is the largest percentage possible of the visible disk of the Sun during the eclipse. Hybrids are generally are close to 0.0.
• Maximum Duration of the Eclipse (Central Durat.)
• Geographical Region of Eclipse Visibility

To make these tables more useable, I have coloured some of the squares - namely the eclipse types. The lines coloured lime green are eclipses visible somewhere in Australia, which is again summarised in the final Table 1.4.

Table 1.1

Solar Eclipses
2008 - 2010

Table 1.2

Solar Eclipses
2010 - 2020

Table 1.3

Solar Eclipses
2020 - 2030

Table 1.4

Upcoming Solar Eclipses Near Australia
2008 - 2030

I M P O R T A N T

When Observing the Sun

The Sun is the brightest object in the sky, and produces so much light and heat that it can be easily felt on the skin. However, under no circumstances should the Sun be observed directly with the eyes and NEVER directly with telescopes or binoculars. You can only safely look at total solar eclipses but only when the Moon completely obscures Sun. You must look away as the diamond begins or ends. Older children should always be prewarned beforehand and be made to listen to responsible adults when they should look away. Check they do so. Younger children are always better kept inside the house if they do not understand what is happening.

!!! W A R N I N G !!!

Using any telescope, the Sun should ONLY be observed by projecting the image onto a white screen or card. (Even this should be for short periods)
Direct viewing of the Sun, by either eye or any other optical equipment, is VERY DANGEROUS without proper eye protection. Otherwise, TOTAL BLINDNESS WILL RESULT, and even glancing will blind you in less than a ten-thousandth of a second.
If your telescope has something called a SUN FILTER - NEVER USE IT !! If this filter were to crack while you are observing the Sun, INSTANT blindness is the only possible outcome.

!!! W A R N I N G !!!

(NOTE : PLEASE READ THE DISCLAIMER BELOW.)

Important Disclaimer

The user applying this data for any purpose forgoes any liability against the author. None of the information should be used for either legal or medical purposes. Although the data is accurate as possible some errors might be present. The onus of its use is place solely with the user. Those not heading the given important warnings on this page do so at their own risk.

Last Update : 09th August 2008

Southern Astronomical Delights
© (2008)

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