GENERAL ASTRONOMY ARTICLES

ECLIPSING BOUNDARY RIDERS

The TRUE WONDER of
the SOLAR ECLIPSE

One of the most wonderful of all the astronomical experiences that an amateur can behold is beyond any doubt is the true magic and of the Total Solar Eclipse. Not only are these are astounding coincidence of geometry, they are also simply beautiful. Several features can be viewed during totality. Most prominent perhaps is the sight of the jet ink-black lunar disk - a depth of black I’ve never seen before. I once looked at the dead heart of the moon during totality and became so mesmerised with the lack of anything. There is no question our ancestors were so trembled with dread and fear when they occurred. Various cultures truly believing the Sun was being swallowed whole whole a portant that the end of the world was at hand. Eclipses of the Sun are very rare events at any particular location on the Earth’s surface. It is calculated that on average that something like 324 years must pass before anothr solar eclipse is seen in the same place on Earth - so it even rarer for two eclipses to be seen near the same place during one’s lifetime.

One of the important questions an observer has to ask is where he or she in positioned within the eclipse path. The unusual immediate reply is of course the centre line, but unfortunately if you did, you would be dead set wrong! Often I have said this to many people and they look at me as if I were totally mad to say this, but once they think about, the more sensible this advise becomes.

AT the CENTRE LINE

The ‘pro-’arguments about the centre line are fairly obvious as it gives you the longest duration of totality. Certainly if you are going for the life-long record for amount of totality in sequential eclipses then this is the place to be. However at the centre line the amount of phenomena visible is much less. Observers will just see the moon covering the solar disk until it reaches second contact, you see the last of the sun’s light disappear in a single flash known as the ‘Diamond Ring’. The diamond is often a small valley on the moon, where the lights gathers between the peaks. This means the light becomes instead of an even arc becomes one small bead of light, and hence the given name of Bailey’s Beads after the person who first notice them. A rough mimic of this is at the last moments of sunset, and you can roughly simulate this by placing a rough surface on a fence and observe through a telescope the projected image as the shadow slowly occultates the artificial disk. Similarly, at third contact, the sun comes back, and we see the Second Diamond Ring. Often you will know the duration of the eclipse - a predicable event, becomes lost during the excitement of totality, but you can visually see when it is about to end. Of course you must be careful at the end because the full stream of sunlight will not only damage your night-vision but you could also lose all of your vision as well! Just before third contact the light on the edge of the sun becomes brighter, and in some instances appears slightly pinkish because of the visibility of the chromosphere. Then all of a sudden appear as a small sliver of light at the side of the moon. Then within half-a-second it is just to bright to look at and you must quickly look away. This is a moment of great disappointment as the eclipse is finally over and sunlight reappears. This disappointment is real, as your preparations, the in organizing your equipment, the weather, getting to the site in the eclipse part and "psyched up"; come to an end. One of the most surprising things is that the final phase after the eclipse are just an anti-climax, and things like worrying if the film will develop occupies your mind. If you are on the centre line this is the end of the eclipse.

Before and after totality, the only safe way to view the eclipse is by projection. You can also uses a solar filter covering the aperture of the telescope, but I have found that unless it is a small telescope you can waste precious seconds trying to remove and then replace the solar filter. If you do want to do this, then the best way is to use your solar filter during up till about five minutes before totality then use projection the rest of the way. Of course once the sun has been completely obscured you can look through the eyepiece yourself. It is completely safe to do so as no sunlight is reaching the eyes, and you can almost imagine the sunlight is radiating sideways from the Sun. Here you can see the inner corona and scattered around the disk are the prominences of the sun. The amount of disk phenomena can vary very much from eclipse to eclipse, depending on the solar activity and happenstance. It is worthwhile doing a quick search of the Web a few hours before to see what sort of prominence activity is occurring. Another is to have your own solar H-alpha filter - something like was seen with Harry Robert’s filter at the 10th SPSP - but this will set you back just as much as getting to and from the eclipse site!

During totality you will also see the co rona, whose brightness is close to the brightness of the lunar surface at night. The corona is the outer atmosphere of the sun and to the naked eye extends between two or four solar radii depending on the solar activity. Telescopically the corona looks, more often than not, like a multitude of thin strands of light stretching away from the centre of the lunar disk. Unfortunately this phenomena is seeing dependent. If the seeing is quite bad, more often than not it is worst in afternoon the total eclipses than morning ones. Also the light may appear more like a pearly sheet of white light showing little detail. On the odd occasions, like the morning eclipse in central Java in 1984, the seeing was that good that the coronal ray structure broken into little beads along a very long string. Each bead was roughly about 1.2" to 1.6"arcsec per piece. The advantage where I observed it was the transparence of the sky up in the mountains and the near cloudless sky, where the only cloud was from the active volcano of Mt. Slumat 15km towards the west which was also puffing out smoke are regular intervals! This 1984 eclipse was simply amazing because we had 80% cloud cover some ten minutes before totality, this evaporated in under one minute, and did not return until ten minutes afer totality. In the tropics such phenomena is not unusual. This was also seen in the 1962 Solar Eclipse in Papua / New Guinea. Just ask the ASNSWI’s Ted Lumley!

Another excitement are the red prominences, and it certainly is fascinating to see them against the black lunar disk. If yu are on the centre line you have enough time to go around the disk to look at each on of them. Yet to see them is by fortune than good planning. Most eclipse observers dream of a huge loop prominence during totality but the probability of this happening is very small. Sometimes totality might match a quite period of the sun and no prominences are visible at all and all you see is the corona. The best way is to check the internet a few hours before totality and you can get an idea of solar activity.

Other than this you will notice the phenomena surrounding you. The wildlife thinks it is again night and any animals around you will become agitated. Even your own body and mind experiences something that I can only describe as electric. It feels like solemnity mixed with a surge of excitement and crossed with a strange kind of euphoria which you don’t come down, if you do see the eclipse, for several hours later. This often is amplified by the onset of darkness, the sharp drop in temperature and the excitement of those around you. If suggest to maximise the experience your first total eclipse should be done as far away as possible from everyone else. If you do share it with someone make sure they are someone close to you because it is so memorable. At the centre line often this is impossible as every man and his dog are placed here. Outside of the centre line you can almost feel lonely.

An observation were surrounding sky that first seems a little strange as its becomes much duller and something akin to how the sky appears about twenty to thirty minutes after sunset or before sunrise. To me the sky seems just a little bluer. At mid-eclipse the bright stars and planets become visible, but more often than not you don't realise until after the eclipse has finished.

The strongest emotional feelings of all that I had was the 18th March 1988 eclipse from Bangka Island (Pulau Bangka) just of the northern coast of Sumatra. I felt so euphoric after the event I drank three double Drambuie's in a row back at the camp to celebrate - and that was for breakfast. I tried sleeping as a a siesta for a few hours but I still had troubles sleeping! Some of my companions got so excited, that rumour had it they stripped of and went skinny-dipping in the ocean after the event!

AWAY FROM the CENTRE LINE

Reading the text above you are probably wondering how can anything be even more exciting, but it certainly can. Not only do you get all the phenomena above (yes there’s more), but you get a little more phenomena before and after totality. Firstly let discuss what happens to the amount of totality you loose.
On the surface of the earth an average eclipse path cuts across a swath about 120-160 kilometres across, but this is dependant on where the eclipse is on the earth. For example, eclipses nearer the poles might be as wide as 800 kilometres. Those nearer the morning and evening parts of the path sometimes are also narrower and are shorter in totality's duration. The general geometry of the eclipse is fairly simple but importantly the duration is dependant on the distance of the moon, and to a much lesser extent on the distance of the sun. The longest total eclipses, when the moon is at perigee, and totality may last as long as 7½ minutes. These are rare with the vast majority lasting about 3 minutes. On other end of the scale, when the moon is at apogee, the apparent lunar disk is smaller than the apparent solar disk. This circumstance produces the annular solar eclipses whose longest duration can be about 13½ minutes.
At first you might think that as you move away from the centre line the duration quickly reduces. Based on the geometry this can be proved to be incorrect, as the duration is determined on the equation duration is sqrt (1-x²), where x is between -1 and 1 - calculating the percentage duration. If x=0, this is the centre line. If x=1, then this is the edge of totality. Graphically the curve is shaped Figure 1 and I have also given Table 1 showing the results for the 2002 Eclipse from Ceduna and just inside the NSW northwestern corner. Look at Figure 1 it is obvious that little difference is observed between the centre line and 50%, and only near 0.85 does the length diminish by one-third the length of the time of totality at the centre line.

Clearly the sacrifice of time has to be balanced against the increased chance to see the phenomena like the Bailey’s Beads or the chromosphere.

NEAR the CENTRE LINE

My personal experience is based on being 200m of the centre line of the eclipse of the 23 October 1976 from Mt Delegate on the Victorian / New South Wales border. This lateafternoon eclipse lasted over five minutes - my first eclipse that got me really hooked on eclipses. I was very very lucky with this eclipse because I saw the entire solar eclipse is the only hole that was in the sky at the time! In fact, most people know of this hole, because the majority of eclipse observers were on a property out site of the southern NSW own of Bombala, and they saw he hole even though they all were clouded out. The failure of all these amateurs taught me two important lessons - one, NEVER EVER PLACE ALL YOU OBSERVERS IN THE ONE PLACE, and two, NEVER EVER LEAVE NO OPPORTUNITY NOT TO BE ABLE TO MOVE either to chase holes or possible clear sky. (I hear they had a "wake" after the eclipse, and I didn't dare go in the property afterwards for the fear of getting lynched by a few thousand amateurs.) This eclipse was quite brilliant and the corona was particularly nice. However wonderful second and third contact was, it was almost boring in the last minute. There were few prominences and the seeing was moderately poor. I also didn't see any Bailey Beads and the chromosphere appeared for a few moments around the contact points. This is where I leaned the real third lesson, NEVER EVER GO TO THE CENTRE LINE. The Fourth Lesson is never listen to local advise on the WEATHER OR LOCAL CONDITIONS BECAUSE THE RULES DO NOT APPLY DURING ECLIPSES. Those who gather in the one place in Bombala, rumoured to have been to get away from the general public, was deemed to be the best place in the area.

NEAR the ECLIPSE LIMITS

Now let me describe the opposite scenario to the centre line. My shortest solar eclipse was the 8 second one in Papua / New Guinea eclipse that occurred on the morning of 23rd November 1984 (22nd in UT). Some 100 kilometres east of Port Moresby when I placed myself merely 175 metres from the actual northern limit. This particular eclipse passed though the East Indies, Papua/ New Guinea, the South Pacific, just north of New Zealand to ending in Antarctica. This solar eclipse began at 7pm in the morning at the beginning of the path and this is similar to the 4th December eclipse when it appearing in southern Africa. In 1984 I was very lucky not to miss totality altogether. I probably still have the record of being the closest person ever to limit of an eclipse who had deliberately placed themselves there! (I know one else who has attempted this stunt, and in terms of he cos of the eclipse it cost me about $130 per second of totality!) So close was I to the limit that I even saw sunlight on the nearby hill just to the north of me! This was one of the most exciting an invigorating astronomical events I have ever experienced. At its end it left me totally exhausted and physical shaking in total disbelief!
I was so very fortunate to measure and visually see some 104 beads, before and after totality, racing twice around 290^o solar disk in about five or six minutes before and after totality. It was the sheer acceleration of the beads in the twenty seconds before and after totality that suprised me. If you listen to me on the audio tape in a height of excitement it still sounds more like I’m describing the running of annual horse race of the Melbourne Cup at twice its normal speed. Beads really came thick and fast! The previous eclipses put me in good stead for doing the most amount of observing in the least amount of time. In less than ten seconds, I did the following;

1) Observing the last of the first round of Bailey’s beads by eyepiece projection.
2) Looking through the telescope at the beginning of totality
3) Take a quick moment to look my surrounds and see sunlight on the northern hill.
4) View the eclipse with my naked eye.
5) Turn again to the eyepiece of the telescope to look at mid-eclipse.
6) Take six snaps of the eclipse through the photographic telescope (cocking the shutter six times mind you)
7) Looking through the telescope so I could see the end of the eclipse a the appearance of the double diamond ring.
8) Go back to projection and time Bailey's beads again.

The most specular thing was seeing, second, and then third contact on the southern side of the disk right up near the poles. Third contact producing the double-diamond ring. Best of all, was my first published photograph showing the chromosphere covering a fair portion of the disk. It was so bright that I saw the chromosphere right to the edge of the photosphere!

Note: This December 2002 eclipse has some relevance for me as it is the next eclipse in the Saros cycle No 142, which are separated by 18 years and 10 days 08 hours (6583.32 days or almost exactly 233 lunations). However at central eclipse it is four seconds longer than the previous one, and is at the end and not the beginning of the eclipse!

MY OTHER ECLIPSEs

All my other total eclipses have been out some 50% to 85% of the eclipse path's width from the centre line.Further discussions about my previous eclipses appear as a seperate page. See My Eclipse Expeditions

TOTAL SOLAR ECLIPSE : 4TH DECEMBER 2002

The following is an example calculation of how long the totality will be as you travel out from the centre line, and how far this is from the centre line. This example is the forthcoming total solar eclipse in South Australia on the 04th December 2002.
If you are uncertain where to go after reading this article, then you should try the “Were to Be” Quiz, which so help guide you to achieve the eclipse observations you want see in the future. Calculations of other eclipse path widths can be calculated as discussed earlier in this page.

C O N C L U S I O N

If you are a corona or prominence freak (I'm not) or totality addict then please go to the centre line.
If you are a chromosphere freak (I am) or Bailey' Beads freak (I am), and love fast packed action, then get as far away from the centre line as far as you dare.

Hope Your Next Eclipse Gets Clear Skies!

References

1."Universe", 35, 8 pg.8-10 Aug (1988)

Last Update : 29th October 2005

Southern Astronomical Delights © (2005)

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