Derby and District Astronomical Society

Aries
The Journal of the Derby and District Astronomical Society
January - April 2006
[Contents]

Solar Eclipses - worth a visit?
By Malcolm Neal

What do we mean by a solar eclipse?

Partial phase of 2005 annular eclipse
The partial phase of the 3rd October 2005 annular eclipse taken from Madrid.
An eclipse may be partial, annular or total depending upon the particular event and where you are viewing it from. A partial eclipse is simply one where the moon is not covering the entire solar disk and happens at every eclipse where the observer is not on the centre line of the eclipse. They are mildly interesting to watch but are not very spectacular. The reason for this is that we see nothing interesting, no corona, no prominences or anything else for that matter. Why? Well because even a tiny portion of uncovered sun, even less than 1% is still so bright to the eye that all other much fainter effects are hidden in the glare. As can be seen from the image at right though there is about 60% of the solar disk covered we can see no spectacular effects!

A total eclipse means that you are on the narrow mid line of the eclipse path. The width of the path varies, as does the length of the total eclipse phase according to the relative earth-moon and earth-sun distances. If the earth moon distance is short i.e. the moon is closer than normal and the earth sun distance is greater i.e. the sun is farther away than normal then the total phase and the total length of the whole eclipse will be longer. The reverse is also true. Totality in the 1999 eclipse was a maximum of 2 minutes and 20 seconds though they can be for only a few seconds or up to about 8 minutes if the geometry of the eclipse is ideal. This does not happen very often.


The 2003 annular eclipse
Image of an annular eclipse in 2003 near Iceland.
What then is an annular eclipse? Here when the eclipse would normally be total we see a fine ring of the sun around the blackness of the moon. Why? Because the moon is at its extreme distance from the earth and the sun is closer, hence the moon appears smaller and the sun larger so the former does NOT cover the latter completely. Annular eclipses are rather more rare than total eclipses but far less spectacular from a simple observational point of view.

At any place on the earth’s surface a total eclipse is quite rare. Because the path is only a couple of hundred miles wide and the path may be a couple of thousand miles long but much may be over oceans, only a few inhabited places are touched by the path of totality. Also because of celestial mechanics the path of each eclipse covers different parts of the earth’s surface. England for instance will not have another total eclipse until 2090, a very long wait for those of us over 50 years old!

Celestial mechanics, the relative movements of the Earth, moon and Sun all combine so that very 18 years, and 10 or 11 and a third days the cycle of eclipses repeats (whether it's 10 or 11 days depends on how many of the 18 years are leap years). Amazingly enough, this period was actually discovered 2,500 years ago, by Babylonian astronomers It is called the Saros, meaning "repetition". A Saros cycle is, then, a series in which similar eclipses happen every 18 years 10/11 and a third days. But of course, eclipses happen more often than that. The explanation is that there are many combinations of circumstances that can produce an eclipse. So, there are, at any one time, 42 Saros cycles running at once! This results in a bit more than 2 eclipses per year though not all are total. There are between 54 and 71 total eclipses per century according to the NASA 6 millennium eclipse catalogue. Partial and annular eclipses occur slightly more often, up to 95 per century and hybrid eclipses that are annular and total along different segments of the path are very rare (only up to 26 per century).


Where do they occur?

They can occur almost anywhere on the planet's surface but few places have two total eclipses in a human lifetime. The map below shows eclipses from 2001 to 2025 and shows only two places on land that have two eclipse paths over them.

Eclipse tracks
Eclipse tracks from 2001 to 2025.


How do you view them?

The answer is carefully. Even a 99% covered solar surface is still blindingly bright - LITERALLY. Projection is by far the safest method, using a telescope or half a binocular with a pinhole cover to let the image through. This is then projected onto a still white card held at a fixed distance and attached to the telescope so they all move together. At totality you can look straight at the hidden sun with the naked eye, through a telephoto camera or even through a telescope, BUT be very careful as time passes in a total eclipse very quickly and you do NOT want to be looking through any lens of any kind when totality stops. A preset timer is a good idea that can be triggered as totality starts. It should emit a warning beep at least 3 seconds before the total phase stops to give you time to get the filters back into place.

Mylar filters in front of the telescope objective do a very good job BUT again they MUST be tested to ensure there are no pinholes or eye trouble will result. Stopping down the telescope is also a good idea. It means you use less mylar, there is less light entering the telescope and so a smaller heating effect on the other optical components will occur.

FILTERS in front of an eyepiece or even worse behind the eyepiece should not be used. They heat up very much and so can shatter with disastrous results.


What to take and where to go

Where to go depends on the eclipse calendar and the depth of one's pockets. For instance March 29th 2006 saw an eclipse crossing Africa, from Ghana to Libya then Turkey and some parts of Asia. Maximum eclipsed time was over 3 minutes in Turkey, 3 minutes 55 seconds at As-Sallum in Egypt, and 3 minute 58 seconds at Musaid in Libya. Of course these times depend on how close to the centre line you can get! For pictures and accounts of the Turkey eclipse by Mike Lancaster and friends click here. What to take usually revolves round how to record the event. I still feel a film camera is better than a digital one [although I disagree! - ML]. A long telephoto lens plus a doubler is good as there is no shortage of light. I used a 500 mm mirror lens and a 2x doubler giving me a 1000mm f16 or f22 lens. This I mounted on a normal tripod and pushed it round to match the movement of the earth. Weight is a problem especially if you are travelling by air. At the 1999 eclipse in Hungary a fellow watcher had taken his driven telescope mount, minus telescope plus a still and video camera. A very good set up but it did mean he had very few clothes because of the luggage weight limit!

Well I hope article has encouraged someone, hopefully, many people to view upcoming eclipses!

Malcolm Neal


For pictures of solar eclipses by DDAS members including the recent 2006 eclipse from Turkey click here.

For more information on past and future solar eclipses visit the NASA Eclipse Home Page.


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