Wednesday, 18 July 2012

Alan Fiala - Eclipse Chaser

I had the pleasure to meet Dr. Alan Fiala in the Soviet Union in 1981. He had joined our eclipse chasing group going to Bratsk, in Siberia. This was an unusual time for an American to be in the USSR and even stranger was this humped scientist from the US Naval Observatory. The customs and immigration people were not amused by his video equipment labeled property of the US Navy. His carefully prepared maps showing his desired set up locations were viewed with suspicion.

Setting up the video recording camera in 1981 in Tasmania for the Annular Solar Eclipse


Dr Alan Fiala attempted to verify the size of the Sun using Solar Eclipses. He was attempting to detect small changes in the solar diameter by timing eclipses and 'beads' from the North or South edge of the eclipse path.

During the eclipse in the USSR he attempted to deviate from the plans of our Soviet hosts. While he was not with our group where we set up, he did not make it to the edge of the path that time.

Alan Fiala's Eclipse Log
Obituary NASA ADS

Saturday, 14 July 2012

Explaining Eclipse Circumstances to the General Public

A recent question came up on the Solar Eclipse Mailing List, an email forum for eclipse enthusiasts. It has to do with relating the circumstances of a partial eclipse to the general public using the fewest terms possible. This is often required for press releases.

Solar Eclipse calculations produce a value called the eclipse magnitude. This is NOT the brightness of the eclipse. The eclipse magnitude is simply a fraction of how much of the Sun's diameter will be covered by the Moon. A magnitude of 0.5 means that the edge of the Moon is at the center of the Sun. The thing is, while magnitude is a simple concept, it requires explanation. As a result a more common term used is obscuration which refers to the area of the solar disk that is covered or obscured by the lunar disk.  If 50% of the solar disk is covered, the actual lunar edge is past the center of the Sun.

Eclipse Magnitude
As can be seen in the figure above, the edge of the Moon is half way across the Sun. Yet more than half of the solar disk is still visible. Thus the Sun is less than half obscured.

The obscuration is how much of the solar surface area is covered. It can be calculated directly from the Magnitude using trigonometry. An explanation for the calculation of obscuration given the magnitude is provided at http://www.jgiesen.de/eclipse/

It is important to remember that the angular size of the lunar and solar disks are slightly different. During an annular eclipse, the angular size of the Sun is larger. During a total solar eclipse the angular size of the Sun is smaller than that of the Moon.

As a general rule of thumb, when the eclipse magnitude is 0.6 (60% of the solar diameter covered) then just about half of the solar disk has been covered.

So which term makes more sense to use when describing eclipse circumstances to the general public?

Some prefer to tell how much area has been covered (obscuration) while others prefer the amount of the solar diameter covered (magnitude).

My personal favorite is to explain both if given the time. Yet the question is how to describe the eclipse circumstances for someplace using the fewest terms that will make sense to the reader. "XX% of the solar diameter is covered" uses few words than "XX% of the solar surface area is covered" but requires an understanding of the term diameter. I'd like to think that most readers of such a press release would know what the term diameter means.

Wednesday, 11 July 2012

40 years ago

It was 40 years ago yesterday that I saw my first total solar eclipse. Yet it does not seem to be that long ago. I can clearly remember and visualize the corona and diamond ring as seen from the decks of the Olympia in the North Atlantic. For that eclipse I had brought along a small telescope (f/10 4.25 Edmund reflector) and a camera with a zoom lens. Even though totality was just a few minutes I managed to finish a roll of film during mid-totality. That allowed me to spend the rest of the time just watching and well, it was quite a view. This is why I strongly recommend not getting lost in the camera equipment for a first eclipse experience. Simply enjoy it!

Here is a report about the cruise from the folks that organized it. Additional links at the bottom of the page tell the saga of how it all came together. As a passenger on that voyage to darkness I can honestly say that they did it right!
http://www.nauticom.net/www/planet/files/jul08olym.html

Pictures and a brief write up of the Eclipse 72


The above picture is me in 1972 on board the ship. Bringing a small telescope of this size is not recommended on a ship unless it has a very wide field of view. To view the eclipse required constant manual correction. However, I can tell you that the view was totally excellent!

Wednesday, 4 July 2012

Eclipse Chasers - Total Addiction

A new book about Eclipses - well, really about Eclipse Chasers - is about to be released at Amazon. You can pre-order the book now. I've not seen it so I really can't comment on it - yet. Take a look at the listing here. While I am not sure I agree with the title, it is catchy and I suppose that is what publishers like in these sorts of things.