[X] My Assistant

[Stu]

People in China will be able to see a total solar eclipse on Friday. Here in the UK we'll see a partial solar eclipse - not a very dramatic one, but still worth watching. Full details here...

Note: these times are for my home in Kendal, Cumbria, but wherever you live in the UK your times won't be drastically different, and a quick Googlehunt will help you track down accurate times... board members in certain other countries will be able to see the eclipse too, but I don't have specific visibility details. Again, just Google it.

This is just a "heads up" to make sure you all put it in your diaries...

[elakdawalla]

All right, I have a question. I'm sitting in a staff meeting at the Society and Lou is talking about his trip to China to see the eclipse. He said that totality lasted only 94 seconds. He then said that next year's eclipse tour will see a much longer eclipse with totality lasting 7 or 8 minutes. So I asked, how does it work that eclipses are sometimes so short and sometimes much longer? Small changes I can understand but that's a pretty huge range. There was discussion about the kinds of things that could influence the duration of totality -- latitude, distance of the Moon from Earth, etc -- but nothing that coherently answered my question. Can any of you help me out?

--Emily

[BPCooper]

All right, I have a question. I'm sitting in a staff meeting at the Society and Lou is talking about his trip to China to see the eclipse. He said that totality lasted only 94 seconds. He then said that next year's eclipse tour will see a much longer eclipse with totality lasting 7 or 8 minutes. So I asked, how does it work that eclipses are sometimes so short and sometimes much longer? Small changes I can understand but that's a pretty huge range. There was discussion about the kinds of things that could influence the duration of totality -- latitude, distance of the Moon from Earth, etc -- but nothing that coherently answered my question. Can any of you help me out?

--Emily

I think I can explain the main idea behind it...

The main factor is the distance between the Earth and moon, and the Earth and sun. If the eclipse occurs when the Earth and moon are closest and at the same time, the sun and Earth farthest apart during their orbits, the eclipse will last the longest as the moon is as large as it can be relative to the sun in that scenario. The maximum theoretical limit for a total solar eclipse is 7 min 31 sec.

By contrast, if the moon is at apogee and the Earth at perigee, the sun will be as far larger than the moon as it can be, creating the other extreme, a maximum duration annular eclipse.

Another factor in this situation is whether they slide passed each other exactly so that their radial centers cross, or if they slide passed each other slightly off axis. This can affect the time as well (just like a lunar eclipse, which is more easily seen... hitting the bullseye produces the longest).

On top of that, I believe your location on Earth an eclipse crosses can also have an effect.

The 2009 eclipse is the longest of the 21st century and will be 6 mins, 38.9 seconds long at the maximum point which is SE of Japan.

The other factor is that each eclipse varies in length along its track, with maximum duration in the middle. At the ends of the track, which occur where the Earth's surface is curving away from the shadow cone, the eclipse occurs at local sunrise or sunset and is shortest in duration because the shadow sweeps along the surface faster here (look at a spherical projection animation). The longest duration last week was 2 m 27 s, in northern Russia. Where I was, in southern Russia, it was 2 m 18 s. In China, less than 2 mins.

I hope this helps and that I didn't leave anything significant out.

In the meantime I have posted more photos and sequences:
http://www.launchphotography.com/Total_Sol...lipse_2008.html