Monday, September 24, 2007

Fall Equinox

Above is a somewhat disappointing result to our analemma experiment.

You may remember back in April or May when someone announced the discovery of an extra-solar Earth type planet. They didn't mean that you could live there or that there was life. It's just one of the first solid planets found outside our solar system. Most of the hundreds of planets we've found have been gas giants big enough to make Jupiter look piddly. Any they were only found because they were so huge.

A co-worker asked me how far a light year was. I explained that it's how far light can travel in a vacuum in one Earth year. Being a science geek I've had years to get used to dealing with these kinds of distances. I had to bring her up to speed. It takes 8 minutes for light to reach Earth from the sun. The nearest star to the sun is are the Alpha Centari stars ~4.5 light years away. There were also charts of our local neighborhood and some other stuff to help explain it.

This led to other questions including how did people know how long a year was thousands of years ago. So we started building an analemma on her office floor.

Here's how it worked. I put a sticker on her window. Having a west facing office we had to wait until 3:00 to see where it's shadow fell. Every Monday, Wednesday, and Friday at 3:00 I'd find the shadow and put a sticker where the shadow fell. What you see above is the result. For awhile things went well. The bottom of the arc falls around June 21-22, the summer Solstice. At that point there's not much of a change from day to day. But late July-early August you can see there's a gap since we only put the sticker every other day and a larger still gap on weekends. Then the clouds came. Big gaps appear as we had a week of rain. A few times we had sun all day but a cloud came at 3:00. A bit of a wobble in the line shows that we didn't always hit it at exactly 3:00. A few times we got as much as 15 seconds late.

Then the sun went behind a tree. We didn't get to the equinox on Sept 21-22. I'm hoping for the dot to return when the leaves fall but I'm not counting on it.

The shape caused by tracking the sun's motion is called an analemma.

Below is a more respectable analemma that I lifted from

I'm hoping that some of my readers will continue the experiment for me. If you have kids, a class, or just a south facing window please continue the experiment. I'd like to see pictures of your analemma along with the time of day you recorded it and your latitude.

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