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Rough Science 4 Death Valley: Ellen McCallie's diary: Impact

Updated Tuesday, 29th August 2006

How would you go about measuring the diameter of a crator on the moon? As this will involve working at night, the first thing Ellen has to do is make some candles to shed a little light on the matter.

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Ellen's candles Copyrighted  image Icon Copyright: Production team

Day 1

Challenge: Measure the diameter of a crater on the moon. We can’t get to the moon - unlike Kathy, Iain and Kate, who are going to Meteor Crater in Arizona—so Jonathan and I are going to make a telescope out of mirrors from the trunk.

Strategy: We've been given good mirrors—something we couldn’t make in three days.
Collect a lot of light—bright so we are able to see detail.
Increase magnification—big mirror and an eyepiece.

Secondary challenge: Make soft-light candles to work by at night while making measurements of the diameter of a crater on the moon.

Strategy: Collect pine resin, make a wick out of sagebrush bark.

Discuss telescope and mount design with Jonathan. We decided there was no need to enclose the telescope because we are using it in the dark. Plus, having it open allows everyone to see exactly how the telescope works. The stability of the mount is crucially important because when we magnify stuff even the littlest shake makes a big difference. We want an accurate measurement of the diameter of a crater on the moon - no wiggle allowed.

Setting up the telescope: First, we found the focal length of the telescope. Then, using similar triangles, Jonathan and I figured out where to put the small (1 inch) mirror, which will divert the image to the eyepiece, where it will be magnified further. (Yes, we are using inches. Some areas of science just haven’t switched over yet.)

The 6 inch mirror has a focal length of 48 inches. Our 6 inch mirror is a concave mirror, a parabolic shape that collects light. Light travels in straight lines, hits the mirror and comes to a point—the focal point of the mirror.

We use strips of an old California car license plate to hold the small mirror in place. Because we are looking far, far away (the moon) and because light travels in straight lines from all points, it is okay to have the thin metal strips in front of the mirrors. They don’t block very much light and they are much better than putting one’s head in front of the mirror—which would make the telescope completely ineffective!

Day 2

Jonathan and I went up to the mine really early. Jonathan worked on securing the eyepiece. I worked on securing the small mirror. We worked and worked, fiddled and fiddled. Detailed stuff, carefully trying various ways to connect everything together. Made mistakes. The whole thing is now working.

Everyone else showed up at 1 pm. We couldn’t believe how fast time was flying and how long it was taking to put things together sturdily. I had to go collect pine resin for the candles, so Jonathan worked on the equatorial mount on his own.

I headed out to collect pine resin. Native Americans used resin to waterproof baskets, seal pottery, make torches, and so on.

When I collected the resin, I ended up in a sticky mess—even my hair. I must have stood up into a bunch of resin. I had to wash my hair in olive oil to get it out. Pine resin is oil soluble, not water soluble.

My hands were a mess. I couldn’t touch a thing without sticking to it.

The pure resin burned liked a sparkler—pretty fun, but not ideal for a candle. So, I tried to boil off the really volatile fractions slowly at low heat. Pine resins are often made up of over 40 different components. The ides was to leave the slower burning parts and create a more stable flame.

It turns out that sagebrush makes a decent wick as long as it isn’t too thick.

The whole deal with candles is not to burn the wick or the actual candle. The goal is to melt the candle (resin in this case) closest to the wick and to have it vaporize (evaporate slowly). The flame is actually the burning vapor.

It was quite hard to tell how long to boil the resin. I made at least 14 candles. Some batches cooled to be hard as rocks. They didn’t burn very well or very long. I think the melting point of the resin in these candles was too high. The resin which I didn’t boil as long made better candles that burned pretty well. I was quite impressed that they worked so well. But I’d liked to have refined them a bit. I’d like to try to make these again. The crucial point is not to heat the resin to its flash point—that’s what is so scary and deadly about fires in pine forests.

Jonathan’s stand is pretty darn good, but the wind is kicking up so the image we see of the moon is shaking. It still looked pretty incredible, very bright and detailed. We could see several craters very clearly. We looked for a crater we could see straight on—not one on the edge of the moon, where there would be lots of distortion.

Archimedes: The next bit for tomorrow is to put a reference line, literally a hair line, in our eyepiece in order to measure how long it takes the crater of Archimedes to pass across the reference line.

Day 3

We couldn’t figure out where to put the hairline to measure off it. Jonathan wanted to drill through it. I took one of the eye pieces apart and then couldn’t get it back together properly. The focal length is in millimetres—the cameraman figured this out for us. Then we got the hairline in the right place—if you ever do this, buy an eyepiece with a reference line already inserted!

Jonathan worked on increasing the stability of the mount and added a bit to make the telescope move more slowly and smoothly.

We took nine measurements of the moon moving across the reference line. It was a little shaky, but with practice, we got used to it. When we repeated the measure, they were really close. Jonathan had the stopwatch. I called out when the moon started to cross the reference point and when it stopped crossing. Then Jonathan and I switched jobs—we got very similar results—whew!

Archimedes was a bit tougher as it is much smaller than the moon. We doubled the magnification and measured it passing the reference line in between gusts of wind. So many nights are still here, but not tonight! Our 20 measures ranged between 2.95 and 3.89 seconds. Most measures hovered at 3.2 seconds.

What was equally amazing is that the candles really worked well, a beautiful orangey glow.

Our final estimate of Archimedes’s diameter was 72 km, plus or minus 10 km. The actual diameter is 82 km. That’s pretty good!

A note about astronomy: I’ve loved looking at the stars all my life. I’ve even gotten a kick out of mapping the path of the moon and the sun across the sky. But none of this came very easily to me, at least not explaining what was going on. I kept trying and observing. Finally, I took a course in which we had to act out what was going on in the sky. My head spun; it was so much to take in. But I got it. Try it!

 

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