[LEAPSECS] ATT drops time

Rob Seaman seaman at noao.edu
Fri Aug 31 12:29:39 EDT 2007


On Aug 30, 2007, at 2:50 AM, Chris O'Byrne wrote:


> As an aside note, those of you involved in teaching physics might

> like to consider putting the output of a GPS 1pps and a radio time

> signal into an oscilloscope. This is what I did when I checked

> against the radio time signal. The radio time source I used was

> Radio Moscow (I was working in the west of Ireland at the time, and

> that was the strongest time signal I could receive with the radio

> set I had). I noticed that the time from Radio Moscow was 11

> milliseconds "late". Of course the reason was because I was about

> 3,300km from Moscow - I had just inadvertently measured the speed

> of light!


I'm sure some of the cleverer folks here have advertently measured
same :-)

To continue an off-topic, but interesting, thread, I've been
volunteering to judge at the regional science fair since a project of
my daughter's to measure the speed of sound failed to impress the
judges that year. It wasn't just that they didn't like the project
(although a dozen judges from Raytheon rated it "best in school") -
it was that they didn't believe a middle school student could measure
the speed of sound - or perhaps they didn't believe anybody could do
such a thing - or perhaps they didn't believe sound has a speed.

There were two parts to the experiment, first using echoes and second
using whistles at either end of a football field (or some larger
space). The echo part used, for example, a large concave office
building fronted by a parking lot. Measure the round trip distance
to the building, measure the echo delay for a number of trials via
stopwatch and whistle, divide. Due to the attenuation of the echo,
this suffered from road noise for the in-town locations she tried,
even on a Sunday morning.

More successful was the two whistle method - A blows whistle, B
replies on hearing first, A (or humble scribe) records total time.
No attenuation of signal, so easy to hear. The complication here is
that the reaction time of the whistlers accumulates. The echo method
has the advantage that the delay at the end averages the same
balancing out the delay at the start (as long as somebody other than
the whistler is working the stopwatch) - which is the same as saying
that there is no delay as the sound bounces off the building. For
the two whistle method, B's reaction time has to be subtracted out.

I think what appealed to the Raytheon judges was that my daughter
plotted her data for different distances (for which a football field
is ideal, albeit a bit small for human reflexes). The speed of sound
is the slope, of course, but the reaction time (or lack) appears as
the intercept. I can't remember if she fit the data, but the visible
scatter gives an ok representation of the error.

In any event, I highly recommend seeking out your local science fair
judging opportunities. They always need folks with a science and
technology background to complement the teachers looking for in-
service credits, etc. I never leave without feeling more optimistic
about the world (quite an accomplishment). Free lunch, too. The
middle school projects seem to need guidance most as a final
opportunity to encourage kids who might be sitting on the fence
regarding pursuing science. Most of the high school projects seem to
come from those who have already committed to a science trajectory.
I chuck the rubric they hand out and simply rank the projects
according to curiosity, creativity, a sense of architectural
elegance, etc. Some of the best are the scruffiest looking projects,
of course. I like to think of some awkward kid like I was feeling
vindicated, and his or her teacher feeling perplexed, at why Trent
and Muffy's shrink wrapped model of a volcano didn't take the prize.

Timekeeping of all sorts seems like a great topic for a science fair
project.

Rob Seaman
NOAO



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