[LEAPSECS] ATT drops time

[Rob Seaman]

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