Is the large telescope at Lick Observatory 3 meters or 120 inches?

"Mt. Hamilton School" wrote:
>
> Is the size of the mirror on the Shane telescope 3 meters or
> 120 inches? During a math project we discovered that 3 meters is
> not equal to 120 inches. It equals 118.11 inches.

Dear Mt. Hamilton School,

I must say first, you guys are way cool! What an appropriate, excellent
exercise (since you spend your schooldays just a couple of hundred
meters away from the mirror of which we speak). Cheers to you also for
not just brushing the discrepancy aside, but asking good questions and
pursuing it. Way to go!

So we looked it up, and the specifications say 120" is the diameter of
the main mirror. That makes sense historically, because in the 1930s
when this mirror was cast, the United States was using almost
exclusively the British system of units -- feet, pounds and seconds.
Now most of the world, led by the science community, has adopted the
more logical decimal systems of mks (meter-kilogram-second) or it's
close cousin the cgs system (centimeter-gram-second). At least we all
agree on seconds! I very much hope that during your lifetimes, the US
will adopt the metric system for general use.

I'm sure you are well aware of the sort of confusion which can arise
between different measurement systems, since we lost the Mars explorer
satellite last month, just because one engineer used foot-pounds when
another expected meters and kilograms. Who needs that?

So why do we call it the "3-meter"? Because in the science community as
well as most of the societies of the world, the metric system is nearly
universally used, so most of the people to whom we speak will understand
how big 3 meters is more easily than understanding how big 120" is. As
you have already discovered, the error of using the 3 meter
approximation is small.

Just keep in mind that the mirror is a physical reality. It just sits
there doing its job. It doesn't care how we measure it, whether in
inches, meters, millipede lengths or light-years. It is what it is, and
(as it happens) it is been one of the most productive astronomical
mirrors ever made!

Thanks for asking,

Rem

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