NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: what is a second?
From: Frank Reed CT
Date: 2006 May 10, 00:36 -0500
"I ask this question of the list, after having looking into time divisions
24 hour day, 60 minute hour and 60 second minute but how the second was
defined eludes me. I have seen that a pendulum of 1 meter length has a period of
a second so if that is how the second was defined I am ok with it.
Still I wonder if the definition of the second might have preceded the
metric system defintions?"
It happened the other way around. You start out by defining a day. There are
some tricky points here. Is it a solar day? The Sun doesn't keep good time
which is why we have to deal with the "equation of time". How about a sidereal
day? The rotation of the Earth is fairly stable. After you've got a day, you
can simply define it to be divided into 24 hours, each with 60 minutes, and
each of those with 60 seconds. Even if you can't measure such divisions
reliably, you can declare them by definition. We can even go further and divide a
second into 60 parts. They used to call those "thirds" (get it?). It would be
fascinating to know when these units of time entered common knowledge. When
did it become common to say, "give me a second" or, "be back in a second"?
Measuring seconds and being able to state reliably that a certain event
occurred at a particular second of time was a slow process historically. In the
pre-clock era, you could use an angle-measuring instrument to measure as
accurately as possible the right ascension of the meridian (a quantity which came
to be known as the "local sidereal time"). After Huygens and the development
of pendulum clocks, it was discovered that a pendulum with an effective
length of about 33 inches counted off seconds as long as the length of the swing
was very short (or the pendulum was constrained to move in a cycloidal arc).
When the metric system was being debated late in the 18th century, some
suggested that a meter might be defined by the length of a pendulum that swings
exactly in one second. Since the swing of a pendulum depends on latitude, this
proposal didn't win out.
-FER
42.0N 87.7W, or 41.4N 72.1W.
www.HistoricalAtlas.com/lunars
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To post to this group, send email to NavList@fer3.com
To , send email to NavList-@fer3.com
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From: Frank Reed CT
Date: 2006 May 10, 00:36 -0500
"I ask this question of the list, after having looking into time divisions
24 hour day, 60 minute hour and 60 second minute but how the second was
defined eludes me. I have seen that a pendulum of 1 meter length has a period of
a second so if that is how the second was defined I am ok with it.
Still I wonder if the definition of the second might have preceded the
metric system defintions?"
It happened the other way around. You start out by defining a day. There are
some tricky points here. Is it a solar day? The Sun doesn't keep good time
which is why we have to deal with the "equation of time". How about a sidereal
day? The rotation of the Earth is fairly stable. After you've got a day, you
can simply define it to be divided into 24 hours, each with 60 minutes, and
each of those with 60 seconds. Even if you can't measure such divisions
reliably, you can declare them by definition. We can even go further and divide a
second into 60 parts. They used to call those "thirds" (get it?). It would be
fascinating to know when these units of time entered common knowledge. When
did it become common to say, "give me a second" or, "be back in a second"?
Measuring seconds and being able to state reliably that a certain event
occurred at a particular second of time was a slow process historically. In the
pre-clock era, you could use an angle-measuring instrument to measure as
accurately as possible the right ascension of the meridian (a quantity which came
to be known as the "local sidereal time"). After Huygens and the development
of pendulum clocks, it was discovered that a pendulum with an effective
length of about 33 inches counted off seconds as long as the length of the swing
was very short (or the pendulum was constrained to move in a cycloidal arc).
When the metric system was being debated late in the 18th century, some
suggested that a meter might be defined by the length of a pendulum that swings
exactly in one second. Since the swing of a pendulum depends on latitude, this
proposal didn't win out.
-FER
42.0N 87.7W, or 41.4N 72.1W.
www.HistoricalAtlas.com/lunars
--~--~---------~--~----~------------~-------~--~----~
To post to this group, send email to NavList@fer3.com
To , send email to NavList-@fer3.com
-~----------~----~----~----~------~----~------~--~---
