It`s about time! - NRC Publications Archive

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It's about time!
Tapping, Ken
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Skygazing: Astronomy through the seasons, 2014-07-08
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IT’S ABOUT TIME!
Ken Tapping, 8th July, 2014
We have been measuring and expressing time in
some way for centuries. However, over that period
our idea of time and how to determine it has
changed a lot. For much of our history, having
some way to “know the time” has been to do with
managing our hours of daylight and in some cases
scheduling religious observances. The day was set
to be 24 hours long, with noon being when the Sun
was at its highest angle above the horizon, which
happens when it lies due south. Midnight was then
when the Sun lay due north, somewhere below the
horizon. Even today, in our industrialized society,
hours of work are mostly set up to make best use
of hours of daylight. Measuring what we can call
Local Solar Time, which is more or less Local
Standard Time, is easy; we can do it using a
sundial. The earliest examples were just sticks or
rocks stuck in the ground. However, over the
centuries sundials became more sophisticated and
more accurate, and could even tell time of year.
Unfortunately, sundials are not portable; they have
to be carefully set up, and are useless in cloudy
weather or at night. In Northern Europe there are
many cloudy days and in winter at latitudes above
45 degrees or so the Sun does not spend much
time above the horizon. So some other means of
monitoring the passage of time were invented.
Initially these were calibrated candles, water
leaking at a steady rate through a small hole,
hourglasses or many other ingenious inventions.
This culminated in the pendulum clock. Mechanical
clocks improved dramatically over the years. Then
electronic clocks came along, using crystals and
then resonances inside atoms. These are so
accurate they led us to a problem that we are
facing today. They are so accurate they can be
used to detect the slowing of the rotation of the
Earth due to tidal effects by the Moon and Sun,
and changes in the rotation rate due to
Earthquakes. Since solar time is linked to the
rotation of the Earth, they can detect changes in it.
This led to the need to periodically adjust our
electronic clocks to keep pace with solar time. We
do this by establishing the solar time by precise
astronomical measurements and tweaking our
electronic clocks by adding an occasional “Leap
Second” whenever the error gets too large.
Although on one side we want our clocks to stay in
step with solar time, so we can continue to work
with the Sun, there are some applications for
which these tweaks are a problem. For example
navigation satellites like to use clocks that don’t
need to be adjusted. Adding an occasional Leap
Second reliably can be difficult. This has led to a
choice we need to make; whether to keep our
clocks as they are, synchronized with the rotation
of the Earth and the rhythm of day and night, or to
let our clocks run at a steady rate. Initially most of
us would not notice the difference, but eventually
this would lead to big differences between Local
Solar Time and official clock time. Then we would
have to decide whether to make a big correction to
get Local Solar Time and clock time back in step,
or to allow the discrepancy to get larger and larger.
Maybe we will need to maintain two time systems
and monitor the difference between them. In an
age where we worry about the basic nature of
space and time, it is interesting to see that our
biggest problem with time is totally practical. That’s
something to think about next time you set your
watch to “The beginning of the long dash”, from
NRC's time transmissions on the radio.
Jupiter is now lost in the sunset glow. However, we
still have Mars and Saturn. Mars, the bright red
starlike object in the south-west needs a big
telescope to enjoy it, but Saturn, a moderately
bright object in the south, is magnificent, even
through small telescopes. Venus rises about 4 am.
The Moon will be Full on the 12th.
Ken Tapping is an astronomer with the National
Research Council's Dominion Radio Astrophysical
Observatory, Penticton, BC, V2A 6J9.
Tel (250) 497-2300, Fax (250) 497-2355
E-mail: [email protected]