Great question! Before we had watches or clocks
(or laptops or cell phones) to read off the time,
people used other techniques to approximate the
time. These techniques use the sun, the moon,
and/or the position of stars to determine about
how far into the day and how far into the night it
is. These techniques still come in handy if you
are outside without a watch.
For example, during the day, you can observe
the track of the sun across the sky. When the sun
is the highest in the sky and at the center of its
track, it is about 12 noon. The actual time
depends also on where you are in your time zone
and whether or not it's daylight savings time. In
the morning, the sun will be in the East and in
the evening, the sun will be in the West. You can
figure out about what time it is by how far above
the horizon the sun is. For example, if you know
the sun rises at about 6 am and sets at about 6 pm
(typical in early spring and late summer), and the
sun is in the East halfway between the horizon and
overhead, then it is about 9 am.
Of course, these techniques can't tell you down
to the minute or second what time it is, and for
that, you do need to refer to a time-keeping
device like a watch.
Since about 1950, we have used a very precise
clock called the "atomic clock" as a reference for
all other clocks. This clock is kept at one of the
US National Laboratories. The atomic clock tells
time to tiny fractions of a second and other
clocks (such as cell phone clocks) are set to
match this reference clock.
You sure can tell time without a clock or a
watch! Time passes regardless of whether you're
keeping track of it or not. While you won't be
able to tell time in seconds or minutes without a
watch, you can still guess the hour or general
time of day. One of the oldest ways of telling
time is the sundial, and it still works. Sundials
work by casting a shadow on a disk marked with
different periods of time. As the sun moves in the
sky, the shadow falls on a different part of the
disk, which gives the time of day. Sundials don't
work well at night, though, as you can imagine.
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