"...Our day is divided into hours. It takes 24 hours to make a day,
and a day is the time the Earth takes to complete one rotation about
its axis....as the Earth rotates on its axis, the time at any given
place changes. By convention, our day starts at midnight. So, as the
Earth rotates, and the location of midnight rotates, the calendar date
changes from one day to the next. But, as we shall soon see, we need
two places where the date changes.
Why do we need two places where the date changes? The answer is
simple. At any given time, we have two dates happening. Yes, we do!
Think about it. When it's 12:01 a.m., November 1st, in Chicago, what
is the date in New York? It's also November 1st. But what is the date
in San Francisco? It's still October 31st, and will be for almost two
hours. See? Different places, different dates.
So, we need some way to divide the Earth into two dates. One way is
obvious: to the west of midnight it's one date, and to the east of
midnight it's the next date. But we still need another dividing line,
because it's only midnight at one meridian at a time. We have a line
running from one pole to the other, dividing the globe between pre-
and post-midnight. But we need another line connecting the poles, to
clearly divide the Earth into two dates. That line has been
arbitrarily set at the 180 degree meridian, zero degrees being also
arbitrarily set at Greenwich, England. The international dateline does
not actually follow the 180 degree meridan exactly, but zigs and zags,
following political jurisdictions. But for simplicity's sake, we may
take it as actually being the 180th meridian. Starting at midnight and
going east to the international dateline, the date is one day ahead of
the date on the rest of the Earth. One divider is fixed in location,
at the international dateline, and the other is moving with the time,
Another way to visualize this is to imagine the globe as having two
half-circles, each anchored at the poles. One half-circle is fixed to
the globe at the international dateline and rotates with it, and the
other is at midnight, always opposite the sun. A new date is born when
the international dateline passes through midnight. The new date
starts out as just a sliver between midnight in the west and the
international dateline in the east. As the date gets older, it grows
in size until, just before the Earth's rotation brings midnight around
again to the international dateline, that date covers nearly the
entire globe. For an instant, that date hogs the entire globe. But
then immediately a new date begins and the old date begins covering
less of the globe, as it is squeezed between midnight, moving to the
west, and the international dateline. In 24 hours, the two
half-circles cross each other again, and yet another new date is born,
as we switch from dates "one" and "two" to dates "two" and "three." 24
hours later, we switch to dates "three" and "four." And so it goes.
From the above explanation, we get an answer to an interesting
question: if a day lasts 24 hours, how do we cram two dates onto the
globe? The answer is strange, but true. A day lasts 24 hours, but a
date lasts 48 hours! Yes, it was November 1st, 1991, somewhere on
Earth for 48 hours. Two dates of 48 hours each, divided by 2, gives us
our 24 hour day. So, it really does all come out okay...."
[ length day "48 hours" ]
[ earth rotation "24 hours" "48 hours" ]
[ earth rotation 24 hours 48 dateline ]
You can also find a lot of answers to questions to do with the earth's
I hope this answers your question!
If you need any clarification on this answer, please ask (before
giving your rating).