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Q: length of one day globally - 24 hours or 48 hours? ( Answered,   3 Comments )
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 Subject: length of one day globally - 24 hours or 48 hours? Category: Science > Astronomy Asked by: mschmotzer-ga List Price: \$35.00 Posted: 28 Mar 2003 06:34 PST Expires: 27 Apr 2003 07:34 PDT Question ID: 182288
 `How long does one day last globally?`
 Subject: Re: length of one day globally - 24 hours or 48 hours? Answered By: jackburton-ga on 28 Mar 2003 07:39 PST
 ```hi mschmotzer, "...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, at midnight. 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...." http://www.vistech.net/users/rsturge/dateline.html ------------------------------------------- Search terms: [ 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 rotation, here: http://image.gsfc.nasa.gov/poetry/ask/arot.html I hope this answers your question! If you need any clarification on this answer, please ask (before giving your rating). regards, jackburton-ga```
 ```I agree: By definition and logic, one day lasts 24 hours, but globally a date lasts 48 hours. Neil```
 ```A day lasts 24 hours (approximately - it depends on which sort of day you are talking about). The notion that it might last 48 hours is just one of terminology. By the same logic, "today" lasts forever.```
 ```Not sure if this is on topic, but xarqi's right--there are different types of days. The type of day mentioned in the opening lines of the answer (the time for one complete rotation of the earth) is actually slightly shorter than 24 h. It is about 23 h, 56 m, 4 s. The reason for this is that in one 365 day year (365 sunrises and sunsets), the earth rotates 366 times.```