 Now before we get into time zones, let's figure out why we even have different time zones on the planet. And so, let's draw a picture of Earth again, and again, let's presume that we're looking at Earth from above the North Pole. So here's the North Pole, and let's say that you're standing somewhere around here on the equator. And we know that in this general direction it's west, and we know that in this general direction it's east. And let's put the Sun in this picture as well. So we'll put the Sun here. Now according to this picture right now for this person standing where this person is at, it's going to be right around Solar Noon. Now Solar Noon doesn't mean that the Sun is right above your head at Zenith Point. That just means that your location, the Sun, is at its highest point that it's going to be. And therefore we call that Solar Noon. Now keep in mind, the Earth rotates on its axis in a nice easterly or counter-clockwise direction. So let's presume that a few hours before this picture, you were standing right here. And let's also presume that a few hours after this picture, you'll be standing right here. Well, a few hours before this picture was taken, if you're here, you have not yet rotated in to noon. So for you it would be before noon. Whereas a few hours after this picture is taken, you're going to have rotated out of noon. And so if you're over here, it's afternoon for you. And again you'll notice that if you're standing here in the west, the Sun is rising in the east, coming from over here, and eventually it'll set in the west. And you'll see that as you rotate with the planet and eventually you're over here on this side, the Sun is now toward the western sky as you're progressing toward the east. So once again the Sun rises in the east and it sets in the west. And so that's sunrise. Early in the day when you're right here, it's before noon. Then you rotate in to noon. And then as you keep rotating, you're rotating to afternoon. So that's why we have different time zones on the planet. You're rotating on Earth and as we rotate on Earth, we'll be rotating in our position relative to the Sun is going to change. And if you're west of a position, you'll be behind in time. And if you're east of a position, then you'll be ahead in time. So let's keep that in mind and let's draw a new picture. So again, here's Earth and now what I'm going to do is take the three-dimensional Earth that we have, the sphere. We're going to rip it apart and basically we're going to plant it on a two-dimensional plane. So here is the prime meridian at zero degrees of longitude. And by the way, our discussion of prime meridian actually now becomes very important when we start talking about time zones. The fact of the matter is, remember, we defined the prime meridian at zero degrees of longitude, but it's a rather arbitrary pick. Why didn't we pick some other longitude? Why is it that we picked this one? This zero, by the way, goes right through Greenwich, London, England. And there's a reason for that and we'll get to that shortly. Okay, let's also draw here 180 degrees and we'll also put 180 over here as well, because remember, we simply pulled the Earth apart and we're just putting it on a two-dimensional plane. By the way, 180 degrees ultimately is going to be known as the international dateline and we'll get to why that's called the international dateline and we'll do that shortly as well. Now recall, this direction is east and this direction is west. From our prior picture, we can now surmise that if you're east of any given position, that is going to be ahead in time and so I'm going to use my little icon here of somebody's head. If you're west of a position, then you're going to be behind in time and I'll leave that icon to your imagination. Yes? Okay. So now that we know east is ahead and west is behind, the next question becomes well how many time zones do we actually have on the planet? Well remember, Earth is rotating in a counterclockwise direction and it rotates one complete rotation in the time span of 24 hours. That 24 hours therefore is 360 degrees of longitude rotation. So 360 divided by 24 equals 15 degrees per hour. So that means that the Earth is rotating on its axis 15 degrees of longitude every hour and if you by the way wanted to calculate how many miles per hour that actually is, all you have to do is multiply 15 times 69 and that's going to be a bit over a thousand miles per hour. So if you're standing on the equator, you're rotating with the planet at about a thousand mile per hour clip. Okay. So we have 24 time zones on the planet. We know that because there's 24 hours in a day and in addition we know that each one of those time zones is 15 degrees of longitude wide and in fact if you go east of the prime meridian, you're going to have 12 hours and if you go west of the prime meridian, you're going to have another 12 hours. So 12 plus 12 is going to equal 24 hours during the course of the day. Yes. Now again, we know that each time zone is 15 degrees of separation and we know that each time zone is 15 degrees of separation because again the Earth rotates 360 degrees in a 24 hour time period and 360 divided by 24 equals 15 degrees per hour. So if we say that the prime meridian is really the central time zone of the planet and we're going to call that GMT for Greenwich Mean Time because again the prime meridian runs through the heart of Greenwich, London, England. What we want to do is figure out where that time zone is east and west and what I mean is the following. We know that each time zone is 15 degrees. So Greenwich Mean Time is going to go from the prime meridian eastward about a 7.5 degrees and it's going to go west about 7.5 degrees as well and now here's our GMT time zone is 15 degrees of longitude wide. Now you can continue on. The next time zone east will be another 15 degrees and then the next time zone will be another 15 degrees and so on and so forth and we can do the same thing west. So here we have another 15 degrees and another 15 degrees and again so on and so forth until eventually we just simply divide the world into 24 15 degree parts. Let's take a look at a quick example. So let's clear this screen. We'll draw this picture again. Here's the prime meridian at 0 degrees of longitude. Here's west which remember is behind in time. Here's east which is ahead in time. 180 degrees, 180 degrees. Remember we talked about San Diego, California. San Diego, California is part of the PST time zone and PST is Pacific Standard Time and Pacific Standard Time is right around here at 120 degrees west longitude. Now to be exact, San Diego is at about 117 west and so there's a three degree difference but remember time zones are 15 degrees wide. Therefore even though we're at 117 degrees west we're still part of the Pacific time zone which by the way leads to another question. Is there a difference between solar time and between political time and the answer is yes. I mean the fact of the matter is just because we are within a 15 degree time zone and just because the time is the same for us as somebody who is three or four degrees east of us doesn't really mean technically that it's the same time. It does according to our watch, don't get me wrong. But if we're just looking at the sun then the position of the sun and the sky is going to be slightly different even if you're within that particular time zone. Okay but for our purposes we're at Pacific standard time. So the question becomes how many time zones are we removed from GMT, Greenwich Mean Time. We already know that we're behind time. So let's say for sake of argument that it's 8 p.m. right here at Greenwich Mean Time. So if you have a friend in England for them it would be 8 o'clock, 8 p.m. Well for us we are 120 degrees divided by 15 degrees per hour equals 8 which means we are eight hours or eight time zones behind Greenwich Mean Time. So if it's 8 p.m. for our friends in Greenwich London England, and by the way it's not just Greenwich London England it's anybody on the planet who happens to live within this particular longitude of seven and a half to seven and a half. So whether they're up here or whether they're way down there it doesn't matter. The fact is time zones are running east to west not north to south. So someone up here is in the same exact time zone as someone who is down there. So if it's 8 hours of time difference between us and between Greenwich London England when it's 8 p.m. for those guys it's going to be noon for us. We're 8 hours behind. Now what about for someone who's way over here at 120 degrees east longitude? Well for them they're 8 hours ahead. So 8 hours plus 8 p.m. would be 8 let me use my fingers here 8, 9, 10, 11, midnight 1, 2, 3, 4. So it would be 4 a.m. for our friends over here at 120 degrees east. Notice it's 4 a.m. So that means it's going to be a different calendar day for those folks over here if in fact it's 8 p.m. for our friends in Greenwich London England. Let's do another example. Again I'm going to draw this same exact picture. Here's zero degrees the prime meridian. We draw 180. Again we know west is behind in time. We know east is ahead in time. Now let's say that we have a friend and this friend lives over here and over here is at about 80 degrees east longitude. And let's say for this friend at 80 degrees east longitude the time is 3 p.m. And we'll just give it a day. We'll say 3 p.m. Friday. And now we want to know what time it is for someone over here who is at 100 degrees west longitude. Yes? So we just simply want to know what time it is here at 100 degrees west knowing that it's 3 p.m. Friday at 80 degrees east. Again all we need to do is just figure out how many degrees of separation there are between 80 and 100. We also know that we're going west of 80 therefore the time is going to be behind in time so we're going to be counting backwards. So degrees of separation between 100 and 80 well if you go from 100 to zero that's 100 degrees. If you go from zero to 80 you're tacking on another 80 so 100 plus 80 equals 180 degrees of separation we divide 180 by 15 because for every time zone that's 15 degrees of longitude and that equals 12. So we know that there are 12 hours separating 80 degrees east from 100 degrees west. So if it's 3 p.m. Friday and we've got to count backwards then we know it's going to be 3 a.m. right here. Is it still Friday? Well yes it is because we haven't crossed over into another day so it's 3 a.m. Friday if you're over there at 100 degrees west. Now you can count backwards you can go 3 p.m. and subtract 12 hours from it you're going to get to 3 a.m. Does that make sense? Yes now by the way there's another way that we could have done this. We counted backwards but we also could have solved the same exact problem counting ahead. Remember we live on a sphere and the fact of the matter is you're not going to get two exact times or two distinctly different times for a singular longitude if you count ahead or behind it doesn't work that way. So let's count ahead instead of behind and we're probably still going to arrive I say probably I know for a fact that we're going to arrive at the same exact answer. So the first question is how many degrees of separation is there from 80 to 180 is 100 degrees and we know that from 180 to 100 that's 80 degrees. So what we now need to do is once again figure out the number of hours separating here to here and then here to here. So if we divide 15 into 100 we're going to get about 7 so we know that there are 7 hours separating 80 from 180. So now we can just simply do the math again we're going ahead in time if we're moving toward the east so if it's 3 p.m. Friday at 80 degrees east we add 7 hours to that then we know it's 10 p.m. right here. Now notice I didn't say whether it was Friday or Saturday we're going to get to the international date line in just a second and describe what it means but very quickly if it's Friday over here then it's actually going to be Thursday on this side. So it's Thursday over here and it's Friday right there so the fact is when we crossed over the international date line we went from Friday to Thursday and again I'll describe why that is in just a second. So we know that it's 10 p.m. Thursday right here right on this side of the international date line we now have to go from 180 to 100 that's 80 degrees we're continually going east here toward the easterly direction therefore we're still counting ahead in time so if it's 10 p.m. we now divide 15 into 80 we get 5 hours so now if it's 10 p.m. here the international date line we add 5 more hours to it and remember we're going ahead still so 10, 11, 12 which is midnight 1, 2, 3 a.m. it's 3 a.m. and you might say well isn't it Thursday? I thought you said it with Thursday but now you're telling me it's Friday well remember we crossed over we went midnight and once you get beyond midnight you're turning the calendar day to Friday so indeed if you're at 100 degrees west it's going to be 3 a.m. Friday if it is 3 p.m. Friday at 80 degrees east now let's go back to this international date line thing again let's clear this board if we draw the international date line right here and if we have this being the east side and this being the west side there is a rule associated with the international date line if you cross over from the west side into the east side you're going to lose a day if you cross over from the east side to the west side you're going to gain a day and that makes perfect sense there are 24 hours in a day you've got the eastern hemisphere has 12 hours the western hemisphere has 12 hours as well and remember the west side is behind in time and the east is going to be ahead in time so if you cross over from the west and you go toward the east you're basically losing that entire day and again let's take a look at this picture right here so here it is and again this is 180 and this is 180 and this is west side and this is east side and remember this whole side is ahead of Greenwich and this whole side is behind Greenwich and so if you're right here 12 hours behind Greenwich you're actually going to be 24 hours behind the very most eastern part of the eastern hemisphere that's 24 hours removed so if you cross over from this west side and you go to the very end of the east side you're giving up this entire 24 hour period of rotation and you're basically giving up your day so you're setting the calendar back it's almost like you're at the front of the line and then all of a sudden your friends ditch you and they throw you back to the back of the line the bouncers do that you've lost your opportunity to get in you've lost the opportunity to cut in front of everybody now on the flip side if you're here on the east and you cross over to the west you're going to have that calendar day all over again now you might be asking yourself well can I perpetually live forever just by hopping back and forth across the international day line and of course the answer is no because you cross over one time you'll get that 24 hour period but if you want to cross over it again you're going to lose that 24 hour period just to cross back over it again to get the 24 hour period so the answer is no the only way that you can ever gain any real time is if you cross it again you don't come back but of course that's not going to happen the curious thing of course is when you're traveling overseas if you cross this international day line you're going to pick up some time in your travels but again when you come home you're pretty much giving it all back does that make sense okay let's do one more example problem and then we'll move on and then we'll ask ourselves well why are we talking about time zones we're talking about longitude and I'll give you the short answer to that in just a second okay so let's do another example problem let's say that you are again located in San Diego, California which is approximately 120 degrees west longitude and let's say that the time is 10pm and the day is a Wednesday let's say that you have a friend in Sydney, Australia is about 150 degrees east longitude and so you want to know what day and time it is for your friend in Sydney, Australia if it's 10pm Wednesday for you and maybe you want to do this because you want to make a phone call but you don't know if they're sleeping or if they're up you just don't know what time it is out there well what we're going to do is once again draw this picture and here is zero degrees here is 180 here is 180 as well and we're going to put us here at 120 west and we know that the time and day is 10pm Wednesday we also know that in this general direction which is west it's behind in time and we know that in this general direction east it's ahead in time and so let's also put in the picture 150 east which is our friends over at Sydney, Australia and again Sydney is in the southern hemisphere but it doesn't matter because remember time zones are running north to south excuse me time zones are all the same north to south but they change east to west so whether this person is here or here or here it doesn't really matter they're all part of that same time zone so the first thing we need to do is just simply determine how many degrees of separation there are from us and Sydney well from us to the prime meridian it's 120 degrees from the prime meridian to Sydney it's another 150 so 120 plus 150 equals 270 degrees of separation we divide 270 by 15 15 goes into 27 once so we'll do the math we'll do the board history here's 120 degrees we know 15 goes into 128 times so there are 18 hours separating us from Sydney, Australia now I can do the math I can count from 10pm and I can add 18 hours to it to figure out what time it is in Sydney but honestly I'd rather not count that many numbers is too much for my little fingers I only have 10 of them and I would have to double duty so I know I'm going to make a mistake but here's the rub I don't need to I know that there are 24 hours in a day so if I count ahead 24 hours I can also count backwards 24 minus 18 equals 6 hours so there are 6 hours separating us from Sydney if we count backwards whereas there are 18 hours if we count forward so for me subtracting 6 hours from 10pm is really really easy so 10pm minus 6 hours means that it's 4pm for our friend in Sydney, Australia the difference of course is if it's Wednesday right here we're crossing over the international date line we're going from the west side to the east side and we know that according to our rules if we cross from the west side to the east side we lose a day so for our friend in Sydney it's not going to be Wednesday it's going to be Thursday so it's 4pm Thursday when it's 10pm Wednesday for us you're good to go make your phone call now by the way you can check this of course we can count ahead if we wanted to count this way so let's do that we need to add 18 hours to 10pm we know that 12 hours added to 10pm gets us to 10am we need to add 6 more hours to 10am so 10, 11, 12, 1, 2, 3, 4 we arrive at 4pm and again because we crossed over midnight we're going from Wednesday to Thursday so everything works out just as it's supposed to when it's 10pm Wednesday for us it's 4pm Thursday for our friends in Sydney, Australia now we've spent a fair amount of time talking about time zones and remember the whole premise behind the time discussion was longitude figuring out what your longitude is remember with latitude we can simply look at the north celestial pole star if you're in the northern hemisphere or the south celestial pole star if you're in the southern hemisphere but I said that with longitude the key was time zones and the clock well again we spent all this time now talking about time zones how does it have anything to do with longitude well it has everything to do with longitude let's take a look at the example that we just used we said that it's 10pm Wednesday at 120 and at that same time at 150 east it's going to be 4pm Thursday but if I had different variables what if in fact we had this what if I said that at 120 west it was 10pm Wednesday and what if I said at some unknown location the time was 4pm Thursday now I'm not telling you where 4pm Thursday exists I'm just telling you that somewhere in the world is 4pm on Thursday can you tell me where 4pm Thursday is located and the answer is yeah we could do it if we know that it's 10pm Wednesday for us and we know that in this general direction time is ahead and in this general direction time is behind and if I say well if it's 10pm for me and I want 4pm seems to be behind 10pm so I'm going to go this way and of course the question is well how many hours are separating 10pm from 4pm well we know it's 6 hours so we know that 4pm is 6 hours behind 10pm how many hours of separation is that well that's 6 hours and we know that for every hour that equals 15 degrees of longitude so we multiply 15 times 60 well 15 times 60 is 90 degrees that means that this person at this time at this unknown longitude is 90 degrees west of us so if we're at 120 we go from 120 to 180 and that's 60 degrees we've got 30 more degrees to go so we're right here at 180 and we go 30 more degrees this way 180 minus 30 now gets us to 150 so again we don't need this person to say hey I'm at 150 degrees east longitude all we need to do is know what their time is and once we know their time we can determine longitude so that's the key let me give you another example let's say that it is 3pm at Greenwich Mean Time or Greenwich London England which we know is at 0 degrees of longitude and let's say that we want to know the location of someone who is at noon 12 noon so we're going to draw this picture again here's 0 degrees the prime meridian we know that this way east is ahead we know that this way west is behind we know that according to our data it's 3pm at the prime meridian the question now is noon is noon behind 3pm or is it ahead of 3pm and we know of course that noon is going to be behind so it's over here so how many hours separate noon from 3pm 3 hours and remember every hour equals 15 degrees of longitude so 3 times 15 is 45 degrees that means that this person where it's noon is at 45 degrees west longitude that's how you do it so again time and longitude are really interchangeable if we say 15 degrees of longitude per hour there's two variables in that statement there's degree of longitude and there's hour or time so this is how time zones and longitude are completely related if you are out at sea and if you want to know what your longitude is you need to have two clocks one clock giving you the time of the destination that you're at and one clock giving you the time of where you left from now this leads to a problem called the longitude problem and there's a real real reason why this is a problem certainly 400 years ago what type of clocks do we have back then we've got pendulum clocks we have sun dials we also have smart people making different types of clocks that use all sorts of different things but the fact of the matter is if we want to tell time on a boat it's very difficult 4 to 500 years ago pendulum clocks don't quite work very well do they and certainly sun dials aren't going to keep track of where you left from it will only give you the time of where you're at so again this leads to something called the longitude problem and if not for the longitude problem you and I probably wouldn't have these watches that we're wearing today so in the next lecture we'll talk about the longitude problem we'll talk about how this problem was solved eventually and inevitably and again once we have a longitude problem solved we now have the ability to figure out our longitude out at sea or anywhere else and now of course we also have the ability to determine our latitude by looking for the north celestial pole star and we'll finish up this discussion therefore next time