Transcription
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 is 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 is west, and we know that in this general direction is 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, it's going to be right around solar noon. Now, solar noon doesn't mean that the Sun is right above your head as zenith. That just means that at 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 counterclockwise, 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 into 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, let's summarize. Early in the day, when you're right here, it's before noon. Then you rotate into noon, and then as you keep rotating, you're rotating to after noon. 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 is Earth. And now, what I'm going to do is take the 3-dimensional Earth that we have, that sphere, we're going to rip it apart and basically, we're going to plant it on a 2-dimensional plane. So, here is the Prime Meridian at 0 degrees of longitude. And by the way, our discussion of the 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 0 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 0, by the way, goes right through Greenwich, London, England, and there's a reason for that. 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 pull 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 Date Line, and we'll get to why that's called the International Date Line, 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 this 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 1000 miles per hour. So, if you're standing on the Equator, you're rotating with the planet at about a 1000 miles 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 to the Prime Meridian, you're going to have 12 hours. And if you go west to 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 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 seven and a half degrees, and it's going to go west about seven and a half degrees as well. And so, 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. 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 3-degree difference. But remember, time zones are 15 degrees wide. Therefore, even though we have 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 3 or 4 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 in 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 the 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 eight o'clock, 8 p.m. Well, for us, we are 120 degrees divided by 15 degrees per hour equals 8, which means we are 8 hours, or 8 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, 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 a 8 p.m. would be 8 -- I'm going to use my fingers here -- eight, nine, ten, eleven, midnight, one, two, three, four. 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 0 degrees, the Prime Meridian. We draw 180, and 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 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. Well, 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 a 100 and 80. Well, if you go from a 100 to 0, that's 100 degrees. If you go from 0 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 gotta 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 solve 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 are going to arrive at the same exact answer. So, the first question is, how many degrees of separation is there from 80 to 180? It's 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, and 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 have 5 hours. So, now, if it's 10 p.m. here at 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 was 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, and so now you're going from Thursday to Friday. So, indeed, if you are 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 get 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 Date 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 and 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 Date 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. Yes? Does that make sense? Okay, let's do one more example problem, and then we'll move on, and then we'll ask yourself, why are we talking about time zones when we're still 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 10 p.m. and the day is a Wednesday. Let's say that you have a friend in Sydney, Australia. Sydney, Australia is at 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 10 p.m. 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 0 degrees -- Prime Meridian -- 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 10 p.m. 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 is 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 laboriously! Here's 120 degrees. We know 15 goes into 120, 8 times. So, there are 18 hours separating us from Sydney, Australia. Now, I can do the math. I can count from 10 p.m. 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. It's 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. Right? So, there are 6 hours separating us from Sydney if we count backwards, whereas there's 18 hours if we count forward. So, for me, subtracting 6 hours from 10 p.m. is really, really easy. So, 10 p.m. minus 6 hours means that it's 4 p.m. 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 4 p.m. Thursday when it's 10 p.m. 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. Let's add 18 hours to 10 p.m. We know that 12 hours added to 10 p.m. gets us to 10:00 a.m. We need to add 6 more hours to 10:00 a.m. So, 10, 11, 12, 1, 2, 3, 4. We arrived at 4 p.m. 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 10 p.m. Wednesday for us, it's 4 p.m. 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 zone discussion was longitude; figuring out what your longitude is. Remember, with latitude, we can simply look at the North Celestial Pole star here in the Northern Hemisphere, or the South Celestial Pole star if you're in the Southern Hemisphere, in order to determine your latitude. 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 it is 10 p.m. Wednesday at 120, and at that same time, at 150 east, it's going to be 4 p.m. Thursday. But what if I had different variables? What if, in fact, we had this: what if I said that at 120 west, it was 10 p.m. Wednesday. And what if I said that at some unknown location, the time was 4 p.m. Thursday? Now, I'm not telling you where 4 p.m. Thursday exists, I'm just telling you that somewhere in the world, it's 4 p.m. on Thursday. Can you tell me where 4 p.m. Thursday is located? And the answer is, yeah, we could do it. If we know that it is 10 p.m. 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 10 p.m. for me, and I want 4 p.m., well, 4 p.m. seems to be behind 10 p.m., so I'm going to go this way. And of course, the question is, well, how many hours are separating 10 p.m. from 4 p.m.? Well, we know that's 6 hours. So, we know that 4 p.m. is 6 hours behind 10 p.m. 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 and 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. But 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 3 p.m. 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 is 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 3 p.m. at the Prime Meridian. The question now is, noon. Is noon behind 3 p.m. or is it ahead of 3 p.m.? And we know of course that noon is going to be behind. So, it's over here. So, how many hours separate noon from 3 p.m.? 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 2 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 did we have back then? We've got pendulum clocks. We have sundials. 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 400 to 500 years ago. Pendulum clocks don't quite work very well, do they? And certainly sundials 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 led 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, and we'll talk about how this problem was solved, eventually and inevitably. And again, once we have the 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 next time.