now let's move on right the moon's orbit this is not going to be to scale but check it out you've got the moon orbiting around earth we all know that we started with it at the beginning of this lecture now you get a little orientation here right you know that the moon's orbit around earth is elliptical okay so there are times in which the moon is actually closer to the earth here or further away from the earth now keep in mind this is not to scale it's actually quite circular but it's slightly elliptical therefore you end up with differences in the time so perigee okay perigee means the moon is close to the earth apogee means uh oh the moon is far away from the earth okay you can see the difference right it's roughly 25 000 mile difference and that affects the gravitational pull so if you think about are again r is smaller here so therefore your gravitational pull would be larger and r is much larger here therefore your gravitational pull will be much less this leads to a 20 percent larger tidal range at perigee perigee and apogee i definitely want you to know those terms okay no perigee and no apogee know what they mean know that perigee means the moon is close to the earth therefore we have larger tidal ranges or apogee that the moon is further away from the earth and therefore we have less significant tidal ranges you need to know those okay another elliptical orbit we got to think about is the earth around the sun okay so there are times when the earth is closer to the sun or further away from the sun we would call this perihelion okay perihelion close to the sun or aphelion far away from the sun notice again right perihelion aphelion if you go back to these ones perigee apogee okay so for the for the moon it's g right but it's the same parry and app right perry and app perry means close at means far away helion means sun and g means moon so there you go perihelion close appealing on close to the sun or far away from the sun in this case appealing on far away from the sun now it is a nearly circular orbit okay but it's three percent further so it's it is a little bit farther all right what this leads to uh are larger larger tidal ranges at you guessed it perihelion okay so for the same reason that perigee has larger larger tidal ranges remember by 20 percent um this case with the earth being close to the sun during perihelion we end up with slightly larger tidal ranges so both elliptical orbits the moon around the earth and then the earth around the sun cause tidal changes all right there's some more factors for you remember there's 200 factors perihelion always happens on january 3rd and uh it's on a 365 and a half day cycle so you know there you go now there's also an effect of declination so what i mean by this is that the bulge right is not always right at the equator or up by the poles and this remember the earth is spinning right the whole time it's spinning so if you look at this this bulge right of high tide while this earth is spinning around if the moon is up here the bulge is going to look just like that okay but the moon just like how we're tilted 23.5 degrees right from the sun the moon is tilted like 28.5 degrees from us so therefore it ranges between 28.5 degrees north and 28.5 degrees south so this bulge all this is saying is that this bulge here the the apex of it can range from 28.5 degrees north or it could bulge all the way down here to 28.5 degrees south okay and then therefore you'd end up with different tides during different seasons okay so the sun we're tilted 23.5 degrees to to the sun our earth is tilted in that way and as we rotate around it like you could imagine right during um uh you know the summer months for us the earth um is tilted towards the sun so the sun is coming in at like 23.5 degrees right and it's hitting us more readily than than anywhere else really is is right here in san diego but um in between there's a moment where uh the equator is the thing that's pointed directly towards the sun and therefore it's getting most of the sun and then it's also way down here at 23.5 degrees south as the earth rotates in its tilted fashion around the sun same case here for the moon and earth except now it's 28.5 degrees and we're just talking about tidal bulges so this major bulge here could actually be down here and if it were down here then the other bulge will be back over here okay so just to let you know here's another factor okay i don't want you to get too caught up in this but this is the effect of what we call declination right so the angle from one kind of part of the gravitational equation m1 and m2 how these angles change all right now let me give you an instance all right so we got to take in all those factors and we got one very bad weekend in january 26 20 through the 28th in 1983. well we had some astronomical factors all right we were near perihelion which means oh yeah right near january 3rd right which is when the earth in its elliptical orbit around the sun is closest to the sun all right we also have perigee on the 28th perigee means that the moon in its elliptical orbit around the earth was close to earth so perihelion and perigee mean that we're going to increase the tidal ranges and it happened to be full moon on the 28th full moon means spring tide so all three factors here leading to large tides okay perihelion perigee and full moon all leading to large tides the meteorological factors was that there was a large storm that came ashore at high tide and that el nino that year which we know about now right warm surface temperatures slosh back onto our west coast of the united states or california um and the trade winds relax right we know that that caused an eight inch rise in sea level so now we've got sea level has been up more than normal eight inches higher we got a big storm and we have all these astronomical factors working against us what do you think is going to happen yep well this is january 27 1983 and this is of crystal pier and you can see that it is broken and shattered and that the waves came in and ripped this pier apart and it caused a hundred million dollars in damage right and i'm sure that all of ob was flooded all because of this storm and all these factors and that's something that we could predict right and we could actually um get people out of the situation tell them hey you know what flooding flooding's gonna happen we need to get you out of here and uh luckily we can predict the tides so real tides right all the tides that we've got uh that we see on the coastline remember 400 different factors i just showed you the factors that i want you to know about but there's a lot going on right so they're they're modified also by coastal shape not just the moon not just the earth and the sun system not just the elliptical orbits not just which lunar phase right it's also modified by coastal shape friction with sea floor also modifies tidal bulges it's also influenced by other planets especially jupiter and saturn how come because their masses are so big okay however these tides can be accurately predicted and there is machine by kelvin right in 1872 this was the first tied predicting machine okay right here that's what it looked like pretty amazing now why are spring tides highest when the moon is near perigee let's get a a little um you know check in with you do you did you hear what i said do you know what i mean now right so let's see what are our options spring times what is spring tides mean spring tides mean that uh right the earth the moon and the sun are all aligned and the moon would either be a new moon or a full moon okay cool and then moon is near perigee okay so moon near perigee perigee means that the moon and the earth are closest to each other during that elliptical orbit so let's let's think about that a process of elimination the moon is moving fastest in its orbit at that time no no we we're not going to talk about changing rates of the orbit nope the moon is farther from earth well we already know this is wrong because perigee means the moon is closest to earth okay see the difference between solar and lunar tides is maximum at this time well no that's just that's just talking random stuff that makes no sense okay d the moon and the sun are lined up well i like that thought so far the moon and the sun are lined up okay hopefully that means that the earth is lined up too i don't know okay e the moon is closer to the earth oh that means yep the y is near perigee perigee means the moon is closer to the earth yes so gravitational force is greatest right is greater there it is why are spring tides highest when the moon is near perigee e the moon is closer to the earth so gravitational force is greater there you go