1 00:00:02,440 --> 00:00:06,956 As I said, there is more moving parts in our model of the cosmos. 2 00:00:06,956 --> 00:00:12,603 There are more things that do not remain fixed on the celestial sphere, and just 3 00:00:12,603 --> 00:00:16,626 rotate with it, but yet, rise and set so are off the earth. 4 00:00:16,626 --> 00:00:20,860 And the next most conspicuous one after the sun, is our moon. 5 00:00:20,860 --> 00:00:24,009 like the sun, the moon moves along the celestial 6 00:00:24,009 --> 00:00:25,452 sphere. It's not fixed. 7 00:00:25,452 --> 00:00:28,107 for the same reason, the moon orbits the Earth. 8 00:00:28,107 --> 00:00:32,031 It orbits the Earth in the same sense that the Earth orbits the sun. 9 00:00:32,031 --> 00:00:36,013 And the same sense that, therefore, the sun appears to orbit the Earth, 10 00:00:36,013 --> 00:00:38,956 moving towards the east along the celestial sphere. 11 00:00:38,956 --> 00:00:41,380 But the moon orbits the Earth much faster. 12 00:00:41,380 --> 00:00:46,444 The moon completes a complete orbit around the earth in what is called a 13 00:00:46,444 --> 00:00:49,643 sidereal month which is 27 and a one-third days. 14 00:00:49,643 --> 00:00:54,440 that means that it's right ascension increases by 48 minutes per day. 15 00:00:54,440 --> 00:00:57,506 Compare this to the sun's measly four minutes. 16 00:00:57,506 --> 00:01:01,304 In other words the moon takes a month to orbit the earth. 17 00:01:01,304 --> 00:01:06,318 The sun takes a year to orbit the earth. A month is a twelfth of a year, four 18 00:01:06,318 --> 00:01:09,840 minutes is a twelfth of 48. The mathematics adds up. 19 00:01:09,840 --> 00:01:13,985 Now, as the moon orbits the Earth, it also rotates about its axis. 20 00:01:13,985 --> 00:01:18,973 We'll discuss next week why that is. But this means that we are always seeing 21 00:01:18,973 --> 00:01:22,211 the same side of the Moon as it rotates around us. 22 00:01:22,211 --> 00:01:25,515 there's the same side of the Moon that faces us. 23 00:01:25,515 --> 00:01:30,697 This means that while there is no such thing as the dark side of the Moon, there 24 00:01:30,697 --> 00:01:34,195 certainly is such a thing as the far side of the Moon. 25 00:01:34,195 --> 00:01:39,377 There are parts of the Moon that are never visible from Earth, and, those were 26 00:01:39,377 --> 00:01:40,498 first, Seen, or. 27 00:01:40,498 --> 00:01:45,408 Indirectly seen by human eyes. When the Soviet lunar space craft first 28 00:01:45,408 --> 00:01:47,560 orbited the moon. So this is why. 29 00:01:47,560 --> 00:01:50,856 When we look up at the moon, every time we see it. 30 00:01:50,856 --> 00:01:55,363 It has this familiar, look. We're always seeing the same side of the 31 00:01:55,363 --> 00:01:58,134 moon. Now, the, moon, therefore, moves to the 32 00:01:58,134 --> 00:02:01,651 east along the celestial sphere by 48 minutes a day. 33 00:02:01,651 --> 00:02:06,724 The sun moves to the east along the celestial sphere by four minutes a day, 34 00:02:06,724 --> 00:02:11,662 which means the moon moves faster to the east along the celestial sphere. 35 00:02:11,662 --> 00:02:15,720 The moon moves 44 minutes per day farther than the sun does. 36 00:02:15,720 --> 00:02:20,590 Which means moonrise relative to sunrise is delayed by 44 minutes a day. 37 00:02:20,590 --> 00:02:25,615 because the moon and sun are in this race, it takes the moon a little bit 38 00:02:25,615 --> 00:02:30,839 longer to complete a full rotation of the celestial sphere relative to the sun. 39 00:02:30,839 --> 00:02:34,080 To come to the same relative position as the sun. 40 00:02:34,080 --> 00:02:39,361 that takes what is called a synodic month, which is about 29 and a half days. 41 00:02:39,361 --> 00:02:43,840 This is the time say, from the moon being in alignment with the sun. 42 00:02:43,840 --> 00:02:48,368 And then the moon, running ahead of the sun, and catching up with the sun, 29 43 00:02:48,368 --> 00:02:51,851 one-half days later. As usual this takes a little bit longer 44 00:02:51,851 --> 00:02:56,031 than the 27 days that it takes the moon to complete a sidereal rotation. 45 00:02:56,031 --> 00:02:59,921 For the same reason that a solar day is longer than a sidereal day. 46 00:02:59,921 --> 00:03:04,275 The difference being more significant here because we're talking about a month 47 00:03:04,275 --> 00:03:07,874 rather than a day. now, the position of the moon relative to 48 00:03:07,874 --> 00:03:11,762 the sun controls two things. One is, when during the day it rises and 49 00:03:11,762 --> 00:03:13,912 sets? If the moon is at the same right 50 00:03:13,912 --> 00:03:18,325 ascension as the sun, then it rises at sunrise and sets at sunset because it's 51 00:03:18,325 --> 00:03:20,984 in the same direction in the sky as the sun is. 52 00:03:20,984 --> 00:03:25,509 Whereas, if the moon say is twelve hours away from the sun, it's 180 degrees away 53 00:03:25,509 --> 00:03:29,300 at right ascension, the moon will rise at sunset and set at sunrise. 54 00:03:29,300 --> 00:03:33,599 So, rising and setting times of the moon change throughout the synodic month. 55 00:03:33,599 --> 00:03:37,955 And, in addition to this of course, the moon's appearance in the skies changes. 56 00:03:37,955 --> 00:03:42,537 It goes through phases, and the best way to see this is the following interesting 57 00:03:42,537 --> 00:03:45,335 simulation. The best way to understand, how the 58 00:03:45,335 --> 00:03:50,030 moon's position relative to the sun gives us the phases of the moon is to just do 59 00:03:50,030 --> 00:03:52,434 it. And to just do it, you need a source of 60 00:03:52,434 --> 00:03:54,782 light. You can use a light bulb, or if you 61 00:03:54,782 --> 00:03:59,190 prefer, you can just use, the sun. Just go outside if you can't find a light 62 00:03:59,190 --> 00:04:00,335 bulb. Anything round. 63 00:04:00,335 --> 00:04:03,999 I have a white styrofoam ball here to play the role of the moon. 64 00:04:03,999 --> 00:04:09,159 And your head, to play the role of Earth. the view from your eyes will give you the 65 00:04:09,159 --> 00:04:14,133 view as seen by people on that side of earth facing the moon of what the moon 66 00:04:14,133 --> 00:04:17,066 looks like. And then, you simply turn around to 67 00:04:17,066 --> 00:04:21,380 perform a complete lunation. to give you a sense of what this looks 68 00:04:21,380 --> 00:04:25,675 like in case you're not going to do it yourself, though I strongly recommend it. 69 00:04:25,675 --> 00:04:30,134 what we have here is a, on the left side of the screen you'll see a setup of in 70 00:04:30,134 --> 00:04:33,777 the studio of me holding a moon and turning around in the relative 71 00:04:33,777 --> 00:04:36,768 configuration. Whereas on the right side you'll see the 72 00:04:36,768 --> 00:04:41,118 image of a gopro camera that's mounted to the Moon, so it shows you the image of 73 00:04:41,118 --> 00:04:44,524 the Moon as I see it. When the demonstration begins, the moon 74 00:04:44,524 --> 00:04:49,054 is, in between the sun and the Earth. It's on the line between sun and Earth. 75 00:04:49,054 --> 00:04:53,699 And so, the illuminated side of the moon, as we can see in this beautiful picture, 76 00:04:53,699 --> 00:04:56,835 faces the sun, and therefore, faces away from the Earth. 77 00:04:56,835 --> 00:05:01,190 I see the dark side of the, the moon. And then moon will be nearly invisible. 78 00:05:01,190 --> 00:05:05,719 As turn to my left to the east. slowly, the western side of the moon will 79 00:05:05,719 --> 00:05:09,087 become illuminated. And we'll see a growing crescent, until 80 00:05:09,087 --> 00:05:12,630 after I've turned 90 degrees. We'll see a waxing quarter moon. 81 00:05:12,630 --> 00:05:18,049 Since the moon is now six hours, or 90 degrees to the east of the sun, it will 82 00:05:18,049 --> 00:05:22,462 rise six hours after the sun. In other words, the waxing quarter moon 83 00:05:22,462 --> 00:05:25,513 rises about at noon and sets about at midnight. 84 00:05:25,513 --> 00:05:30,445 As I continue to turn to the, the east, the illuminated part of the moon will 85 00:05:30,445 --> 00:05:35,572 grow and become gibbous until at last, when I am twelve hours away from the sun. 86 00:05:35,572 --> 00:05:40,894 I will see a full round moon illuminated. we're going to have to switch cameras at 87 00:05:40,894 --> 00:05:45,437 some point, to give you the full view, because of studio limitations. 88 00:05:45,437 --> 00:05:49,561 Don't get confused by that. Twelve hours to the east to the sun, the 89 00:05:49,561 --> 00:05:52,289 full moon rises as sunset and sets at sunrise. 90 00:05:52,289 --> 00:05:56,499 And so the full moon is the only moon that is really up all night and only 91 00:05:56,499 --> 00:06:01,361 during the night and you'll note that to give us a view of the full moon, I had to 92 00:06:01,361 --> 00:06:05,986 tilt the moon's orbit I'm holding way above my head, we'll get back to that in 93 00:06:05,986 --> 00:06:09,038 a second. As I continue to turn to the east now the 94 00:06:09,038 --> 00:06:11,916 western part of the moon is losing the sunlight. 95 00:06:11,916 --> 00:06:14,733 I see the eastern part of the moon illuminated. 96 00:06:14,733 --> 00:06:18,810 It is a waning gibbous moon. And then when I reach 90 degrees to the 97 00:06:18,810 --> 00:06:23,367 sun again I have a waning quarter moon. The waning quarter moon which is six 98 00:06:23,367 --> 00:06:27,984 hours to the west of the sun will therefore rise six hours before the sun. 99 00:06:27,984 --> 00:06:31,161 In other words rise about at midnight, set about noon. 100 00:06:31,161 --> 00:06:35,936 This is the moon we see in the morning. Finally, as I continue turning to the 101 00:06:35,936 --> 00:06:39,293 east, the moon becomes closer to the sun in the sky. 102 00:06:39,293 --> 00:06:42,914 only the eastern edge of the moon is illuminated. 103 00:06:42,914 --> 00:06:47,589 I find a waning crescent moon. And after a full synodic orbit is passed, 104 00:06:47,589 --> 00:06:52,593 the moon is back in line with the sun. And again, I see only the dark side of 105 00:06:52,593 --> 00:06:54,963 the moon. And we're back to new moon. 106 00:06:54,963 --> 00:06:59,698 So what I hope you saw and I do encourage you to do it yourself. 107 00:06:59,698 --> 00:07:02,650 It's really fun and you can show it to your 108 00:07:02,650 --> 00:07:06,615 friends and family, is that over the course of a senotic 109 00:07:06,615 --> 00:07:11,656 month as the Moon orbits the Earth relative to the Sun the shape of the 110 00:07:11,656 --> 00:07:14,614 visible part of the Moon changes in the sky, 111 00:07:14,614 --> 00:07:19,588 as well as because of the relative position of Moon and Sun the rise time. 112 00:07:19,588 --> 00:07:25,099 So the New Moon when the Sun and the Moon are roughly at the same right ascension 113 00:07:25,099 --> 00:07:30,410 and the Moon is completely dark in the sky rises at sunrise and sets at sunset. 114 00:07:30,410 --> 00:07:37,399 the waxing quarter moon, when the moon is about six hours of right ascension to the 115 00:07:37,399 --> 00:07:41,138 east of the sun, rises six hours after the sun. 116 00:07:41,138 --> 00:07:47,722 So the waxing quarter moon rises at noon and sets around midnight, and is visible 117 00:07:47,722 --> 00:07:52,055 all afternoon. The full moon, where the moon is twelve 118 00:07:52,055 --> 00:07:54,808 hours of right ascension ahead of the sun, 119 00:07:54,808 --> 00:07:57,889 in other words on the opposite side of the sky. 120 00:07:57,889 --> 00:08:03,198 The full moon is the only time that the moon rises at sunset and sets at sunrise. 121 00:08:03,198 --> 00:08:06,410 And the waning quarter moon, where the moon is 122 00:08:06,410 --> 00:08:10,778 six hours to the west of the sun, or eighteen hours to the east, the moon 123 00:08:10,778 --> 00:08:15,024 being six hours to the west of the sun, rises six hours before the sun. 124 00:08:15,024 --> 00:08:18,786 In other words, rises about at midnight and sets about at noon. 125 00:08:18,786 --> 00:08:21,395 And the waning moon is visible all morning. 126 00:08:21,395 --> 00:08:25,520 So when you see the moon in the daytime, you should not be surprised. 127 00:08:25,520 --> 00:08:31,204 But should you ever see a full moon at noon, something has gone terribly wrong, 128 00:08:31,204 --> 00:08:35,133 so. Both the phases and the periodic change 129 00:08:35,133 --> 00:08:41,911 in moon rise and set times are completely understood in terms of this model where 130 00:08:41,911 --> 00:08:46,436 the moon reflects sunlight and what we see depends on the angle between the 131 00:08:46,436 --> 00:08:50,485 moon, the sun and the earth. You can stimulate this, you can go to the 132 00:08:50,485 --> 00:08:55,011 stimulation page of the University of Nebraska Lincoln and get less freedom 133 00:08:55,011 --> 00:08:59,060 version but I encourage you to construct take a light pall and a. 134 00:08:59,060 --> 00:09:02,285 Ball of some sort and make yourself a moon. 135 00:09:02,285 --> 00:09:07,162 So, this describes the moon's motion and the changes in the phase. 136 00:09:07,162 --> 00:09:13,539 you notice that at new moon and at full moon, I had to sort of adjust the moon. 137 00:09:13,539 --> 00:09:19,616 I was moon, orbiting the moon about my head, not in a horizontal orbit but in a 138 00:09:19,616 --> 00:09:25,468 tilted orbit, so that it went below the sun at new moon, and way high above the 139 00:09:25,468 --> 00:09:29,669 sun at full moon. So, this brings up the question of, what 140 00:09:29,669 --> 00:09:34,627 is the moon's declination? Indeed, the moon roughly orbits the Earth 141 00:09:34,627 --> 00:09:39,101 along the ecliptic. But it deviates from the ecliptic by five 142 00:09:39,101 --> 00:09:42,842 degrees. for comparison, the sun and the moon are 143 00:09:42,842 --> 00:09:48,343 about the same size in the sky. And both of them are about half a degree, 144 00:09:48,343 --> 00:09:53,330 so that the moon is, distant from the sun by up to ten times 145 00:09:53,330 --> 00:09:57,786 their radius. And so typically the moon passes well 146 00:09:57,786 --> 00:10:02,820 below or above the sun when its for example a new moon 147 00:10:02,820 --> 00:10:08,777 And so we have these two circles, now the ecliptic and the orbit of the moon around 148 00:10:08,777 --> 00:10:12,223 the Earth, the tilt between them is five degrees. 149 00:10:12,223 --> 00:10:18,037 The analog of the equinoxes, the hinges at which these two circles are hinged, is 150 00:10:18,037 --> 00:10:22,919 now called the nodes and the line connecting them, the line of nodes. 151 00:10:22,919 --> 00:10:28,374 And so the moon is at most five degrees away from the sun, which means among 152 00:10:28,374 --> 00:10:34,260 other things that as the sun moves north and south along the ecliptic, the by 23.5 153 00:10:34,260 --> 00:10:38,372 degrees in the summer. North and 23.5 degrees south in the 154 00:10:38,372 --> 00:10:42,451 northern winter. the moon, which is at most five degrees 155 00:10:42,451 --> 00:10:46,249 away from the sun pretty much follows the ecliptic. 156 00:10:46,249 --> 00:10:51,806 So the moon too, is higher in the sky in the summer, and lower in the sky in the 157 00:10:51,806 --> 00:10:52,580 winter. But. 158 00:10:52,580 --> 00:10:56,405 It generically is not exactly the same as the sun. 159 00:10:56,405 --> 00:11:01,836 It is only at exactly the same declination of the sun when it is along 160 00:11:01,836 --> 00:11:07,880 the line of nodes, just as the sun is on the celestial equator at the equinoxes. 161 00:11:07,880 --> 00:11:14,076 So twice in each of it's rotations about the Earth, the Moon lies along the lines 162 00:11:14,076 --> 00:11:17,748 of nodes. And in that case the Earth, the Sun and 163 00:11:17,748 --> 00:11:23,021 the Moon are in the same plane. Interesting things happen, of course, 164 00:11:23,021 --> 00:11:28,422 when the moon is in the line of nodes. And, it is either full or new. 165 00:11:28,422 --> 00:11:34,387 Let's see what happens then. In our first limation, I kept the moon 166 00:11:34,387 --> 00:11:39,222 way north, or up in this image from the sun when it was full and way south of the 167 00:11:39,222 --> 00:11:42,744 sun when it was new. what we're going to do now is see what 168 00:11:42,744 --> 00:11:47,580 happens to the full moon when the line of nodes is aligned with the direction to 169 00:11:47,580 --> 00:11:50,744 the sun. In other words the moon when it is twelve 170 00:11:50,744 --> 00:11:55,460 hours away in right ascension from the sun, is it exactly the sun's declination. 171 00:11:55,460 --> 00:11:59,303 And we start our demonstration with the waxing gibbous moon. 172 00:11:59,303 --> 00:12:04,428 And it waxes until it, is becoming full. And then, what we see is that, from the 173 00:12:04,428 --> 00:12:08,976 eastern side of the moon, a fuzzy shadow covers the surface of the moon. 174 00:12:08,976 --> 00:12:14,165 Of course, this is the shadow of my head. it would not be as shaggy were it the 175 00:12:14,165 --> 00:12:17,368 real Earth. But this is exactly the geometry for a 176 00:12:17,368 --> 00:12:20,700 total lunar eclipse. The Moon enters the shadow from. 177 00:12:20,700 --> 00:12:25,378 The west moving east and therefore it moves out of the shadow starting with its 178 00:12:25,378 --> 00:12:29,763 eastern edge first so when you see a lunar eclipse take place and then the 179 00:12:29,763 --> 00:12:33,740 moon uncover you're literally watch the moon orbit around the earth. 180 00:12:33,740 --> 00:12:37,828 About two weeks later or two weeks earlier, the Moon is now at the Sun's 181 00:12:37,828 --> 00:12:42,086 right ascension, we're getting a new Moon and the Moon is still at the same 182 00:12:42,086 --> 00:12:44,925 declination as the Sun and we'll see what happens. 183 00:12:44,925 --> 00:12:47,708 We pick up the story with a waning crescent Moon. 184 00:12:47,708 --> 00:12:51,966 The Moon moves in front of the Sun. When it is at the Sun's right ascension 185 00:12:51,966 --> 00:12:55,032 and declination the Moon in fact is obscuring the Sun. 186 00:12:55,032 --> 00:12:58,099 And remember we are on the day time side of the Earth. 187 00:12:58,099 --> 00:13:02,584 This is mid-day because it is a new Moon, we shouldn't be able to see it at all. 188 00:13:02,584 --> 00:13:05,695 In fact we don't. But then we don't see the sun either, we 189 00:13:05,695 --> 00:13:09,770 get complete darkness, in the middle of the day, and as the moon continues to 190 00:13:09,770 --> 00:13:12,827 move to the east, then we see this weird glow on the moon. 191 00:13:12,827 --> 00:13:17,010 This is an artifact of a shiny Gopro camera but even this is instructed. 192 00:13:17,010 --> 00:13:21,553 The lens of the camera is still eclipsed. But the bits of the camera on the right 193 00:13:21,553 --> 00:13:23,965 hand side are already shining in the light. 194 00:13:23,965 --> 00:13:27,443 And they're, you can see their reflection of that off the moon. 195 00:13:27,443 --> 00:13:31,706 This is what would happen if people living a few thousand kilometers to the 196 00:13:31,706 --> 00:13:34,399 west of you. Erected a huge shiny tower on earth, 197 00:13:34,399 --> 00:13:38,942 during a total solar eclipse, you might see the reflections from that shiny tower 198 00:13:38,942 --> 00:13:42,644 on the surface of the moon. And then, as the moon continues to move 199 00:13:42,644 --> 00:13:45,730 east, the sun is again revealed and it's bright mid-day. 200 00:13:45,730 --> 00:13:50,685 What did the model show us? Well, the moon's orbit is tilted five 201 00:13:50,685 --> 00:13:54,789 degrees with respect to the sun's orbit, the ecliptic. 202 00:13:54,789 --> 00:14:00,441 And so there are two points along the moon's orbit at which it is on the 203 00:14:00,441 --> 00:14:03,791 ecliptic. Now, as the sun orbits the earth or the 204 00:14:03,791 --> 00:14:08,883 earth orbits the sun there's going to be two times a year where the direction of 205 00:14:08,883 --> 00:14:14,142 this line of nodes coincides with the direction of the sun because the sun 206 00:14:14,142 --> 00:14:19,192 makes a complete circle and, at two antipodal points along the circle it 207 00:14:19,192 --> 00:14:24,171 coincides with the line of nodes. This gives us twice during an average 208 00:14:24,171 --> 00:14:29,571 year something called and eclipse season. And, the alignment is, need not be 209 00:14:29,571 --> 00:14:35,112 absolutely perfect because neither the sun nor the moon are point like objects. 210 00:14:35,112 --> 00:14:39,320 But there are two sort of one, one and a halve month periods, 211 00:14:39,320 --> 00:14:44,085 during each year. And in those one, one and a half month 212 00:14:44,085 --> 00:14:48,889 periods, eclipses may occur. May occur that is, when the moon is 213 00:14:48,889 --> 00:14:53,499 either full or new. So that it is at the correct part of its 214 00:14:53,499 --> 00:14:56,515 orbit, at the right ascension, either of the sun 215 00:14:56,515 --> 00:15:00,316 or opposite that of the sun, and now at the right declination. 216 00:15:00,316 --> 00:15:02,933 So twice a year we have an eclipse season. 217 00:15:02,933 --> 00:15:07,670 And during an eclipse season, you have usually between two or three eclipses. 218 00:15:07,670 --> 00:15:11,222 One of one kind and flanking it two weeks on either side. 219 00:15:11,222 --> 00:15:14,961 Two of the other kind. So you could can have a solar eclipse 220 00:15:14,961 --> 00:15:19,323 flanked by two lunar eclipses. Or a lunar eclipse flanked by two solar 221 00:15:19,323 --> 00:15:25,051 eclipses by the time, the next eclipse of either type would have occurred the next 222 00:15:25,051 --> 00:15:29,258 new moon for example. the sun is now to far from the line of 223 00:15:29,258 --> 00:15:34,265 nodes and you no longer get an eclipse. So typically no more than three per 224 00:15:34,265 --> 00:15:37,469 season and this should happen twice per year. 225 00:15:37,469 --> 00:15:42,343 In fact like everything else that is tilted the line of nodes wobbles. 226 00:15:42,343 --> 00:15:47,027 In other words the tilt between. The moons orbit and the ecliptic is 227 00:15:47,027 --> 00:15:52,760 always five degrees but its orientation wobbles and as is with the case with the 228 00:15:52,760 --> 00:15:58,241 earth it actually wobbles to the west Because it is orbiting to the east and so 229 00:15:58,241 --> 00:16:03,825 the two persesses to the west very slowly to the west once every eighteen point six 230 00:16:03,825 --> 00:16:08,811 years or so and this just means the eclipse year the year during which two 231 00:16:08,811 --> 00:16:13,841 eclipse seasons happen. is a little bit shorter than the full 232 00:16:13,841 --> 00:16:16,969 orbital year of the Earth. Only 346.6 days. 233 00:16:16,969 --> 00:16:23,524 And so, during each of those two eclipse seasons a newer full Moon might lead to a 234 00:16:23,524 --> 00:16:28,590 solar or a lunar eclipse. so new moon during eclipse season means 235 00:16:28,590 --> 00:16:34,474 the moon is in front of the Sun and at the right declination so you can have a 236 00:16:34,474 --> 00:16:37,380 solar eclipse. A full moon means the. 237 00:16:37,380 --> 00:16:42,600 Earth is between the moon and the sun, and the Earth's shadow will obscure the 238 00:16:42,600 --> 00:16:46,402 moon, just as my shaggy head obscured the moon in our model. 239 00:16:46,402 --> 00:16:51,365 Now, we need to get a handle on the relative sizes and distances of things to 240 00:16:51,365 --> 00:16:54,394 understand the difference of the two phenomena. 241 00:16:54,394 --> 00:16:58,906 So, the moon and the sun are almost precisely the same size, in angular 242 00:16:58,906 --> 00:17:02,450 terms, in the sky. If you think about it, the sun is much 243 00:17:02,450 --> 00:17:06,253 larger than the moon. From our small angular proximation, we 244 00:17:06,253 --> 00:17:10,121 know that it is much farther. The ratio of their sizes is almost 245 00:17:10,121 --> 00:17:15,427 precisely the ratio of their distances. So that both the moon and the sun appear 246 00:17:15,427 --> 00:17:19,672 in the sky to be about the same size. An interesting coincidence. 247 00:17:19,672 --> 00:17:24,447 What this allows is, when you have perfect alignment during new moon, the 248 00:17:24,447 --> 00:17:27,300 moon is able to completely obscure the sun. 249 00:17:27,300 --> 00:17:32,067 When you superpose two discs of the exact same size in the sky. 250 00:17:32,067 --> 00:17:36,478 But this requires perfect alignment. So perfect, that in fact, it will only 251 00:17:36,478 --> 00:17:41,192 obtain for a small region on Earth, if you move a little bit away from that 252 00:17:41,192 --> 00:17:44,576 region on Earth. Then the alignment is no longer perfect. 253 00:17:44,576 --> 00:17:47,718 The alignment is only perfect at one point on Earth. 254 00:17:47,718 --> 00:17:50,740 And when I say one point, I mean, a region on Earth, 255 00:17:50,740 --> 00:17:57,033 whose size is up to about 250 kilometers, that's a very small part of a planet with 256 00:17:57,033 --> 00:18:02,113 a radius of 6400 kilometers. And in that region where the moon's 257 00:18:02,113 --> 00:18:07,724 shadow completely obscures the sun, you get what is called a total eclipse. 258 00:18:07,724 --> 00:18:12,653 And we have here a beautiful time series of a a total eclipse. 259 00:18:12,653 --> 00:18:18,567 Notice that as time progresses in this, the moon seems to obscure the sun from 260 00:18:18,567 --> 00:18:22,102 the left. Whereas in mine, and then leave the sun 261 00:18:22,102 --> 00:18:27,441 to the right, whereas in my model, this was happening the other way around. 262 00:18:27,441 --> 00:18:33,218 The reason is that this eclipse in 2001 was observed near Zimbabwe, and, near 263 00:18:33,218 --> 00:18:38,484 Zimbabwe, you're in the southern hemisphere, which means, east and west 264 00:18:38,484 --> 00:18:43,750 are still the same, but looking, since the sun is, now to your north, 265 00:18:43,750 --> 00:18:50,266 The, the moon entering the sun from the west moving to the east is now from the 266 00:18:50,266 --> 00:18:55,096 left to the right. So north to the southern hemisphere 267 00:18:55,096 --> 00:19:01,460 viewers this time it makes sense to you. The moon is coming from the west to the 268 00:19:01,460 --> 00:19:07,286 east across the surface of the sun, it obscures the sun completely for a few 269 00:19:07,286 --> 00:19:11,135 minutes and then. Is seen to move away and what we're 270 00:19:11,135 --> 00:19:16,790 watching again is the moon orbiting the earth although also this place Zimbabwe 271 00:19:16,790 --> 00:19:22,022 is being moved along the surface of the earth by the earth's west to east 272 00:19:22,022 --> 00:19:26,900 rotation and moves out of the shadow as we'll see in a moment and 273 00:19:26,900 --> 00:19:30,613 noteworthy, when the moon completely obscures the sun, 274 00:19:30,613 --> 00:19:36,079 a region around the sun that appeared completely dark before, is suddenly seen 275 00:19:36,079 --> 00:19:38,058 to be brilliantly luminous. 276 00:19:38,058 --> 00:19:41,077 This is called the corona, the crown of the sun. 277 00:19:41,077 --> 00:19:46,088 It is not visible when, we see the, the, the, the sun itself, or the sun's disc, 278 00:19:46,088 --> 00:19:51,355 because the sun's disc is so bright that it blinds us to the, brilliants of the 279 00:19:51,355 --> 00:19:54,374 corona. But once you obscure the sun's disc, you 280 00:19:54,374 --> 00:19:59,689 can see that the region around the sun's disk is illuminated when we talk about 281 00:19:59,689 --> 00:20:04,879 the sun, we'll try to understand what this glowing crown is and how to observe 282 00:20:04,879 --> 00:20:08,671 it away from eclipses. A more common phenomenon than this 283 00:20:08,671 --> 00:20:14,060 beautiful totality is when the new moon occurs, the line of nodes are not exactly 284 00:20:14,060 --> 00:20:19,382 aligned with the sun and so the moon is a little bit south or a little bit north of 285 00:20:19,382 --> 00:20:23,766 the sun, and then the moon would have passed below or above the sun. 286 00:20:23,766 --> 00:20:28,897 Above if it were south since we're in the southern hemisphere, and below, if north, 287 00:20:28,897 --> 00:20:31,684 or the other way in the northern hemisphere. 288 00:20:31,684 --> 00:20:36,878 And then you will find a partial eclipse where not the full surface of the sun is 289 00:20:36,878 --> 00:20:39,602 obscured. But only, some fraction thereof. 290 00:20:39,602 --> 00:20:44,543 And this is far more common, because it requires a less sensitive alignment to 291 00:20:44,543 --> 00:20:48,778 give you a sense of, what the, the shadow of the moon on the 292 00:20:48,778 --> 00:20:53,053 earth looks like. here's a beautiful image taken from the 293 00:20:53,053 --> 00:20:57,970 Mir spacecraft on August eleventh, 1999. And, this is an image of earth, 294 00:20:57,970 --> 00:21:01,580 and what we're seeing on Earth is the moons shadow. 295 00:21:01,580 --> 00:21:06,787 So, the moon's shadow as I said, obscures the, the moon can obscure the entire sun 296 00:21:06,787 --> 00:21:11,618 in this inner circle of a radius of about 250 kilometers, up away, we see this 297 00:21:11,618 --> 00:21:15,006 partial shadow. These are regions where if you look up, 298 00:21:15,006 --> 00:21:19,774 you can see part of the sun, but, some fraction of the sun is obscured by the 299 00:21:19,774 --> 00:21:22,534 moon. You can see around it a little bit, and, 300 00:21:22,534 --> 00:21:25,608 so, this is called the Penumbra or Partial Shadow. 301 00:21:25,608 --> 00:21:29,749 People here see a total eclipse, people here see a partial eclipse. 302 00:21:29,749 --> 00:21:33,058 And, now remember, that underneath. This, shadow. 303 00:21:33,058 --> 00:21:39,248 This earth is rotating so that this shadow is effectively moving along the 304 00:21:39,248 --> 00:21:45,768 earth at some 1000 kilometers per hour and so each, individual location only 305 00:21:45,768 --> 00:21:49,470 gets a short period of totality. Now. 306 00:21:49,470 --> 00:21:54,517 This is what happens if alignment is perfect and if the moon and the sun are 307 00:21:54,517 --> 00:21:58,372 indeed exactly the same size in the sky. How can they change? 308 00:21:58,372 --> 00:22:02,194 Does the moon shrink? No, but the moon's orbit around the earth 309 00:22:02,194 --> 00:22:06,571 is not completely circular. The moon is sometimes a little closer to 310 00:22:06,571 --> 00:22:09,160 earth, and sometimes a little farther, when. 311 00:22:09,160 --> 00:22:12,760 Complete alignment occurs and the moon is on the. 312 00:22:12,760 --> 00:22:16,662 Farther part of its orbit, it's just a bit farther from Earth. 313 00:22:16,662 --> 00:22:21,396 Since its size didn't change, its apparent size in the sky is a little bit 314 00:22:21,396 --> 00:22:24,530 smaller. it's then smaller than the sun in the 315 00:22:24,530 --> 00:22:27,153 sky. And we get what is called an annular, 316 00:22:27,153 --> 00:22:30,736 annular eclipse. because you seen an annulus of sun, a 317 00:22:30,736 --> 00:22:34,702 ring of sun surrounding the moon. This shadow here is the moon. 318 00:22:34,702 --> 00:22:38,792 We see some of the, corona, and even some of the chromosphere 319 00:22:38,792 --> 00:22:44,158 around the sun but what we also see here is a little bit of a disc of the sun, the 320 00:22:44,158 --> 00:22:49,130 photosphere as it's called and the moon not completely obscuring it in this 321 00:22:49,130 --> 00:22:52,010 beautiful image of an annular solar eclipse. 322 00:22:52,010 --> 00:22:58,862 This is what happens when alignment, during eclipse season occurs at new moon. 323 00:22:58,862 --> 00:23:03,934 During full moon, what happens is that the earths shadow prevents sunlight from 324 00:23:03,934 --> 00:23:06,695 hitting the moon and the moon becomes dark. 325 00:23:06,695 --> 00:23:11,510 The moon as I said, is moving to the east, it enters earth's shadow from the 326 00:23:11,510 --> 00:23:14,014 west. Again, the eclipse can be total or 327 00:23:14,014 --> 00:23:17,161 partial and Depending on the quality of the 328 00:23:17,161 --> 00:23:20,025 alignment. And, you can get a penumbral eclipse, 329 00:23:20,025 --> 00:23:25,004 where the moon is only in partial shadow. In other words, where, from the moon, you 330 00:23:25,004 --> 00:23:29,634 can see some part of the sun. But, some of the sunlight is blocked by 331 00:23:29,634 --> 00:23:32,521 the Earth. Just as pieces of the Earth were in 332 00:23:32,521 --> 00:23:35,973 partial moonshadow. then the moon just slightly dims. 333 00:23:35,973 --> 00:23:40,304 It's kind of hard to even notice it. But when you get a total eclipse, 334 00:23:40,304 --> 00:23:45,074 when some part of the moon's surface is completely obscured from the sun, 335 00:23:45,074 --> 00:23:49,468 then this is the beautiful image you see. Now this, I'm not sure, surprise you. 336 00:23:49,468 --> 00:23:52,669 I mean, perhaps some sunlight is reaching over here. 337 00:23:52,669 --> 00:23:55,870 Maybe I didn't time this photo to precise totality. 338 00:23:55,870 --> 00:24:01,413 But where is the light coming from that allows us to see the moon this side of 339 00:24:01,413 --> 00:24:04,928 the moon at all. This side of the moon is, in fact, in 340 00:24:04,928 --> 00:24:09,946 total Earth shadow, you cannot see sun, the sun, from this area of the moon. 341 00:24:09,946 --> 00:24:13,181 What you can see in the sky is a bright, glowing Earth. 342 00:24:13,181 --> 00:24:17,793 Remember, the Earth is bigger than the moon, so it, handily obscures the sun. 343 00:24:17,793 --> 00:24:22,586 It's much easier to arrange a total lunar eclipse than a total solar eclipse. 344 00:24:22,586 --> 00:24:26,600 And also, a solar eclipse, a lunar eclipse, when it occurs, is visible, 345 00:24:26,600 --> 00:24:30,434 since the moon is dark, to anybody on Earth who can see the moon. 346 00:24:30,434 --> 00:24:34,849 So whenever it occurs during your nighttime, you can see the moon eclipse. 347 00:24:34,849 --> 00:24:39,879 It's a far less delicate arrangement than the solar eclipse which only blocks out 348 00:24:39,879 --> 00:24:42,640 the sun for a small fraction of the earth. 349 00:24:42,640 --> 00:24:46,490 but the source of this red illumination of the moon. 350 00:24:46,490 --> 00:24:51,867 the answer is here on the slide. Comes from light defracting through 351 00:24:51,867 --> 00:24:56,226 Earth's atmosphere. Why light defracting through the Earth's 352 00:24:56,226 --> 00:25:01,820 atmosphere would give the moon this famous wine red color is something we'll 353 00:25:01,820 --> 00:25:06,179 have to investigate. This is a picture actually taken at our 354 00:25:06,179 --> 00:25:10,684 observatory, in the, solstice eclipse of, December 2010. 355 00:25:10,684 --> 00:25:15,047 And so, this is Pretty image and lunar eclipses are 356 00:25:15,047 --> 00:25:19,195 easier to find and observe. I encourage you to enjoy them. 357 00:25:19,195 --> 00:25:24,361 A few more fun facts about the moon, while we're discussing the moon. 358 00:25:24,361 --> 00:25:28,320 So two well known things that people appear to observe. 359 00:25:28,320 --> 00:25:33,377 One is that when you see the moon rising or setting, it's near the horizon, it 360 00:25:33,377 --> 00:25:36,792 appears larger. This, it turns out, is a psychological 361 00:25:36,792 --> 00:25:39,682 illusion. Taking pictures of the moon with a 362 00:25:39,682 --> 00:25:44,674 camera, you can measure its angular size. And in fact, if anything, the strange 363 00:25:44,674 --> 00:25:49,929 optical effects make it appear a little bit smaller in the sky near the horizon. 364 00:25:49,929 --> 00:25:54,789 But there are various psychological theories, near, the comparison to other 365 00:25:54,789 --> 00:25:59,662 objects nearby, angular corrections. I'm not an expert on the psychology. 366 00:25:59,662 --> 00:26:05,196 But, it does appear to us that the rising full moon seems huge when it's on the 367 00:26:05,196 --> 00:26:08,278 horizon, and small when it's high in the sky. 368 00:26:08,278 --> 00:26:13,295 But, this is completely in our heads. On the other hand, there is this other 369 00:26:13,295 --> 00:26:18,287 famous illusion, where, which is called seeing the old moon in the new moon's 370 00:26:18,287 --> 00:26:21,111 arms. Which is, that when you see a crescent 371 00:26:21,111 --> 00:26:23,708 moon, where only a fraction of the moon is 372 00:26:23,708 --> 00:26:27,998 illuminated, then you can sometimes imagine to yourself that your mind 373 00:26:27,998 --> 00:26:32,963 completes the full disk of the moon, and you can see the part of the moon that is 374 00:26:32,963 --> 00:26:35,109 not illuminated by sunlight. And, 375 00:26:35,109 --> 00:26:38,725 this one is a physical effect. Here's a camera capturing it. 376 00:26:38,725 --> 00:26:42,894 Here's the illuminated side of the moon, so the sun is down that way. 377 00:26:42,894 --> 00:26:47,491 This side of the moon is dark, sunlight cannot reach it, and yet I see it, so 378 00:26:47,491 --> 00:26:52,211 there's sunlight hitting it, and there is a reason, of course, is that when the 379 00:26:52,211 --> 00:26:55,350 moon is a crescent, if you are on the moon, 380 00:26:55,350 --> 00:27:01,015 set up the configuration in your head, you would see that there would be a full 381 00:27:01,015 --> 00:27:04,959 Earth in the sky. And viewing the, the full Earth in the 382 00:27:04,959 --> 00:27:10,051 lunar sky means that there is bright Earth light illuminating the moon. 383 00:27:10,051 --> 00:27:15,788 So the light that is hitting the moon, by which we are seeing this moon is Earth 384 00:27:15,788 --> 00:27:22,055 light, in other words light that was emitted by the sun, reflected by the 385 00:27:22,055 --> 00:27:24,917 earth, hit the moon, reflected off the moon and 386 00:27:24,917 --> 00:27:29,259 came in the appeture of this camera. So the fact that you can see the dark 387 00:27:29,259 --> 00:27:32,956 part of the moon as a crescent is not psychological, it is true. 388 00:27:32,956 --> 00:27:37,768 You can only see this when the moon is a crescent basically because once the moon 389 00:27:37,768 --> 00:27:41,759 becomes too bright, it blinds us, it dazzles us and we can't see this. 390 00:27:41,759 --> 00:27:44,499 Also, the larger the illuminated part of the 391 00:27:44,499 --> 00:27:47,723 moon, the less of the face of the Earth is illuminated. 392 00:27:47,723 --> 00:27:52,560 Of course, when we have a full moon, then people on the moon would see a new Earth. 393 00:27:52,560 --> 00:27:56,920 The phases are complementary. And so for both of those reasons, we only 394 00:27:56,920 --> 00:28:01,461 see this when the moon is a crescent. So we've made some progress in 395 00:28:01,461 --> 00:28:06,800 understanding astronomical phenomenon. The way they change periodically. 396 00:28:06,800 --> 00:28:12,515 We've got the sun, we've got the moon. Let us close this discussion with some 397 00:28:12,515 --> 00:28:16,801 interesting relations between astronomy and time keeping. 398 00:28:16,801 --> 00:28:22,516 I've told you that units of time kept being defined in terms of the Earth's 399 00:28:22,516 --> 00:28:25,900 rotation and orbit. This is not a coincidence. 400 00:28:25,900 --> 00:28:29,058 We want our time to match what's going on. 401 00:28:29,058 --> 00:28:30,240 We want. Six a.m. 402 00:28:30,240 --> 00:28:36,050 On our clock to be solar sunrise, because that's the time we go out and plant, 403 00:28:36,050 --> 00:28:40,842 things, work in our fields. And so, our 24 hour days are adjusted to 404 00:28:40,842 --> 00:28:45,295 be the mean solar day. Are months that, twelve months into which 405 00:28:45,295 --> 00:28:49,264 we traditionally divide the year, are approximately lunar. 406 00:28:49,264 --> 00:28:52,886 the synodic lunar month is 29 and a half days. 407 00:28:52,886 --> 00:28:57,830 Our months are a bit longer than that. But, and this allows for the rare 408 00:28:57,830 --> 00:29:03,123 phenomenon of two full moons. Falling in the same month, which is what, 409 00:29:03,123 --> 00:29:07,250 colloquially, is called a blue moon. It's a rare phenomenon. 410 00:29:07,250 --> 00:29:12,373 It requires a full moon right at the beginning of the month, but it does 411 00:29:12,373 --> 00:29:15,717 happen. Our definition of a year is designed to 412 00:29:15,717 --> 00:29:18,208 match the orbit. A year is 365 days. 413 00:29:18,208 --> 00:29:23,720 the Earth, orbits the sun once. every 365.2564 days, a little over a 414 00:29:23,720 --> 00:29:28,108 quarter of a day, This is sidereal orbit, in other words, 415 00:29:28,108 --> 00:29:34,387 this is the time that it takes the Earth to return to the same position in the sky 416 00:29:34,387 --> 00:29:40,666 relative to the sun, or the time it takes the sun to return to the same position in 417 00:29:40,666 --> 00:29:44,525 our sky, relative to the stars. Hence, sidereal orbit. 418 00:29:44,525 --> 00:29:50,577 The first thing we observe is, that the year is not, unfortunately, an integral 419 00:29:50,577 --> 00:29:53,073 number of days. This is a problem. 420 00:29:53,073 --> 00:29:59,073 It means that, Since we, our days turn over every 24 421 00:29:59,073 --> 00:30:06,557 hours, that every four years You're timekeeping if you have a 365 day 422 00:30:06,557 --> 00:30:10,288 year. Then every four years, you are off by a 423 00:30:10,288 --> 00:30:14,274 day relative to the Earth's orbit. So who cares? 424 00:30:14,274 --> 00:30:19,786 Well, accumulate those days for 180 intervals of four years. 425 00:30:19,786 --> 00:30:26,741 And now, you are off by 180 days which is half a year which means that now January 426 00:30:26,741 --> 00:30:32,101 corresponds to northern summer. This is very inconvenient if you're 427 00:30:32,101 --> 00:30:37,868 trying to plan agriculture and we have a solution to this, right, this was 428 00:30:37,868 --> 00:30:43,787 discovered by Julius Caesar, or in his time, and it was his legislation that 429 00:30:43,787 --> 00:30:48,188 added leap years. Once every four years we add another day, 430 00:30:48,188 --> 00:30:53,727 making that year 366 days long. The average year is now 365 and a quarter 431 00:30:53,727 --> 00:30:59,418 days long, and now we never drift more than a day off from having our orbit 432 00:30:59,418 --> 00:31:03,613 match our calender. However, this is not completely precise 433 00:31:03,613 --> 00:31:06,719 enough. In fact what we want our calendar to 434 00:31:06,719 --> 00:31:11,096 match is the seasons. The seasons have to do with the relation 435 00:31:11,096 --> 00:31:16,602 between the earth's position relative to the Sun and not the stars, but the 436 00:31:16,602 --> 00:31:22,249 direction of the tilt of the celestial north pole or the terrestrial north pole. 437 00:31:22,249 --> 00:31:25,990 And remember that, that wobbles to the west, 438 00:31:25,990 --> 00:31:29,999 rather than to the east. So a, a complete rotation. 439 00:31:29,999 --> 00:31:36,136 Where, between solstice and solstice, or equinox and equinox, is a little bit 440 00:31:36,136 --> 00:31:40,473 shorter than the sidereal year, rather than longer. 441 00:31:40,473 --> 00:31:46,119 This is called the tropical orbit. The mean time between solstice and 442 00:31:46,119 --> 00:31:51,201 solstice is 365.2422 days. Remember that the precession is very 443 00:31:51,201 --> 00:31:57,283 slow, takes 26,000 years. So it's not a big effect over a year, but it does make 444 00:31:57,283 --> 00:32:01,494 the mean tropical year a bit shorter than 365.25 days. 445 00:32:01,494 --> 00:32:07,421 This was understood in the sixteenth century and led to the correction from 446 00:32:07,421 --> 00:32:13,191 the Julian to the Gregorian calendar, correcting for the deviation between 447 00:32:13,191 --> 00:32:17,636 0.2422 and 0.25 required removing some of the leap years. 448 00:32:17,636 --> 00:32:21,346 That is why on Centuries that are not millennia, in 449 00:32:21,346 --> 00:32:26,209 other words years whose number divides 100 but not 1,000, we do not add a leap 450 00:32:26,209 --> 00:32:29,326 year. We do not add a 29th day to February, 451 00:32:29,326 --> 00:32:33,878 those years are only 365 days long. The average year is a little bit less 452 00:32:33,878 --> 00:32:38,491 than 365 and a quarter days long. In fact it's close enough to this number 453 00:32:38,491 --> 00:32:42,918 on average that it will be a millennium before we have to make another 454 00:32:42,918 --> 00:32:45,973 correction. And so we definitely use astronomical 455 00:32:45,973 --> 00:32:51,104 phenomena to adjust our clocks and our calendars, not for any silly reason but 456 00:32:51,104 --> 00:32:54,320 because astronomical phenomena govern our life, 457 00:32:54,320 --> 00:32:58,820 and we need our, timekeeping to match that. 458 00:32:58,820 --> 00:33:02,935 It's quite an elaborate universe we're starting to build around us. 459 00:33:02,935 --> 00:33:06,314 We have the celestial sphere where the stars are fixed. 460 00:33:06,314 --> 00:33:11,105 we have a solar sphere that rotates relative to the celestial sphere about 461 00:33:11,105 --> 00:33:14,300 this tilted axis. So that the sun can move along the 462 00:33:14,300 --> 00:33:17,432 ecliptic. We have a lunar sphere tilted relative to 463 00:33:17,432 --> 00:33:21,180 the ecliptic around which the moon moves a little bit faster. 464 00:33:21,180 --> 00:33:26,867 Moreover, all these, tilted trajectories are also wobbling to the west, one very 465 00:33:26,867 --> 00:33:31,572 slowly, every 26,000 years. The moon's a little bit faster, every 2.6 466 00:33:31,572 --> 00:33:34,451 years. This is very elaborate, but it does 467 00:33:34,451 --> 00:33:38,945 explain everything we see. The alternations of day and night, the 468 00:33:38,945 --> 00:33:42,737 phases of the moon, eclipses, seasons. Almost everything. 469 00:33:42,737 --> 00:33:46,880 Let's go to Athens and see what we might have been missing. 470 00:33:48,260 --> 00:33:52,342 We're back to our favorite picture of the sky in Athens. 471 00:33:52,342 --> 00:33:58,155 And I've allowed the software to show us the ecliptic and we see the 23.5 degree 472 00:33:58,155 --> 00:34:03,275 tilt, relative to the celestial equator. We the, see the intersection of the 473 00:34:03,275 --> 00:34:07,773 ecliptic and the equator here at the prime meridian in Pisces. 474 00:34:07,773 --> 00:34:12,568 We still live in the age of Pisces and We see that the part of the ecliptic 475 00:34:12,568 --> 00:34:15,836 that's visible at night is mostly the part north of the equator. 476 00:34:15,836 --> 00:34:19,461 That's reasonable because, end of November, the sun is well south of the 477 00:34:19,461 --> 00:34:21,962 equator. The part of the ecliptic that lies south 478 00:34:21,962 --> 00:34:24,260 of the equator is what we see during the day. 479 00:34:24,260 --> 00:34:27,115 So far, so good. the other thing you'll notice. 480 00:34:27,115 --> 00:34:30,447 And I'm sure if you, tried to run your own simulation. 481 00:34:30,447 --> 00:34:34,730 And certainly, if you went outdoors, you will have realized by now that I 482 00:34:34,730 --> 00:34:37,050 fudged. I suppressed some things in the 483 00:34:37,050 --> 00:34:41,214 simulations I've been showing. In particular, the two brightest objects 484 00:34:41,214 --> 00:34:44,367 in the sky were omitted from my discussion up to now. 485 00:34:44,367 --> 00:34:48,115 One of these, the brightest of them is this waning gibbous moon, 486 00:34:48,115 --> 00:34:51,030 which we see here. A waning gibbous moon, remember, 487 00:34:51,030 --> 00:34:52,982 is a moon that is past full. 488 00:34:52,982 --> 00:34:57,832 And so, it rises after sunset, and at 9:00 p.m., we're still seeing it 489 00:34:57,832 --> 00:35:03,736 in the eastern sky, so we expected that. the moon happens to lie very close to the 490 00:35:03,736 --> 00:35:07,743 ecliptic right now. It could have deviated, remember, by as 491 00:35:07,743 --> 00:35:12,717 much as five degrees. we also have, the next brightest object 492 00:35:12,717 --> 00:35:16,083 in the sky is the beautiful planet, Jupiter. 493 00:35:16,083 --> 00:35:21,797 and we have not brought up planets. And once we allow that, we see that 494 00:35:21,797 --> 00:35:27,042 scattered along this ecliptic are a few others, non-star objects, 495 00:35:27,042 --> 00:35:33,304 Neptune, Uranus, and the asteroid, Ceres. And, if we look at this image over time. 496 00:35:33,304 --> 00:35:38,940 We would notice that, like the moon and the sun, the planets move as well, 497 00:35:38,940 --> 00:35:43,815 which means we're going to have to start next week by adding even more moving 498 00:35:43,815 --> 00:35:47,508 parts to our universe, and enriching what we know, 499 00:35:47,508 --> 00:35:52,385 and eventually leading us to a much, much deeper understanding.