Full transcript
0:00the last part of this unit is gonna
0:01focus on the different types of eclipses
0:04that we can have and we're actually
0:05going to start with the so called lunar
0:07eclipses in this case you can think
0:10lunar 4 Moon and Eclipse means we're
0:12eclipsing the moon
0:14we're actually blocking out the moon
0:16that's what a lunar eclipse is all about
0:18and it actually happens when the earth
0:21is between the Sun and the moon what
0:24happens is the shadow of the Earth from
0:27the Sun beaming its light on us we end
0:30up with a shadow and it actually passes
0:32across the moon and blocks out the light
0:36so this is the one case I was referring
0:38to before where you wouldn't have half
0:40of the moon lit and it's only because
0:42our shadow is blocking out the light so
0:44lunar eclipses the earth is between the
0:47Sun and the moon and our shadow actually
0:49blocks the light and goes across the
0:51moon now that being said there is
0:54different types of lunar eclipses you
0:56can get and that's because our shadow
0:58actually has slightly different regions
1:00the so called Umbra and penumbra the
1:04Umbra you can think is the inner shadow
1:06the center of the Earth's shadow the
1:10penumbra is the outer region of that
1:13shadow so the type of Eclipse we get
1:16really does depend on how that shadow
1:20pass across the moon well if the moon
1:23goes through the inner part the Umbra
1:26then we get a full total eclipse in the
1:30case of only part of it going across the
1:34Umbra then we get a partial clip where
1:36we'll see part of the moon blocked out
1:38much like it shows in this image however
1:41if the moon only is blocked by the outer
1:45shadow we may not really observe it in
1:47fact if you look at this image you can
1:48see it's quite hard to say even in the
1:51peak of this type of Eclipse you might
1:54not even notice that this Eclipse is
1:56occurring because no it doesn't get
1:59blocked there's not quite the same disc
2:01that's blocking it in like these two
2:03cases so it's not quite as spectacular
2:05an event and is much more hard to notice
2:07and it's all because of the shadow of
2:10Earth an inner shadow outer shadow
2:13thank Umbra again for enter and then
2:15penumbra I sometimes think it was put
2:17like partial or the outer part of the
2:20shadow now these lunar eclipses are
2:26quite spectacular we're going to talk
2:28about solar eclipses in a second we're
2:30going to see a lunar tips can be more of
2:33a shared experience in a way much more
2:35of the world gets to see it
2:37you can think births shadow is very
2:39large and can actually block the moon
2:41for a large portion of time in fact a
2:44total lunar can eclipse cannon lasts as
2:47long as about an hour and 40 minutes so
2:50it is a pretty spectacular event and
2:52lasts long time and a large portion of
2:54the world can see it all at once so it's
2:57a nice shared experience we're gonna see
2:59in the case of a solar eclipse tend to
3:01be quicker and it's a smaller path well
3:04you see what I mean in a second so lunar
3:07eclipse again you can see it impacts a
3:09huge portion and also it's worth
3:11mentioning as the Earth rotates during
3:14that time even more people get to see it
3:17but one thing that you may have seen or
3:21heard about is the idea that the moon
3:24might get this red appearance even
3:28during Eclipse it can get this reddish
3:31color and the reason is that whole
3:34refraction idea we were talking about
3:36earlier light bending as it goes from
3:39one medium to another well it's a light
3:41sure is being blocked by the Sun and
3:43this image shows it very nicely the
3:45sun's blocking the light the moon's
3:47there the lights not hitting it but some
3:50of the light gets refracted in our
3:52atmosphere and will actually end up
3:53bending towards the moon and we won't go
3:58into it in detail in this particular
3:59course but the red light would Bend more
4:03and actually we will look a little bit
4:05when we talk about the idea of
4:07spectroscopy so we'll get there but red
4:11light would Bend more and it would thus
4:14give the moon this reddish color so it
4:17doesn't get totally dark in those
4:19circumstances and you can see these
4:21images show some examples of that okay
4:24but what about this so
4:27called solar clips so solar well I mean
4:30we're actually gonna talk about that
4:32more on the next slide I guess and
4:33define it but really right now we happen
4:38to live in a time where we have a happy
4:40coincidence the Sun and the moon
4:42happened to have the same angular size
4:46if you remember angular size depends on
4:49how big something actually is and how
4:51far away it is with respect to the
4:55viewer you know in this case us well
4:58when we look at the Sun it happens to be
5:01about 400 times larger than the moon
5:04roughly in terms of diameter it also
5:06happens to be about 400 times further
5:08away so that means they have roughly the
5:12same angular size it's a very happy
5:15coincidence later on we'll see that the
5:17moon the distance it is from us has
5:19changed over time so we just happen to
5:21live in a time where it is about the
5:23same distance now why is it important
5:27well in the case of a solar eclipse
5:30again in this case you can think solar
5:33for Sun the Sun is being eclipsed much
5:36like this image shows we actually block
5:38out the light of the Sun with the moon
5:41the moon comes in between us and the Sun
5:45and it cast a shadow on us it actually
5:49blocks the light of the Sun from us well
5:55the reason that works is because they
5:57had the same angular size if they
6:00weren't we'd get a different effect
6:02depending and I mean earth if the moon
6:05was closer it could still eclipse it
6:07because they'd have a larger angular
6:08size than the Sun so it just means that
6:11if the moon was far enough away it
6:13wouldn't be the same angular size and
6:17thus wouldn't be able to block the Sun
6:18and this is actually gonna be something
6:20we're talk about more in a second thing
6:23is what we end up having is the same
6:26kind of Umbra and penumbra region the
6:29moon shadow just like the Earth's shadow
6:32has an inner Umbra region and an outer
6:34pun number region and the type of
6:37eclipse we see will actually be impacted
6:40by that
6:41and we'll come back to the idea in a
6:42little bit and show some examples but
6:46what you can see is where the shadow
6:49lands on earth that's the place where
6:52the eclipse would occur and in this case
6:55the shadow is much smaller compared to
6:58the lunar case and travels very quick
7:00you can see it talks about 1500
7:02kilometers an hour so even if you happen
7:04to be lucky enough for the inner shadow
7:07to pass across you the Umbra region and
7:10for you to see a total eclipse it would
7:13only last about 7 minutes at most so
7:16it's not quite the same experience as
7:19the solar eclipse and it's not quite as
7:20shared in fact you can see the shadows
7:21much smaller dot there so not that many
7:24people get to see it and often a lot of
7:26these clips is might happen to occur
7:28across water so might just be that the
7:31fish are lucky and get to see it so