Full transcript
0:00the next thing we want to consider is
0:01the idea of seasons and why we have
0:04because there's actually quite a bit of
0:06misconception around it most commonly
0:09people think we have different seasons
0:11because of the different distance we are
0:13from the Sun and now it's true that we
0:16do vary our distance from the Sun but
0:19it's not by a very large amount it's
0:21only about 3% and well for thus those of
0:25us in North America we're actually
0:27closer to the Sun during our winter than
0:30during our summer so clearly that's not
0:33the case so what's the reason well it's
0:36because of the tilt of our axis of
0:39rotation
0:40remember that's that big metal spike if
0:42we jabbed it through the earth and we're
0:44rotating spike that's the thing we're
0:47rotating about well you can see that's
0:51our big metal spike in the image right
0:52here and it's tilted at about 23.5
0:56degrees so that means how we are facing
1:00the Sun changes from time to time if
1:04we're in the summer for North America
1:06our part of the planet is actually
1:10tilted more towards the Sun and then in
1:12our winter we are actually tilted away
1:14from the Sun and thus it's the opposite
1:17and the other side of the planet well we
1:21also have these vernal equinox autumn
1:24equinoxes the idea is that equinoxes for
1:27equal and in fact remember vernal
1:29equinox was where we defined our zero
1:32point for our right ascension so it's
1:36important idea it's that time of day a
1:38year that March 21st where the Sun is it
1:41gives us an idea a zero point for where
1:43to compare and the reason these are
1:45equal or equinoxes in the name is
1:47because you can see that tilt doesn't
1:50matter in this case the tilt doesn't
1:53actually go towards or away from the Sun
1:55so both both the north and south
1:58hemisphere are getting the same amount
2:01of sunlight at these two times great
2:05well let's keep going why does that
2:08really account for the seasons again I
2:09mean we're tilted towards the Sun
2:11well that has a couple different effects
2:15for one thing the angle of the sunlight
2:17is actually different in those two
2:19different times in our summer the
2:21sunlight is coming more directly on us
2:24in fact let's go back to that image for
2:26a second and you can see it be more
2:29directed at us
2:30whereas here you'd be more or again
2:33leaning away if you will and so the
2:35sunlight actually gets spread out over a
2:37bigger angle the same amount of sunrays
2:39being spread out over a larger area
2:41means it's not gonna heat up as much so
2:44in the winter the Sun that gets more
2:46spread up and that's not as effective
2:48but even more than that the sun's
2:51position if we were going back to the
2:53idea of the celestial sphere we could
2:56think of how the Sun is rotating around
2:57us I remember of course it's not
2:59actually rotating around us we're
3:01rotating relative to it but where it
3:03would appear in the sky changes
3:05throughout the year for us in North
3:09America during the summer months is much
3:11higher up and thus as it goes around
3:15due to our rotation let's just go the
3:18proper way of explaining it if we think
3:20of how it seems to go across the sky due
3:23to Earth's rotation well during the
3:26summer it has further to travel so it's
3:28out longer we have a day with more
3:31sunlight by the time we're in the winter
3:35the Sun is much lower
3:37from our perspective and thus it's not
3:39out for as long so not only is the
3:42sunlight more direct but it's also out
3:44for longer and that's why in the summer
3:46it's warmer nothing to do with the
3:48distance from us to the Sun again we're
3:51actually further in the summer it's all
3:54about how we are tilted relative to
3:57orbiting around the Sun now the strength
4:01of this effect depends on where you are
4:04if you're in the northern hemisphere of
4:06course versus the southern hemisphere
4:08it's completely different our summer
4:09their winter the opposite but even how
4:14much you notice the seasons really
4:16depends on where you are and if you look
4:18at this image right here you can see
4:21where this is the two different times
4:22one we're tota towards the Sun one will
4:24toss it away
4:25but if you look at see the equator the
4:28effects are not gonna be as strong the
4:31sunlight still gonna be fairly direct in
4:33both of these cases and this amount of
4:36sunlight is gonna be very similar so
4:38they won't notice the seasonal change as
4:40drastically as someone who is very far
4:43north in fact if you were say really far
4:45north you can get to certain times of
4:47the year where the Sun won't go down
4:49it's so high up in the sky it won't go
4:52down for months and months and once at a
4:55time so you might have six months of
4:57sunlight six months of darkness
4:58depending on where you are it really
5:01depends on how far north and south on
5:04our planet you are and how big of an
5:07effect you notice from season to season
5:11of course there is a couple real-world
5:14complications your book points to and I
5:16felt like it would be important to talk
5:17about them for one thing when people
5:19talk about how long the days are well
5:21our days are actually longer than
5:24science would normally predict I mean if
5:27you take all the things into account no
5:28science will get it right but if you're
5:31just looking at where the Sun is in the
5:34sky from one time to another our days
5:38would seem longer and it's because of
5:40this idea of a refraction refraction is
5:42the bending of light when it travels
5:44from one medium to another
5:46and if you've ever gone to the beach or
5:49had your legs in water you'll probably
5:52notice it it when you look down it looks
5:54like your legs kind of get cut off in a
5:55weird way it's because the light bends
5:57when it was into the water so this same
6:01thing happens in our atmosphere and what
6:03ends up happening is as say the Sun is
6:05setting some of the light gets bent and
6:08so we actually see that sunlight for
6:10longer than you would predict also we
6:15talk about summer solstice is when we'd
6:17have the most sunlight on us summer
6:20solstice for us in North America around
6:22June 21st we get the longest day of
6:26sunlight I mean the days of course are
6:28still the same length but the amount of
6:30sunlight we have in that day is the
6:32longest yet it's usually not the warmest
6:35day of the year I mean June 21st is not
6:37the day you're picturing going ah that's
6:38gonna be the war
6:39they usually sound like july/august is
6:41when you really are starting to enjoy
6:43the beach why well because the earth
6:48actually has to heat up sure we have
6:49tons of sunlight and it's very direct
6:52and out for long on the summer solstice
6:54but we still have the world is a little
6:58cold all that sunlight hitting the
7:01ground hitting the water especially if
7:03you're at like a lake that water's gonna
7:05take a while to warm up so summer
7:08solstice will not be the warmest day of
7:10the year but it will be the day that has
7:12the most sunlight for those of us in
7:14North America