Full transcript
0:00if you've ever looked at the night sky
0:01you've probably noticed that the moon
0:04changes through the course of a month
0:05how it appears to us changes quite
0:08drastically it goes through these so
0:10called phases like this image shows well
0:13what's going on I mean seems like it's
0:17probably the same moon up there so why
0:19is it seeming to change so drastically
0:21for us well one important thing to keep
0:26in mind is that the moon does not have
0:29its own light source the only reason we
0:32see the moon the only reason it's
0:33illuminated is because of the Sun the
0:36sun is shining on it and we are seeing
0:39the reflected light from the Sun the
0:41moon reflects that light we see it and
0:43thus we see the moon and I mean I might
0:46seem weird because it's at night and the
0:48Sun doesn't seem to be there but the
0:49sun's still there it's just we aren't
0:51seeing it on our side of the planet and
0:52so it's light is still reflecting off
0:54the moon well another important thing to
0:58keep in mind is that the moon always has
1:00half of its face lit except for one
1:03really extreme case an eclipse which
1:06we'll look at a little bit later but
1:08it's safe for us to say that the moon
1:11always has half of it illuminated it's
1:15just are we seeing that illumination
1:18that is what's gonna explain the phases
1:21so okay let's take a look so here's good
1:24old Earth and this image is gonna help
1:26us because it shows the moon at various
1:28phases and again half of the moon is
1:31always lit well at the so-called new
1:34moon when the moon appears dark so
1:37that's what this image is showing by the
1:38way here's earth here's the sunlight and
1:40here's the moon as it's going around us
1:43and you can see always half the moon is
1:46lit but this outer ring is what we would
1:49see so this is showing the new moon and
1:52at that time it looks dark to us well
1:55half the moon is still lit it's just
1:57that the half is facing away from us
2:00here we are on earth somewhere on here
2:03maybe not where I was pointing is that
2:05looks like water but a Mariana boat but
2:08you would be looking at this and not
2:10seeing the lit side of the Moon
2:13you would see a dark spot if you could
2:15maybe pick it out at all because the lit
2:19side is not facing us well as we
2:23progress through the month still half is
2:26lit but as the moon orbits around us we
2:30start seeing more of that lit face and
2:33in but at this point right here a few
2:36days in we would be at the crescent
2:39phase Crescent you can see it actually
2:41looks very small crescent shape and
2:44waxing crescent in this case and one
2:47thing that's important to keep in mind
2:48is that we're gonna have back Singh as
2:50it gets larger and waning as it gets
2:53smaller so I think it's on wane off wax
2:56as it gets larger wane as it gets
2:58smaller because we're gonna actually see
3:00in fact you can kind of see it already
3:02that these two sides are basically
3:04mirrors in a way it goes through the
3:06same phase except in Reverse on this
3:08side but we'll get there in a second so
3:10just had new moon and again within say
3:13maybe about three four days we're gonna
3:15be in the waxing crescent and then about
3:18a week in we're gonna hit the first
3:21quarter which is a weird name because
3:24about half of the moon is lit from our
3:25perspective but it's called the first
3:28quarter or about a quarter of the way
3:30through the phases so it kind of makes
3:31sense for Matt's perspective but again
3:33half the moon's lit we're just seeing
3:35part of that face that's lit by the time
3:39we get over here we're entering the
3:42waxing gibbous a man remember again
3:44waxing cuz it's getting larger we're
3:46seeing more of the lit face over time
3:48and so it starts looking like this
3:50most of the face is actually lit and by
3:53the time were about two weeks in we're
3:56gonna see the so-called
3:57full moon again only half the lace faces
4:01lit in fact if you look at this image
4:03right here and this one it's the same
4:05moon it's just that what we're seeing we
4:08are now seeing the side that is lit we
4:10see the full bright moon because that's
4:13what's facing us and then over the
4:16course of the next approximately two
4:17weeks we're gonna go in Reverse through
4:20those phases waning gibbous third
4:22quarter waning crescent and
4:26eventually end up back at New Moon so
4:29there's always one half of the moon
4:32illuminated it's just are we seeing that
4:35how much of it is actually being facing
4:38us impacts very significantly what we
4:40see and now if this concept seems
4:43difficult to you or just having some
4:44trouble picturing it I would strongly
4:46recommend checking out the one YouTube
4:49video in the additional resources it's
4:52kind of a bit silly but kind of fun and
4:54the guys do a really good job of
4:56demonstrating it
4:57they actually basically shine a light on
5:00the other the one guy's face and they
5:02demonstrate this very clearly so it
5:05might be helpful to look at that but
5:09there's also a few other weird things
5:13going on with our Moon for one thing we
5:18actually have the moon tidally locked to
5:21us this is a term we'll come back to
5:22later but the idea is it goes through
5:24so-called synchronous rotation
5:27synchronous rotation this is a fancy way
5:30of saying that the moon actually does
5:32one full rotation in the time it takes
5:36to orbit us once so this arrow would be
5:41let's say someone's standing on the moon
5:43and we're assuming that they hopefully
5:45have enough air to survive the full
5:48month it's gonna take otherwise there's
5:51a lot sadder story but if we look this
5:55person would always be facing earth at
5:58every single point of that month the
6:02same face of the moon is always facing
6:04us and in fact you might have heard the
6:06idea the Dark Side of the Moon
6:08there's a side of the moon we don't see
6:10from Earth the far side of Moon we don't
6:13see from Earth we didn't see it until we
6:15actually put things in space to actually
6:17take pictures so the dark side of the
6:20Moon is always facing away it's this
6:22idea synchronous rotation again in the
6:24time it takes to go one orbit around
6:27Earth it spins once on its axis great
6:32well how long does it take well we're
6:35back to the idea of sidereal month and
6:37solar month
6:39sidereal month is how long it takes to
6:41do a full 360 just like in the case of
6:45the sidereal day around 360 in this case
6:48a sidereal month is the time it takes
6:50for the moon to do a 360 orbit which is
6:55by definition the same time it takes for
6:57to do a 360 spin on its axis great
7:01that's about twenty seven point three
7:02days if you want to see the real number
7:04the book goes really precise in the
7:06details but approximately twenty seven
7:08point three days the problem is that's
7:10not what we're used to as a solar month
7:12for us to go back from one new moon to
7:14another new moon is actually the solar
7:17month and is twenty nine point five days
7:20and the reasoning here is incredibly
7:23similar to the reasoning for the
7:25sidereal and solar month during the time
7:28that the moon takes to orbit around us
7:31approximately a month
7:32well we've traveled along our orbit
7:35around the Sun and for the moon to
7:37therefore be actually lined up back with
7:40the Sun so from New Moon to New Moon it
7:43has to go more than 360 degrees so it
7:47takes more than the sidereal day our
7:50sonata k-- or solar month is a little
7:54bit longer as a result