Phases and Motions of the Moon

imagine if you could watch the moon over the course of several weeks you would see that light would gradually be added to the face of the moon and then after between the full phase if the light would seem to come off so these are the familiar phases of the Moon now before we describe the phases of the Moon let's just clarify one rather important point about the moon and that is we always see the same side of the Moon it doesn't matter what phase it's in if you look at the features on the moon surface we're always seeing the same near side of the Moon so a really good way to compare the near side and the far side is to look at them side by side you notice that the near side has these light these light regions but also many darker regions these are known as moiré and the far side is dominated by highland regions so the question now is why do we only see one side well we will eventually learn that this all has to do with something called tidal forces but practically what does it mean to only see just one side of the moon let's take a simple illustration I've rendered the moon with the far side in gold and the near side just as the normal coloring of the moon so we have the earth of the center and if we could imagine for a moment the moon revolving around the earth but we're going to let the moon stay still in other words we're letting the moon revolve but we're not allowing it to rotate on its axis so if the moon doesn't rotate on its axis then over the course of a about a month or so we should expect to see all sides of the moon both the near side and the far side on the other hand if the moon executed several rotations on its axis as it revolved around the earth once then we would also be able to see both the near and far sides of the moon so how is it that we could only just see the near side of the moon no matter where it is in its revolution around the earth and the answer is timing the rotation period is exactly equal to its orbital period so you see that as the moon is revolving around the Earth the moon is also executing a single rotation on its axis and this keeps the near side of the moon facing the earth throughout the entire lunar orbit and we call this type of rotation synchronous rotation and it's for this reason that we always see the same side of the Moon facing the earth therefore the far side by definition is forever hidden from our view as we saw the previous images if we wanted to actually view the far side of the Moon you have to go there or at least send a spacecraft there to take a picture and send it back now let's take a more realistic view here so we're going to put ourselves in space and you notice that we have both the earth and the moon has a daytime side and a nighttime side but as the moon executes its rotation around the Earth we can only see at any given time a portion of the nearside because of the illumination from the Sun so let's just imagine for a moment you're standing on the earth and you're looking toward the incoming sunlight and suppose the moon just happens to be between the Earth and the Sun in this case so we're looking at that first image of the moon that is to the right of the earth if we were to look up take a look at the moon and somehow squint and be able to actually see the moon which we never could really expect to do we wouldn't see a moon the moon would be in silhouette and we call this phase of the moon the new moon phase likewise the appearance of the moon is purely the result of where it happens to be in its orbit with respect to the incoming sunlight and so you notice that the phases where the light is being added to the moon we call those the waxing phases whereas when the light is being removed from the moon we call those the waning phases so we have waxing crescent through first quarter to waxing gibbous to full and then we repeat the cycle but we reverse it we go from give us to third quarter to Crescent and then back to new again so let's imagine that we are now standing in place of the earth and that is the main view that we see here we're starting out with the moon between us and the Sun so therefore this is new if you look at the inset on the lower left of your image we have the Earth and Moon obviously not to scale but certainly within the same configuration we have the earth and then we have the moon and you could see the sunlight is illuminating both the earth and the moon and the sunlight would be coming in from the right so let's begin the movie let's just watch the moon revolve around the earth through one complete lunar cycle so here we're in the new phase we simply don't simply don't see the moon but you notice that as the moon makes its way to about the one or two o'clock position in the inset we're beginning to see just a little sliver of that illuminated side so this is the waxing crescent now when the moon gets to about 1/4 of the way around in its revolution around the earth we call this phase first quarter a lot of times people call this the half moon but in fact we're seeing just well one quarter of the moon's entire surface so the moon is continuing to move beyond first quarter it's not quite full we call this phase waxing gibbous now you see as the moon is coming around to basically opposite the Earth from the Sun we can now see the entire illuminated near side of the Moon or rather the entire near side of the Moon is fully illuminated we call that the full moon phase so as the moon begins to come around it's losing some of that light as seen from Earth we call this waning gibbous and as the moon makes its way about three-quarters of the way around in its orbit we give it the unimaginative name of third quarter so the moon has gone three-quarters of the way around in its orbit and we are seeing just one half of the near side of the moon you notice if you look carefully you'll see that there still is a near Side of the Moon the moon didn't disappear it's just that it's in silhouette so now we're in waxing and the light continues to wane off the moon so to speak as seen from Earth until we eventually come around and we lose the very limb of the illuminated side and we're back to a new moon phase once again now as we think about how long this takes well the answer is it's about a month but about a month is really the key phrase here let's explain how this works so we have a diagram of the earth and moon and we see the Earth's orbit illustrated we see the Sun and I'm also going to point a reference to the vernal equinox right and we're just going to imagine the vernal equinox as a fixed location among the stars and we will be able to measure our position relative to the vernal equinox no matter where we are in our orbit so in this illustration I've chosen to place the moon on the left and that would mean that we're looking at the moon we see the fully illuminated side of the near side of the Moon so we have the full moon of course but imagine what would happen if the moon were to make one revolution around the earth well it can do that but at the same time the earth is also moving about its own revolution around the Sun so when the moon comes back to its starting location with respect to the vernal equinox it isn't quite full yet it's almost full but it's not quite full it's now a waxing gibbous phase moon so this period of time is called a Saudi riyal month and satiereal literally means the Stars so this is a month with respect to the Stars it allows the moon to make one full revolution around the earth this comes out to about twenty-seven and a third days that means if we want to get back to our starting phase which in this case was the full moon well then we have to allow the moon to orbit and indeed the earth to orbit a little bit more so we need a couple of extra days of motion and this gives us what's called a synodic month or a solar month literally with respect to the Sun so this is what gets the moon back to the start Faye's and it wouldn't matter if we chose the new moon phase as our start phase or first quarter phase as our start phase from any given phase to any given phase it's going to take twenty nine and a half days to get there so there's a difference that when we say a month the first question we have to ask ourselves is well what do we mean by month so let's imagine the phases now we're going to take a split screen view you might say we have the earth and the moon on the Left we have the phase of the moon as it appears on the right and you notice I've drawn a symbol representing the direction to the Sun and the second symbol representing the direction to the vernal equinox so we start out with a new phase and we proceed to the waxing crescent as we saw and again now we're just repeating what we saw before but with a slightly better understanding of how these phases evolve and the sequence of the phase so the main reason for the phase is simply an alignment or a geometric view of the moon with respect to where it happens to be with respect to the Sun so let's put all this together and now we're going to imagine ourselves looking at the moon I'm using Stellarium and I've zoomed out to a very wide field of view which is way beyond what the human eye normally sees the moon and the Sun are going to be very tiny in the sky so I correct for that by enlarging the moon by about 10 times now let's take a look at our inset so this is a overhead view we're kind of looking at this slightly in perspective our hometown of Baltimore is right there and that means that North is toward the top of the globe South is toward the bottom east and west to the right and left accordingly so because the earth is rotating on its axis that means that the sunlight is coming in from the direction indicated and that means that given our location we're kind of rotating from the night side of the earth into the daylight we are therefore experiencing dawn and so the Sun is exactly where we expect it to be it's on the eastern horizon it's just rising and because the moon is between the Earth and the Sun the moon is presently at the new phase we have our vernal equinox we know that the earth is going to continue in its orbit as the moon revolves around the earth and we're seeing the moon in the new phase by the way I've just gone ahead and just added that dotted line just to help us see the moon let's go ahead and get the movie started and we'll just watch and see what happens so we noticed that the earth is rotating the it's carrying us from the day lit side to the night time and we come back to a new day but because the moon has traveled it a little bit in its orbit the moon is now somewhat east of the Sun and you could just see it setting just after sunset and we are now in the waxing crescent phase so we're gonna let some more time go by we've were still well within waxing crescent and as we get closer to first quarter you're going to notice that the moon is rising later and later and later so much so that by the time the first quarter phase actually arrives the Sun will be on the meridian just as the first quarter moon is rising let's take a look so there's the Sun on the meridian and the first quarter moon just rose so that means when the Sun sets the first quarter moons on the meridian and then the first quarter moon sets around midnight so now we're getting into the waxing gibbous phases so we're gonna let the moon just keep moving right along here and when you notice that more and more light is getting added to the moon and the moon is rising later and later and later so now the waxing gibbous moon rises in the afternoon now as we approach the full phase notice that the moon is getting to the opposite side of the earth in other words if you were standing let's say at the North Pole on earth you'd look in one direction and see the Sun and you'd look in the opposite direction and see the moon so to put ourselves back in Baltimore we're just gonna watch right now North America is in the night side of Earth if you look at the inset and now in North America comes around and we're at the full moon that means the full moon rises just at sunset see there so the Sun just said it the Sun just set and the full moon rose so let's watch that again so the Sun sets and the full moon rises that means the full moon transits the meridian at midnight and the Sun is now setting and because we're now past the full moon you will notice that the moon rises after the Sun sets so the moon is rising later and later and it's going to be usually past most people's bedtime before the full moon will before any moon will rise the waxing the waning gibbous moon interestingly if we notice in the waning gibbous phase the waning gibbous moon is rising very late at night long after the Sun is set and that means it's setting long before the Sun sets so we have the waning we have the waning gibbous moon setting sometime in the morning and as we come up on third quarter notice that we will have to wait until midnight so there goes North America and if you see our home town coming at midnight the third quarter moon just Rises and the Sun is up and the third quarter moon sets at noon so now we're getting into the waning crescent phases let's watch now and see how the moon starts to catch up to the Sun you might say you notice that it's still maintaining that eastward drift we saw at the beginning that the moon was moving to the left on your screen which is the eastward direction but because the moon has now come around so to speak it's catching up and still moving in the eastward direction but it's now getting closer and closer to the Sun in the sky pretty soon the moon comes back to it's a new phase and therefore the new moon is going to rise with the Sun and it's going to transit the meridian approximately at the same time as the Sun and it'll set at sunset so let's watch that now and there we go so a full lunar cycle as seen from our home now if you listen to Pink Floyd's Dark Side of the Moon you may have noticed that after the very end of the last track there's a voice overheard that says there is no Dark Side of the Moon really as a matter of fact it's all dark and I think it's a great way to wrap up this presentation on the Moon because we have shown that there is no such thing as a dark side in other words yes there's a dark side but the near side of the Moon is not always illuminated sometimes it's dark so how old the moon is dark at some point or another

Channel: Launch Pad Astronomy

Views: 199,422

Keywords: Astronomy, Moon

Id: 1CC0P7y90PU

Length: 16min 24sec (984 seconds)

Published: Sat Feb 17 2018