On Monday September 16, my fellow blogger Alex Autin posted a captivating NASA video called Rotating Moon from LRO on her wonderful bog …things I LOVE!. You should visit and follow her blog especially if you are a fan of space exploration and astronomy. The video showed the Moon undergoing full rotation, not something we get to see from Earth. It is really amazing to see that kind of lunar motion. I was working on a draft of this post at about the same time. I wanted to illustrate the different ways the Moon moves from our vantage point on Earth and tell why we see it that way. So, consider this just an extension of her post from earlier in the week.
The following mesmerizing animation shows the libration and phases of the Moon at hourly intervals during 2013. Turn up the volume. Click play and watch for a while. The moon has some very intriguing motions which we normally don’t notice. This animation is a product of the NASA/Goddard Space Flight Center Scientific Visualization Studio. Your tax dollars are at work helping to understand and appreciate the world around us.
The link to Goddard above the video will take you to the site where it and many other materials related to this animation are located. Of particular interest is the first thing on that page called Dial-A-Moon. Enter any date and time for 2013. Click Update to see the appearance of the moon then.
Why does the moon move like that? Several things are going on at the same time.
Join me below for answers. I think you will find this an interesting journey.
Phases of the Moon
One of the more obvious and understandable motions is the orbit of the Moon around the Earth. It appears as the changing phases we see in the top animation. As it orbits, we see it in a different place each successive day or night. The Moon travels toward the east in the sky and takes about one month per orbit. The Sun always illuminates half of the Moon. The illuminated portion of the Moon that we can see from Earth varies dependent on the Moon’s position in orbit. The animation below shows why we have a changing view. It is not to scale. The view is looking down from above the north pole of the Earth.
Phil Hart Blog w/other great content in astronomy
The animation also shows that the Moon rotates and presents the same side toward Earth. This is synchronous rotation, also known as tidal locking. It is a common phenomenon for moons to be tidally locked with their planet. Sixteen of Saturn’s moons are tidally locked.
As water is raised three feet to higher tides in open ocean by the gravitational force between Earth and Moon, land surface does the same by a smaller amount. On Earth, land tides of up to a foot are recorded. On the Moon, they are much larger and can reach 100 feet on the side facing Earth. Jupiter causes 330 ft land tides on its nearest moon Io. This high land tide on the Moon had a slowing effect on its rotation over the millennia until it locked into a synchronized rate. Tides are also slowing the rotation of Earth. It’s one of the reasons for adding a leap second to the clock every now and then.
Orbital Distance of the Moon
The Moon does not orbit Earth in a perfect circle. The distance from the Earth varies between 28 and 32 diameters of Earth, seen to scale in the animation below. The closest distance reached during each orbit is called perigee. The farthest distance is called apogee. We see plotted below the distance between the Earth and the Moon as it makes two orbits. The variation in the Moon’s distance is quite pronounced. All sizes and distances and shading are to scale in this view. If you were standing on the Moon looking at Earth, you would see the Earth also going through phases. But, the Earth would not change its position in the sky of the Moon. The Sun and stars would rise and set slowly with each monthly orbit, not Earth. Colonies on the far side of the Moon will never see Earth.
Since the Moon is closer during part of its orbit, it is necessarily going a little faster, too. It travels a little farther to the east each unit of time. During apogee, eastward progress is a little slower each unit of time. The video at the top of the post takes into account those faster and slower speeds. It appears as a left-right wobble of the face of the Moon with each orbit cycle.
The Moon looks largest to us at perigee. The difference between the apparent sizes at apogee and perigee during 2012 can be see in this composite image. Many people claim it also looks bigger when it is low to the horizon. That effect is an optical illusion.
Orbital Inclination of the Moon
We have accounted for the motion which changes the phases we see. We have also accounted for the left-right wobble by the fact the Moon doesn’t orbit at the same speed all the time. It speeds up and slows down cyclically. What about the apparent up-down tipping motion in the animation at the top of this post? The answer to that question lies in the fact that the plane of the orbit of the Moon is different than the plane of the orbit of Earth by about 5˚. This illustration shows a portion of the orbit of Earth with the plane of the orbit of the Moon maintaining the same orientation.
As a result, during half of each orbit of the Moon, it is above the plane of Earth’s orbit. We see it as much as 5˚higher in the sky. During the other half orbit, it is below by as much as 5˚. During each month of orbit, the Moon appears higher and lower in a cyclic way. This up-down tipping motion is captured in the NASA animation at the top of this post.
Incidently…you might have noticed in right part of the figure above that the shadows of the Moon and Earth do not usually coincide, or align. There are times during our orbit when the Earth, Moon, and Sun alignments have the small possibility for alignment, an eclipse. They are shown in the two parts of the illustration in the left of the figure. The intersection (dotted line) of the planes of the orbits aligns with the Sun and Earth. The Moon might be directly in line with the Sun and cause a solar eclipse. Or, it might be directly in line with the shadow of the Earth for a lunar eclipse. But, the Moon is rarely in the right place at the right time for an eclipse. For that reason, we don’t see them every orbit. It is more common for lunar eclipses because the shadow of the Earth is rather large. This NASA animation exaggerates it.
Thank you for taking the time to join me. Watching the Moon night to night is peaceful and reassuring. It has predictable behavior. When I first watched the animation at the top, I was intrigued. I knew the motions were present. As I analyzed them, it seemed a good topic for this post. Maybe you did, too. Why don’t you take a few minutes to watch it again with the sound up. Relax. Enjoy a few moments of peace. Be thankful for our neighbor in the sky.