The Magellanic Clouds

By Betty Robinson,

Our home galaxy, the Milky Way, has two satellite galaxies in orbit about it. They don’t look like the typical pinwheel, or spiral, images of galaxies we are familiar with. The satellite galaxies look like clouds, and they have come to be known as the Magellanic Clouds, named after the Portuguese explorer Ferdinand Magellan. While in the Southern Hemisphere during Magellan’s first trip around the world, from 1519 to 1522, he and the crew observed these celestial objects. However, the Indigenous peoples in the Southern Hemisphere had been observing them for thousands of years. They are known simply as the Large Magellanic Cloud (LMC) and the Small Magellanic Cloud (SMC). They can only be seen in the Southern Hemisphere, and you don’t need binoculars to see them! Scientists think that the SMC is actually orbiting the LMC.

The bright object to the left of the Small Magellanic Cloud is a globular star cluster in our own galaxy. The star cluster is called 47 Tucanae. Credit: ESO/S. Brunier – ESO, CC BY 4.0,

The Magellanic Clouds are among the closest galaxies to the Milky Way: The LMC is about 160,000 light years away from us, and the SMC is farther, about 190,000 years away. (The closest galaxy is the Sagittarius Dwarf Elliptical Galaxy, at 65,230 light years.) They are about 75,000 light years apart. Recall that a light year is the distance that light travels in one year, about 9.7 trillion kilometres. They are a lot smaller than the Milky Way, too. The Milky Way is about 20 times the diameter of the SMC, and about 10 times the diameter of the LMC.

This manipulated image shows where the Magellanic Clouds are in relation to the Milky Way. If we could look down on everything from above, this is what we would see. Credit: Nina McCurdy / Nick Risinger / NASA. Not to scale.

Because of their loosely defined shape and their size, the Magellanic Clouds are classified as irregular dwarf galaxies. The Milky Way is classified as a spiral galaxy. Another galaxy shape is elliptical. There are further subclassifications.

While the Magellanic Clouds are thought to have been formed at the same time as our own galaxy, about 14 billion years ago, they have only been in orbit around the Milky Way for about 1.5 billion years. The Milky Way’s gravity would have “captured” the two galaxies. In fact, for the clouds to orbit the Milky Way, it would take about 4 billion years. Since they have only been in orbit for 1.5 billion years, we might be seeing them after their initial capture. They may not have even made one complete orbit yet.

Further, there are gravitational, or tidal, forces at work, that result in a lot of tugging and pulling between the two clouds and between the clouds and the Milky Way. Scientists think the LMC-SMC system could be on a collision course with the Milky Way, although the collision won’t happen for another 2.4 billion years. The collision could disturb the supermassive black hole at the centre of our galaxy, causing it to consume more gas and other matter and increase in size. Stars closer to the black hole could get kicked out of the galaxy.

So, if something is classified as a galaxy, does it automatically have the same composition as all other galaxies? Nope. Apart from shape and size, the Magellanic Clouds differ in two more important ways from the Milky Way: the clouds have more hydrogen and helium than our home galaxy does, but less metal. There is a similarity, though: the Magellanic Clouds and the Milky Way both contain a range of very young stars to very old stars. This points to a long history of stellar formation.

In February 1987, Canadian astronomer Ian Shelton discovered a supernova in the LMC from the University of Toronto observatory in Chile. This is the closest supernova visible for study since the invention of the telescope. Astronomers continue to observe the supernova remnant with ground-based telescopes and telescopes in space.

Mercury, a Planet of Extremes and More

By Betty Robinson,

Mercury has been called a planet of extremes for many reasons. Of all the planets,

    • it’s the closest to the Sun.
    • it’s the smallest; in fact, it’s just a little larger than the Moon.
    • it’s the fastest; it only takes 88 Earth days to go around the Sun once.
    • it has the longest day: it takes 58.6 Earth days for Mercury to rotate once.
    • it has the smallest axis tilt, less than a degree; so it’s upright.
    • it has the most elliptical orbit. (See the table and diagram in Extra Information at the end.)
    • it has the highest orbital inclination. (See the table and diagram in Extra Information at the end.)

The planet Mercury. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie


Mercury doesn’t have an atmosphere per se. There are atmospheric gases—oxygen, sodium, hydrogen, helium, and other gases—but they form a thin, inconsistent exosphere above the planet. An exosphere is the layer of atmosphere farthest from the surface of a planet. The space between the exosphere and Mercury’s surface is a vacuum.

Mercury is not volcanically active, there is no vegetation, no life, so how does Mercury keep adding gases to its exosphere? And Mercury’s gravity is low, so how can Mercury hold its exosphere in place?

Some impacts from meteoroids vaporize some of materials from the meteoroids themselves, as well some of Mercury’s surface rocks. The vaporized material goes off into the exosphere or off the planet. However, the major source of gases is through the interaction of Mercury’s magnetic field and the solar wind; the magnetic field also helps keep the tenuous exosphere in place. See below.

Magnetic Field

I first became interested in Mercury when I learned about its gravitational field. The strength of Mercury’s magnetic field is about 1% the strength of Earth’s magnetic field, so not strong at all. In addition, Mercury’s magnetic field is really offset: it’s three times stronger in the northern hemisphere than in the southern hemisphere. Earth’s magnetic field is more or less consistent in both hemispheres.

The solar wind is a thin plasma (charged particles), and it is more intense near the Sun than farther out. So, because Mercury is so close to the Sun, Mercury’s magnetic field interacts a fair amount with the fast-moving solar wind. Sometimes, as a result of this interaction, magnetic tornadoes form. These tornadoes aren’t like tornadoes on Earth: On Mercury, there is no dust, no air, no clouds, no weather. Magnetic tornadoes are invisible. They carry the solar wind plasma to the surface of the planet. When the charged particles in the plasma hit the surface, they cause atoms to fly off the surface into space and into the exosphere.

Double Sunrise/Sunset at Perihelion and Retrograde Motion of the Sun

As I read more about this planet, I became more intrigued. At one point in Mercury’s orbit around the Sun, the Sun undergoes retrograde motion: for an observer on Mercury’s equator about to experience daylight, the Sun rises, sets, and rises again. That point is when Mercury is closest to the Sun, or perihelion. (The point at which a planet is farthest from the Sun is called aphelion. See the diagram showing perihelion and aphelion at the end.) For an observer about to experience nighttime, the opposite happens. Check out these animations of this phenomenon: (double sunrise) (double sunset)

An observer at noon, at perihelion, would see the Sun stop, move eastward, then westward.

If you could be on Mercury’s surface, at the equator, at noon, at perihelion, then you could see the Sun in retrograde motion. The Sun’s real motion does not change. This is just a phenomenon that we would see if we were in the right place at the right time.

This phenomenon only happens on Mercury. Why does this happen?

To start, here’s a reminder of how planets move, thanks to Johannes Kepler (1571–1630). Kepler identified three laws of planetary motion. We just need the first two, but I have included the third law for completeness:

Law 1: Planets move in ellipses (not perfect circles), with the Sun at one focus; that is, the Sun is not the centre of the ellipse. (See the table and diagram at the end.)

Law 2 (Summarized): Planets move fastest at perihelion and slowest at aphelion.

Law 3: The time, T, it takes a planet to complete one orbit, squared, is proportional to the mean distance, r, from the Sun to the planet, cubed: T 2 µ r3.

Now, let’s look at Mercury: As per law 1, when a planet is at perihelion, it is moving faster than at any other time in its orbit. At perihelion, Mercury is moving about 1.5 times faster than it moves at aphelion. Note: Mercury’s speed around the Sun changes, but its rate of rotation does not change. Mercury’s rotation is pretty slow, about 58 Earth days to rotate once.

Also, since it only takes Mercury 88 Earth days to go around the Sun once, and since Mercury’s rotation is so slow, the Sun is in the Mercury sky for 176 Earth days every year (Mercury’s solar day). The 176 days is about three times longer than the time it takes for Mercury to rotate once on its axis.

Next, let’s look at the Sun from Mercury: As Mercury goes around the Sun, the Sun is in the sky for 176 days. During that time, the Sun moves, on average (remember how Mercury changes speed at different points), 2° west every Earth day. At aphelion, Mercury has slowed down, so the Sun’s movement appears to speed up a bit, to about 3° west every day.

Now as Mercury gets closer to perihelion, it starts to speed up, but its slow rotation doesn’t change. The opposite to what happens at aphelion happens at perihelion: Mercury has sped up, so the Sun’s movement appears to slow. As Mercury moves close to perihelion, arrives at perihelion, and moves past perihelion, the Sun stops, then moves eastward instead of westward, and starts moving westward again. As Mercury continues to slow down after perihelion, the Sun’s apparent movement returns to about 2° west until Mercury reaches aphelion again, when the Sun speeds up to about 3° west every day.

So over a period of about 8 days before and after perihelion, an observer on the equator, at the terminator on Mercury, would see the Sun rise in the east, set, then rise again.

And throughout the entire orbit, the size of the Sun from Mercury appears to change: On average, the Sun appears to be over three times larger than what we see on Earth. So at perihelion, it would appear larger. At aphelion, it would appear smaller. Of course, the size of the Sun does not actually change.

Mercury’s Tail

There is some sodium in Mercury’s exosphere, but not that much. What makes the sodium mentionable is the fact that it absorbs and emits light quite easily, so it’s more visible than other elements. The solar wind is constantly interacting with Mercury’s magnetic field. The wind pushes gases, including sodium, from Mercury’s surface and exosphere away, and they form a tail behind the speeding planet. The ultraviolet light in sunlight causes the sodium to glow, or fluoresce, a yellowish colour, so under favourable conditions we can see a yellowish tail trailing behind Mercury.

Mercury’s tail. The sodium particles in the tail fluoresce, causing a yellowish glow. Credit: Andrea Alessandrini

Why Don’t We Send Probes to Mercury More Often?

Mercury is truly an intriguing planet. But so far, only two spacecraft have visited Mercury, both American: Mariner 10 (1973–1975) was the first spacecraft to study Mercury, and it only took 147 days to get there. However, Mariner 10 did not go into orbit around Mercury; it just did flybys. Messenger (2004–2015) was an outstanding mission. It took seven years to get to Mercury. The difference here is that Messenger didn’t just do flybys; it spent four years in orbit around Mercury.

The next mission to Mercury is the ESA spacecraft called BepiColombo. It was launched in October 2018 and will begin orbiting Mercury in December 2025, another seven-year mission. I’ll be following BepiColombo for sure.

An engineering model of BepiColombo, seen in the Science Museum, London. Photo by Randy Attwood

So why haven’t there been more spacecraft sent to Mercury, especially compared with missions to planets beyond Earth?

Mercury is tough to get to. Imagine that you are at the top of a high hill and you start running down the hill. Halfway there you decide to stop running. You will likely be unsuccessful and go tumbling head over heel down the rest of the hill. This is a rough comparison to what it’s like to send a spacecraft to Mercury. Mercury is so close to the Sun that a spacecraft en route to this tiny planet has to more or less head straight for the Sun. The closer it gets, the more the Sun’s gravity pulls it in. So spacecraft heading to Mercury with the purpose of going into orbit around it need to use gravity effects from Earth, Venus, and Mercury to slow down. This help is called a gravity assist. To do that takes longer.

And once the spacecraft gets there, it has to be able to endure the extreme heat and cold: Mercury’s daytime side can reach a temperature of 430 °C; the nighttime side can reach –180 °C. So spacecraft designers face design challenges to protect the spacecraft as well as the electronics in the instruments.








Extra Information


Eccentricity, e, is a measure of how elliptical a shape is. The closer e is to 0, then the closer the shape is to being a circle. In the table below, we can see that Venus’s orbit is the closest one to a circle, and Mercury’s orbit is the most elliptical.

Left: a circle with 0 eccentricity; middle: an ellipse with an eccentricity of 0.2; right: an ellipse with an eccentricity of 0.3. Not to scale.

Source: RASC Observer’s Handbook 2021

Perihelion and Aphelion

At perihelion, a planet is closest to the Sun. At aphelion, a planet is farthest from the Sun. Not to scale. Exaggerated to highlight differences.

Orbital Inclination

If we could view the planet orbits from the side, we could see how much they are tilted.

The orbital inclination is how much the orbit is tilted. Earth’s orbit is almost completely flat. But Mercury’s orbit is tilted a fair amount.

Source: RASC Observer’s Handbook 2021



Sound on Mars

I am getting excited about the upcoming Perservance landing on Feb. 18, 2021. Not just because of the helicopter, 25 cameras, and the focus on searching for evidence of ancient microbial life; but also because I have learned that Perservance is also carrying two microphones so we can “hear” Mars.

With such a thin atmosphere on Mars, I began to wonder if we can even hear sound on Mars. Sound needs a medium to travel through from the source to our ears. But the density of Mars’s atmosphere is only about 1% the density of Earth’s atmosphere. Is there enough matter for the sound waves to transmit? And Mars’s atmosphere is about 95% carbon dioxide; ours is 78% nitrogen, 21% oxygen, and the rest various gases. Does the high percentage of carbon dioxide have any effect on sound, assuming it can travel through the thin atmosphere? Finally, does temperature have an effect? It’s extremely cold on Mars. Nights can be as cold as –90 °C at Jezero Crater, Perseverance’s landing site.

(Note: Light is a combination of waves and particles. But we can see light, and we can send radio waves through the vacuum of space to Mars for communication purposes because light and its components are electromagnetic radiation, which doesn’t need particles to travel.)

How Do We Hear Sound?

When we hear something, our eardrums are detecting vibrations. Eventually, the vibrations get to the brain. And the brain interprets the vibrations and tells us that we are hearing something; it will identify the sound if possible. The vibrations need a medium to travel through. By definition, a vacuum is an absence of matter—no matter, no particles, no sound waves, and no sound.

Are Sounds on Mars the Same as on Earth?

Turns out, we can definitely hear sound on Mars. It will just be different than what we hear on Earth.

  • The effect of the thin atmosphere on sound quality is a muffled, lower-frequency sound.
  • The carbon dioxide atmosphere has the effect of absorbing the higher frequencies in a sound, so any sounds with high frequencies would arrive at our ears without those high frequencies.
  • The temperature definitely has an effect: sound travels slower in colder temperatures. The average speed of sound through air on Earth is about 330 metres/second. On Mars, the average speed is about 240 metres/second. So a slower speed means sounds generated on Mars will take a little longer to reach our ears.

At , you can listen to a few Earth sounds and then hear how they would sound on Mars.

Left: the SuperCam microphone; right: the Entry, Descent, and Landing (EDL) microphone

One of the microphones on Perseverance (the EDL microphone) will hopefully pick up the sounds as Perseverance enters the Martian atmosphere and lands. The other microphone is part of an instrument called SuperCam. SuperCam will fire a laser at rocks, and should generate a sound when it hits a rock. The sound it makes could help the team at home understand more about the target rock. The microphone could also pick up any background noise on Mars, such as wind.

Here’s a video of a demonstration: Mars’s thin atmosphere is a partial vacuum. So I created a (temporary) partial vacuum in a mason jar, added a sound source, and lit the tea light to get a change in atmospheric pressure. The loud pop from the jar means the pressure has changed. After the change,  you can still hear sound quite clearly!

Ingenuity Mars Helicopter

By Betty Robinson,

NASA’s newest rover—Perseverance—is scheduled to land on Mars on Feb. 18, 2021, at about 3:30 p.m. EST. Perseverance will be looking for evidence of past microbial life, as well as studying Mars’s geology. An ingenious passenger onboard Perseverance is a helicopter. That’s right. A helicopter. The first aircraft on another planet! It has the appropriate name Ingenuity Mars Helicopter.

Perseverance will be the fifth rover to land on Mars. The others are Pathfinder, Spirit, Opportunity, and Curiosity.  Credit: NASA

Aircraft on Earth are designed, naturally, to work in Earth’s gravity and atmosphere. None of these are similar to Mars conditions. Mars has a little over one-third of Earth’s gravity, and its atmosphere is about 1% the density of Earth’s atmosphere (and a completely different gaseous composition). Consequently, the atmospheric pressure on Mars is next to nothing. And it’s cold on Mars. Much, much colder than on Earth. Nights can be as cold as –90°C at Jezero Crater, where Perseverance is going to land.

The purpose of Ingenuity is to test the waters for future flight missions on Mars and other bodies in the solar system. Ingenuity doesn’t carry any science instruments so it won’t be doing any experiments. It does have two cameras, though: one black and white, and one colour. However, Ingenuity’s first challenge may be to survive one night on Mars. Fortunately, it has a heater.

Artist’s illustration of the Ingenuity Mars Helicopter. Ingenuity is about 0.5 metres high. Credit: NASA/JPL-Caltech

So how does an Earth-designed helicopter work in Mars conditions? For starters, Ingenuity has to operate on its own. It cannot be directly controlled from Earth like we control drones, for instance, because of the length of time it takes to get signals to and from Mars. It will, however, get the initial command to start, and other commands, from Earth via Perseverance.

Ingenuity’s blades are 1.2 metres long and made of carbon fibres. They rotate at about 2,400 rpm, which is eight times faster than the average helicopter rpms on Earth. The longer blades and faster rotation speed are required because of the almost non-existent atmospheric pressure on Mars. If the air is thin, as in higher altitudes on Earth, the aircraft needs to go faster because there is less air under and above the wings. So with such a thin atmosphere on Mars, the higher rotation speed and longer “wingspan” are needed.


For an aircraft to take off, it needs to be able to overcome gravity/weight. To achieve this, the aircraft needs more lift force than weight. Air moving under and over the aircraft’s wings provides the lift. This force acts upwards to counteract the downward force of gravity/weight. Ingenuity is quite light, only 1.8 kg. The light weight is helpful with overcoming Mars’s gravity.


Gravity pulls everything downward, toward Earth’s centre. When talking about weight in science, weight is a force and refers to the mass of an object times the force of gravity. For Earth, the force of gravity, or g, is 9.8 m/s2. For Mars, the number is 3.7 m/s2. So someone with a mass of 50 kg has a weight of 50 kg x 9.8 m/s2 = 490 N (newtons) on Earth and 50 kg × 3.7 m/s2 = 185 N on Mars. A newton is the unit of force. On Earth, Ingenuity’s weight is 1.8 kg × 9.8 m/s2 = 17.6 N. On Mars, its weight is 1.8 kg × 3.7 m/s2 = 6.7 N. Considerably less.


For an aircraft to move forward, it needs thrust. An increase in thrust allows the aircraft to accelerate and overcome drag (see below). An aircraft’s engine provides the thrust. The thrust on Ingenuity is provided by rechargeable solar-powered batteries. With its reduced mass and Mars’s reduced atmosphere, thrust will be less challenging than thrust on Earth.


The force working against thrust is called drag. It is caused by air resistance and acts in the opposite direction to the motion. The amount of drag depends on the object’s shape, the atmospheric density, and the object’s speed. With such a thin atmosphere on Mars, there is little drag. That can be a good thing. Thrust must be greater than drag for the helicopter to take off, so with little drag there can easily be more thrust. And when landing, thrust is easy to reduce so that the speed decreases and thrust becomes less than drag.


Watch landing online:

News briefings and launch commentary will be streamed on

Reflection and Making a Cellphone “Hologram”

By Betty Robinson,


The property of reflection may well be the property of light we are most familiar with and dependent upon. After all, without reflection we wouldn’t be able to see. Light, either from a natural or an artificial source, leaves the source and bounces off an object (reflects). If the light reaches our eyes, we can see the object.

Once we understand the various properties of light, such as reflection, we can develop technologies that use the properties. For example, we use reflection for mirrors, telescopes, and microscopes. Some makeup manufacturers even use an understanding of reflection to produce makeup that makes our skin look smoother and more perfect than it really is.

We can also have fun with understanding light—how about “holo” nail polish? There will have to be a separate posting on that, because the science is more involved than simple reflection. But the cellphone “hologram” does, in fact, use simple reflection.

Thanks to Randy Attwood for help making the video.

Making a Cellphone “Hologram”

There are many sites on the Internet that describe how to do this. Just search for “make a phone hologram.” Here’s what I did to make the cellphone “hologram” in the video.


You need a piece of clear plastic; it must be relatively rigid but doesn’t need to be 100% rigid. Some sites recommend a clear CD case, but that can be tough to cut using an Exacto knife, especially if you want to involve kids. I used the clear plastic covering, or lid, from a box of Christmas cards. You could also use a transparency, if you have one lying around. You’re going to cut out 4 pieces to make a trapezoid, which needs to be able to support itself when the pieces are taped together. So keep that in mind while choosing your clear plastic material. And it must be large enough that you can cut all 4 trapezoids from it. For an idea of size, see the image below; it has dimensions on it.

  • Pencil
  • Ruler
  • Graph paper is helpful but not necessary.
  • Scissors or Exacto knife (have kids use scissors; adults can use the Exacto knife)
  • Clear tape
  • Cellphone with Internet


  • Draw four trapezoids on the clear plastic. See dimensions below. Don’t change the dimensions. We want to end up with a specific angle of reflection.

  • Cut them out.
  • Tape them together, with a slanted side attached to another slanted side. You want to end up with a free-standing object, so all sides are joined.

  • On the cellphone, search YouTube with the term “phone holograms”; for example, the video I used is from here: 
  • Make sure the brigthness setting on the cellphone is at its highest.
  • Have the cellphone on a table or desk such that people can crouch down and be eye level with the flat phone. Have the video on the cellphone ready to play. Place the structure, small opening on the bottom, on the phone over the video. Crouch down to be eye level with the phone. Start the video, and turn out the lights and watch the “holograms.” They are faint and best seen in a darkened room.

I keep using quotes when I refer to the cellphone “hologram” because these are not real holograms. They are light reflections. They just look like holograms, which makes them so cool. You need lasers, mirrors, and wave interference to make a real hologram.

The Science

The dimensions of the trapezoids are such that the sides are designed to be at a 45º angle to the surface of the phone. When you reflect an image in a transparent screen (the clear plastic) at that angle, you see a “virtual” image, which is just a reflection of what the cellphone screen is playing.

You can have your kids or students view the images from each side of the plastic trapezoid. They may be able to figure out that they see the image of the video on the side they are watching.

Fluorescence and Colour Models

By Betty Robinson,


Some materials will glow when lit with an ultraviolet (UV) light. But when you take the light away, the glowing stops. A phosphorescent material will continue to glow after the light source is removed.

Let’s look at olive oil. The molecules in olive oil absorb the energetic UV light, from, say, a UV pen light. And the molecules get “excited.” When they are “excited,” they release energy often in the form of visible light (heat is released, too). In the case of olive oil, the light is red. As you can see in the video, the light emitted by the canola oil molecules is greenish, and the light emitted by the rock salt molecules is purplish.

Thanks to Randy Attwood for help making the videos.

Colour Models

In a novel I read recently (The Last Widow, by Karin Slaughter), one of the main characters described making a UV light from a cellphone. Will, the character in the book, coloured the light on his cellphone with a blue Sharpie, covered the blue with see-through tape, then coloured over the blue with a purple Sharpie, then covered that with tape. He then turned on the cellphone’s flashlight and used the “UV light” to read a message written with urine. (Urine is fluorescent under a UV light, but it would have been invisible to the bad guys in the story in white light.)

While an exciting application of science as well as a clever addition to the plot, unfortunately, the science does not pan out. It might have worked if Will used filters, such as filters used with a camera, because some light gets through the filter and the rest is absorbed. You can’t make UV light with a material that is coloured by pigment, such as paint, fabrics, and printer ink. The light just gets absorbed. I tried it anyways, by the way. It didn’t work. Some light got through the Sharpie markings but it certainly wasn’t UV. (But the blue colour from the Sharpie came right off the cellphone light; it may be permanent on some materials, but not the glass light.)

There are two models of colour that describe what I wrote above: the additive model of colour and the subtractive model of colour.

According to the additive model, we “add” the three primary colours of light to produce any colour of light, including white. Varying the amounts of each colour of light will give different hues and colours.

With the subtractive model, we are “subtracting” different colours through the processes of absorption and reflection: a red sweater is red because of the red pigment in the dye used to colour it. The sweater absorbs all the colours except red, and the red is reflected to our eyes.

Lunar Libration

By Betty Robinson,

The Moon only presents one half, or 50%, of its surface to us, what we call the near side of the Moon. (We never see any more than a small amount of the far side of the Moon from Earth because of the tidal lock between Earth and the Moon, but that’s for another blog post.)

However, we can actually see about 59% of the Moon’s surface over time, due to a phenomenon called libration. Libration is the real or apparent “wobbling” of an object, in the left-right direction and/or in the up-down direction. As a result of lunar libration, we get to see more of the Moon’s surface. Libration is noticeable to any observer, as shown in the two images.

The lunar libration predicted for January 2021 is based on data collected by NASA’s Lunar Reconnaissance Orbiter (LRO) satellite, which has been orbiting the Moon since 2009. We can see both the left-right libration and the up-down libration in these two images. On Jan. 1, 2021, Mare Crisium looks to be a bit higher and not fully visible as compared to the image for Jan. 25, 2021. Credit: NASA Scientific Visualization Studio and LRO

In the two images, notice how the size of the Moon seems slightly smaller on Jan. 25 than on Jan. 1. This isn’t a manipulation of the images. The difference is because the Moon’s path around Earth is not a perfect circle; it’s elliptical. Think of an oval, or a slightly flattened circle, with Earth positioned closer to one end than the other. So at one point the Moon is slightly closer to Earth, and at the opposite point it is slightly farther from Earth. The Moon’s elliptical path is important to the concept of left-right libration.

On Jan. 1, the Moon is 386,460 kilometres from Earth. On Jan. 25, the Moon is 397,371 kilometres from Earth. The difference is about 11,000 kilometres. So, because the Moon is farther away from us on Jan. 25 than on Jan. 1, it appears to be smaller in the sky to us, just as any object far away from us appears to be small. The size of the Moon itself does not change.

Back to the photos. So, what is happening here? Why can we see different positions of the Moon’s features? Why don’t we always just see the same features in the same spots?

Left-right Libration

The Moon rotates on its axis, just like Earth does. They both rotate west to east. So, if you could look down on Earth and the Moon from high above the north pole, you would see them rotating in a counterclockwise direction.

One Earth rotation takes 24 hours, our definition of a day. One Moon rotation takes about 27 days. It also takes the Moon just over 27 days to go around Earth once. As a result, we only see one side of the Moon. It seems like the Moon isn’t rotating at all, but it is.

Here is where the elliptical path comes in: in the late 1500s, a famous astronomer called Johannes Kepler figured out that objects such as Earth, the Moon, and the planets do not orbit in circles, but ellipses. Kepler then figured out that, because of the elliptical orbits, the objects move at different speeds, depending on where they are relative to the Sun, or Earth in this case. For example, when the Moon is at its farthest point from Earth, its speed is just under 1 kilometre per second. When it is at its closest point to Earth, its speed is just over 1 kilometre per second. (1 kilometre per second = 3,600 kilometres per hour.)

After the Moon’s closest approach to Earth, the Moon is still moving faster in its orbit. But the Moon’s rotation speed doesn’t change and it can’t keep up with the faster orbital speed. As a result, on the right side of the Moon we see a bit of the Moon’s far side before the Moon completes its full rotation.

After the Moon’s farthest distance from Earth, the Moon is still moving a bit slower in its orbit. So the Moon’s rotation is a little faster than its orbital speed. As a result, on the left side of the Moon, we see a bit of the Moon’s far side.

Up-down Libration

If you think of a line coming out of the centre of the Sun and going to the edges of the solar system, many planets would appear to sit on or near that line. The planets and other objects are said to be co-planar. Some don’t, though. Mercury, for instance. Mercury’s orbit crosses the imaginary line at an angle of about 7 degrees. And Pluto’s orbit is at an angle of about 17 degrees to the line. The Moon’s orbit is not co-planar with Earth’s orbit. The Moon’s orbit is inclined about 5 degrees to Earth’s orbit.

The concept of up-down libration is related to this 5 degree tilt. Also of note is the tilt of the Moon itself. Earth is tilted about 23.4 degrees, which causes our seasons. The Moon is tilted only about 1.5 degrees, so no seasons on the Moon! Anyways, back to libration. It’s so easy to move from topic to topic in astronomy because everything is so interesting!

As the Moon orbits Earth, sometimes it’s above Earth’s orbit, and sometimes it’s below Earth’s orbit. As a result, the Moon’s north pole is sometimes tipped toward Earth, and sometimes it’s tipped away from Earth. So at times we can see a little more of the south side as the Moon goes above our orbit, and a little more of the north side as the Moon goes below our orbit.

There are other factors that affect how much of the Moon’s surface we can see, such as our latitude on Earth and the Moon’s precession. But the main two factors are the left-right and up-down libration.

Watch the Moon with binoculars over a month, and try to see different positions of its features as shown in the NASA images.