Intro to Astronomy Assignment

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Investigating Astronomy
Timothy F. Slater, Roger A. Freedman

Chapter 1

Predicting the Motions of the Stars, Sun, and Moon

Welcome to the first week of SCIN134, Introduction to Astronomy. In these audio lectures, I will prompt you to go to the next slide in the PowerPoint presentation. You can also read the script of the lecture in the notes field in PowerPoint. Your textbook is Investigating Astronomy by Timothy Slater and Roger Freedman. These lectures are NOT intended to be a replacement for reading the textbook, as they only highlight information, and do not go into all the detail that the text does. I recommend reading the chapter first, and then view/listen to the lecture. Reading a science textbook is not like reading a novel; you often need to reread sections. Active reading skills are important. Ask yourself questions about the reading as you’re doing it (writing down your questions is really helpful). If you still have questions after reading and viewing and listening to the lecture, reach out for help to your instructor and classmates in the forums or via messages. It can also help to look for additional information on the web. Sometimes it is useful to read different people’s explanations of topics.

This week, we will be covering chapter 1. We’ll talk a little about the scientific method and then move on to what we can see in the night sky.

The Scientific Method

Hypothesis: A testable idea
Theory: A description of nature, based on a great deal of data.

A theory explains what we see.

Exploring the physical world using

observation, logic, and skepticism

You may have learned about the scientific method in science classes before college. It is often taught as a somewhat unbending series of steps. In reality, the order of the steps isn’t so set in stone. The most important thing to remember about the scientific method is that it is a way of exploring the world around us by observing, using logic, and being skeptical. Being skeptical doesn’t mean not believing something. It means that in order to be accepted as valid, an idea must be based on a significant amount of solid data. Especially important to note are two words that have very specific meanings in science. First is hypothesis (the plural is hypotheses), which means a testable idea. When scientists want to study something, they either start with an hypothesis or they come up with an hypothesis as a result of observations. Then, they proceed to test the hypothesis either through experimentation or further observations. In astronomy, we can’t really experiment in a lab in the same way that scientists in other fields can (you can’t set up a little solar system in your lab and change things about it to see what happens). Instead, astronomers must rely on observing the universe as it is. So, whether we experiment or observe, the data we collect allow us to come up with a theory. This word is really important in science, and it has a very specific meaning. It is a description of something in nature, based on a lot of sound data, that explains what we see AND successfully predicts the results of future experiments or observations. It must fit ALL of the available data, and if future good data doesn’t fit the theory, the theory has to be changed (sometimes thrown out all together). Note how much this contrasts with the everyday usage of the word theory. People might discredit someone’s idea by saying, “Oh, that’s just your theory.” But in science, theory is a VERY strong word, and is not thrown around lightly. When an idea in science is identified as a theory, that means that there is a significant amount of confidence that it is the correct explanation.

A Quick Guide to Objects in the Sky

This slide shows a quick overview of the kinds of things that we’re going to be studying in this class. If you start at the top left, you see a diagram of the major bodies of our solar system, with the Sun at the center and the planets orbiting around it. All of these planets except the two innermost ones, Mercury and Venus, have objects orbiting them, called moons. If you see the word Moon capitalized, then it is referring to Earth’s Moon. All the other planets that have moons have more than one (some have many more than one). Not pictured in this general diagram are the smaller objects that also orbit the Sun. There are asteroids, which are rocky bodies mostly found in the inner solar system. Then there are Trans-Neptunian objects, which are bodies of rock and ice that are beyond Neptune, and comets, which are mostly icy bodies that primarily live in the outer solar system. Comets can venture into the inner solar system, though.

While our Sun is the largest object in our solar system, it is just one of many, many stars (it’s a sort of average one in terms of size, temperature, and age). Almost all of the points of light we see in the night sky are stars. Stars are born in clouds of gas and dust called nebulae (singular is nebula). When stars end their lives, they can form a variety of objects (based on their size). Some explode as supernovae (singular is supernova). The largest stars can form black holes when they die, and we’ll definitely talk more about these strange objects later in the class.

Stars are grouped together in massive conglomerations called galaxies. Our Sun is one of about two hundred billion stars in our galaxy, called the Milky Way Galaxy. Some galaxies are smaller, with millions of stars, while others are much larger (the largest has 100 trillion stars). There are about 100 billion galaxies in the known universe, grouped into clusters. We’ll learn more about all of these objects later in the course.

ConceptCheck:

Which is held in higher regard by professional astronomers, a hypothesis or a theory?

Explain your answer.

Let's do a quick check of your understanding. Before you go on to the next slide, pause the audio and think about this question. Be sure you can explain your answer. If you're not sure, go back to the second slide and/or review section 1–1 in your textbook. The answers to concept checks are at the end of each chapter in the book, but I highly encourage you to fully think out your answer before looking there. Just looking up an answer doesn’t help you to really learn an idea.

Constellations and Asterisms

Asterisms: Recognizable, dot-to-dot patterns

Constellations: One of 88 sections of the sky

I'm sure you've heard of constellations. We often think of these as being the shapes recognized among the stars by people of long ago. Today, the constellations have a more specific meaning to astronomers. While at their heart, they are still the shapes that people imagined, astronomers actually recognize constellations as being sections of the sky. There are 88 of these sections covering the entire sky. Another word, asterism, is used for familiar, dot-to-dot patterns in the sky, that may or may not relate to the original 88 constellations. Here we see one of the most famous constellations, Orion the Hunter. The center image shows the boundary of the official constellation. So anything within that boundary, as seen from Earth, is said to be “in” the constellation of Orion, even if it is not part of the shape imagined as the hunter. On the right, you can see an actual photograph of part of this region of the sky. The three stars in a row marking the belt of Orion are very easy to see. The red star marking his right shoulder is Betelgeuse, one of the largest stars visible from Earth (it is 800 times the diameter of the Sun, but obviously much further away).

Using Asterisms to Navigate
the Sky

These images show some of the asterisms I mentioned. In the top left image, you see the asterisms known as the Big Dipper and the Little Dipper. These are both parts of constellations. The Big Dipper is part of the constellation Ursa Major, the Big Bear. The dipper is only the back end and tail of the bear, but as the most noticeable part of the constellation, it is called an asterism. The Little Dipper is part of the constellation Ursa Minor, the Little Bear. In this case, the dipper and the bear pretty much match up. These are very important asterisms to know if you live in the northern hemisphere, as they can help you find your direction using the sky. If you find the Big Dipper, you take the two stars in the end of the bowl furthest from the handle and point away from the bottom of the bowl of the dipper. About five times the distance between those two stars away, you will find the end star in the handle of the Little Dipper. That star is Polaris, the North Star. It is called the North Star because the north axis of the Earth's rotation points almost directly towards it. So, as the Earth rotates on its axis, Polaris does not appear to move along with the rest of the stars (it actually does move a little, and makes a tiny circle throughout the course of one day). If you are facing Polaris, you are facing north, and the other directions shouldn't be too hard to figure out.

We can also use the handle of the Big Dipper to find other stars. We can think of the curved handle of the dipper as an arc to help us remember the name of the next two stars, because we can follow the arc to Arcturus and then speed on to Spica. Arcturus is the brightest star in the constellation Boötes the Herdsman. Try as I might, I can't imagine a herdsman in the sky among those stars, so I like to imagine an ice cream cone with Arcturus as the drip of ice cream coming out the bottom of the cone. Spica is the brightest star in the constellation Virgo the Maiden. The arc from the Big Dipper through Arcturus and on to Spica is sometimes called the Spring Arc because it is visible all night long during the spring months.

In the bottom left image, you see a triangle connecting three stars: Deneb, Vega, and Altair. This is called the Summer Triangle because it is visible all night long during the summer months. Those three stars are the brightest stars in the constellations Cygnus the Swan, Lyra the Harp, and Aquila the Eagle. Cygnus is also known as the asterism of the Northern Cross (the Southern Cross is a constellation in the southern hemisphere that can help you find the southern equivalent of the North Star, though there doesn't happen to be a visible star at that point).

The right-hand image has a triangle marked also, but I prefer to find a squashed circle in this area of the sky, which I have outlined (squashed is definitely a good word for it). This asterism, combining seven bright stars, is known as the Winter Circle. Based on what you learned about the Spring Arc and the Summer Triangle, I'll give you one guess why this is called the Winter Circle. And yes, there is a shape for the Autumn, the Autumn Square. This is the only one where the stars are all part of the same constellation, Pegasus the Flying Horse. The Winter Circle is one of my favorites, because it marks the brightest area of stars in the night sky all year long. It also contains examples of many stages in the lives of stars, which you will learn about later in this course.

One last thing to note about these images is the background of faint stars that you can see in the two bottom images. That is the Milky Way Galaxy, or rather our inside view of it. The stars we see as individual points of light are only the nearest ones to us in our galaxy. The rest of the 200 billion stars in the Milky Way are too far away for our eyes to resolve them as individual points of light (and some are blocked by dust and gas in the intervening space), so we just see their combined light as this glowing band stretching across the night sky.

ConceptCheck:

If Jupiter is reported to be in the constellation of Taurus the Bull, does Jupiter need to be within the outline of the bull’s body? Why or why not?

Here's the next check of your understanding of a key concept. Pause the audio to think about your answer to this question before you move on to the next slide. If you are not sure, review slide 5 and/or section 1–2 in your textbook.

All of the observed celestial motions can be described

if our planet spins once each day,

and orbits around our Sun each year.

The Sun, the Moon, and the constellations appear to rise in the East and set in the West, everyday…

and, the constellations shift over the course of the year.

(Different at/near the North and South Poles)

I mentioned before that the North Star, Polaris, will make a small circle over the course of a day as the Earth rotates. You can see this in this photograph of “star trails.” You can take an image like this yourself, by putting a camera on a tripod and leaving the shutter open for a number of seconds or minutes, depending on how dark your sky is. As you see here, each star forms an arc. This photograph was taken in the northern hemisphere in Hawaii (from the Gemini Observatory on Mauna Kea). Polaris, the north star, is almost at the center of these arcs (“almost” because it’s about a degree off from being exactly where the north axis of the Earth points). If you could get this image for 24 hours, each of these arcs would be a complete circle. Because the Sun gets in the way during the daytime, you can't actually get an image of the full circle. All the motions that we see in the sky can be explained by the fact that our planet spins on its axis once every day, and orbits around our Sun once every year. The Sun, the Moon, and the constellations appear to rise in the east and set in the west everyday, and the constellations visible at night shift over the course of the year. Incidentally, this is slightly different at or near the North and South Poles, where the stars appear to spin around as if on a merry-go-round (counter-clockwise in the north and clockwise in the south), and daytime and nighttime last for six months apiece.

Daily Motion and the Earth’s Rotation

Half of the Earth is ALWAYS lit by the Sun.

The Earth spins, changing which part is lit by the Sun.

Let's talk about the Earth's rotation. Half of the Earth is always lit by the Sun. As the Earth spins, the part of the Earth that is lit by the Sun changes. We are, of course, familiar with this phenomenon, as we experience day and night everyday. If you look at the bottom left diagram, you see the Earth as seen from above the North Pole with the sunlight coming from the left. The eastern hemisphere of the Earth, including Asia seen here, is experiencing daylight, while the western hemisphere (we see North America, Western Europe and Northern Africa here) is experiencing night. The red dot indicates a person in California at about 8 PM local time, with the constellation Cygnus straight overhead (which, by the way, indicates that the date is October-ish). As the Earth rotates from west to east, in 4 hours we would see the image on the right, where the person in California is now experiencing midnight (being directly opposite the Sun), and now the constellation Andromeda is straight overhead, with Cygnus further west.

Yearly Motion and Earth’s Orbit

The Earth orbits the Sun―in an almost-perfect circle.
The side turned toward the Sun sees its light.
The side turned away from the Sun sees a changing pattern of stars.

Now let's look at some changes we see as a result of the Earth's orbit around the Sun, which is very close to being a circle. As we saw on the previous slide, the side of the Earth facing the Sun is in daylight, and the side turned away from the Sun sees a changing pattern of stars. This slide shows a local time for California of midnight (if you can see the tiny North America there). At the bottom of the image, the month is July and the constellation Cygnus is straight overhead at about midnight (remember, Cygnus is part of the Summer Triangle, which is visible all night long in the summer, so in July it is halfway across the sky or straight overhead, at midnight). Two months later, in September, the Earth would be in the middle position on this image and at midnight the constellation Andromeda would be at its highest point. After another two months, the constellation Perseus would be at its highest point at midnight in November.

Imagine a giant “celestial sphere” surrounding Earth.

Project the equator and poles into space.
The point directly above you in the sky is the zenith.
Objects near the North celestial pole seems to move in a circle, never setting: circumpolar.

One way to imagine the sky is as a giant “celestial sphere” surrounding Earth. If you project the Earth's equator into space that would become the celestial equator. Projecting the North and South Poles of the Earth into space creates the North Celestial Pole and the South Celestial Pole. Wherever you are on Earth, the point directly above you in the sky is the zenith. This imaginary celestial sphere is sort of how the ancients viewed the universe, with the Earth at the center. We now know that this is not the case, but the celestial sphere is still useful as a way of thinking about the motions we see in the sky. In the bottom image, we are imagining a little person standing somewhere in the middle of the United States (it may be hard to see on this slide, but the same image is on page 10 of your textbook). To this observer, who is at a latitude of 35° north, the North Celestial Pole (with Polaris very nearby) is located 35° above that observer’s northern horizon. The person can't see it, of course, but the South Celestial Pole is 35° below their southern horizon. The stars within a 35° radius circle around Polaris and never appear to set for this observer. Those stars are called circumpolar, and are visible all night long (conveniently, from most locations in North America, this includes the stars of the Big Dipper, so no matter what time of year or time of night, the Big Dipper can be used to find north). Similarly, for this observer at 35° north latitude, stars within a 35° radius circle around the South Celestial Pole will never rise. So, for those of us in North America, Europe, and Asia, if we want to see the beautiful stars of Crux the Southern Cross, we have to travel further south. That also includes being able to see the nearest galaxies to our own Milky Way, the Large Magellanic Cloud and the Small Magellanic Cloud, which are near the South Celestial Pole.

Motions of the Celestial Sphere

Objects near the celestial pole seem to move in a circle, never setting: circumpolar.
Your latitude impacts how the stars appear to move.

As you saw on slide 8, stars near the north celestial pole appear to move in a circle. If the full circle a star traces out during a 24-hour period is completely above the horizon, that star is said to be circumpolar: it never sets. But the view changes with your latitude. Looking at either the North Celestial Pole or the South Celestial Pole, the stars appear to move in circles, as in the photograph on slide 8. Now, what about if we are looking towards the east or west, rather than north or south? In the image on the left, we see the stars are moving in lines at an angle to the horizon. The angle depends on your latitude. At middle latitudes, you would see something like the image on the left, which shows stars setting in the west. The merry-go-round effect at the poles that I mentioned before is shown in the center image, which was taken near the North Pole, where the stars are moving in lines parallel to the horizon. On the right, we see the view from the equator, where the stars rise in the east perpendicular to the horizon and set in the west, also perpendicular to the horizon. In all of these cases, the stars do still appear to move in circles, as the motion is based on the circular rotation of the Earth, but the full circle may or may not be completely above the horizon.

ConceptCheck

People in which of the following cities in North America experience sunrise first: New York, San Francisco, Chicago, or Denver?
If the constellation of Cygnus rises along the eastern horizon at sunset, at what time will it be highest above the southern horizon?
If the Earth suddenly rotated on its axis three times faster than it does now, then how many times would the Sun rise and set each year?
Where would you need to be standing on Earth for the celestial equator to pass through your zenith?

And here we are at another concept check. Pause the audio to review these questions. Make sure you're clear on the answers before moving forward. If not, review the previous slides and section 1–3 in your text.

The Earth is tilted on its side.

The Earth’s North Pole is always tilted toward the North Star. This will not change in your lifetime.
Earth does not change its tilt “toward” or “away” from the Sun.

Remember that Earth orbits the Sun in an almost-perfect circle. Don’t let this picture fool you!

Here is a very important thing to know about the Earth. It is tilted on its side. The Earth's North Pole is tilted toward the North Star, Polaris. While this is not always the case, it will not change in your lifetime. It changes on a time scale of about 26,000 years. The Earth does not change its tilt toward or away from the Sun, the tilt is always pointing the North Pole towards the North Celestial Pole. So, as the Earth orbits around the Sun, the relationship between the tilt of the Earth and the Sun changes, as shown in this diagram. When the North Pole is tilted toward the Sun, it is summer in the northern hemisphere. At the same time, the South Pole is tilted away from the Sun and it is winter in the southern hemisphere. You see that in the left-hand image of the Earth in this diagram. The right-hand image of the Earth in this diagram shows a time six months later. The North Pole is still pointed towards Polaris, but that means that the North Pole is now tilted away from the Sun, which makes it winter in the northern hemisphere. And, at the same time, the South Pole is now tilted toward the Sun, and it is summer in the southern hemisphere.

This is a really important point. Many people think that summer is caused by the Earth being closer to the Sun, but the Earth’s orbit is an almost perfect circle, so the difference in distance between its closest point and its furthest point is not that big. Also, if the distance were the reason, then how could the seasons be opposite in the northern and southern hemisphere? They couldn’t, but they are, so distance is not the reason for the seasons.

The angle and hours of sunlight change during the year.

The Northern Hemisphere’s Winter:

The North Pole cannot spin into the sunlight.

Light at the Tropic of Cancer is “weak.”

The Northern Hemisphere’s Summer:

The North Pole cannot spin out of the sunlight.

Light at the Tropic of Cancer is “strong.”

The reason we have seasons is that the angle of sunlight and the number of hours of daylight change throughout the year. This image is two times of year superimposed. Imagine the Sun is at the center (not to scale, obviously). On the left is the Earth at the winter or December solstice, and on the right is the summer or June solstice. These terms tend to be a bit northern-centric, as what is often called the winter solstice is actually the first day of summer in the southern hemisphere.

Let’s look first at the image on the left, that’s the December solstice. The northern hemisphere is tilted away from the Sun, and the southern hemisphere is tilted toward the Sun. As the Earth rotates during this time of year, the North Pole cannot spin into the sunlight, and so it is experiencing 6 months of night. The rest of the northern hemisphere experiences the Sun’s rays hitting the ground at more of an angle (we’ll see this in an image on the next slide), so it’s northern winter. But at the same time, the South Pole is in constant daylight for those same 6 months. The spinning of the Earth can’t bring the South Pole away from the sunlight. The rest of the southern hemisphere is experiencing more direct sunlight during the day, and it’s southern summer.

In the image on the right, the situation is reversed. The North Pole is in constant daylight, and the rest of the northern hemisphere is experiencing more direct sunlight, so it’s northern summer. The South Pole is in constant darkness, and the rest of the southern hemisphere gets less direct sunlight, so it’s southern winter.

The angle and hours of sunlight change during the year.

When the sunlight hits the ground directly, it heats the ground more efficiently.

When the sunlight hits the ground for longer periods of time, it gets hotter!

The tilt of the Earth causes sunlight to hit the Earth more directly, and for a longer periods of time, during Summer.

Summer Solstice

(June in the north/December in the south

Winter Solstice

(December in the north/

June in the south)

So, here is the key concept: the angle and hours of sunlight change during the year. The tilt of the Earth's axis causes the angle of sunlight hitting the ground to be larger or smaller. When the angle is larger, the Sun gets higher in the sky at midday, and is therefore hitting the Earth more directly and for longer periods of time. This is summertime. So, not only is there more sunlight (a longer day), but the sunlight is hitting the ground more directly and the same amount of energy is concentrated in a smaller area. Conversely, in the winter sky the Sun is lower in the sky at midday, and so the same amount of energy is spread out over a larger area and less ground heating happens. Furthermore, the Sun is in the sky for less time (a shorter day), so less ground heating happens for that reason also.

The Sun’s Path on the Celestial Sphere

The Sun appears to cover one constellation after another, along the ecliptic.
In reality, the changes we see in the Sun’s position occur because WE are moving.
Bonus: The eight planets also appear to travel on the ecliptic.

Let's think about the celestial sphere again. Remember this is an imaginary way of thinking about these motions. From our position inside the imaginary celestial sphere, the Sun appears to move through one constellation after another along a path called the ecliptic. The constellations along the ecliptic are the ones known as the zodiacal constellations, and so the Sun’s apparent motion through these constellations was used in ancient times as the basis for astrology, which was an interpretation of the motions of objects in the sky and their supposed influence on humans. We now know that it is really the Earth that is moving, and the motions of objects in the sky have no control over human behavior. The planets also appear to move along the ecliptic, and that is simply due to the fact that the planets orbit the Sun mostly in the same plane. Incidentally, the 26,000 year time frame that I mentioned two slides ago for the change in where the Earth’s axis of rotation points has already caused astrology to be even more out of step with reality. You may have been told that because of your birthday, you have a particular “sign,” which in ancient times would have represented the position of the Sun on the celestial sphere in the month you were born. Because the Earth’s axis of rotation wobbles like a spinning top, the Sun’s position on the celestial sphere is a constellation and a half “off” from where it was when astrology was first invented.

Equinoxes and Solstices

When the ecliptic and celestial equator intersect, day and night are each 12 hours long: the equinox.
When the Sun reaches its most Northern and Southern points in the sky: the solstice.

As the ecliptic, the Sun's path along the celestial sphere, is tilted because of the 23.5° tilt of the Earth's axis, it intersects the celestial equator twice per year. Those two times are called the equinoxes, the vernal (or spring) equinox in March and the autumnal equinox in September. So as not to be northern hemisphere-centric, you can also refer to them as the March equinox and the September equinox as on this slide, but the northern hemisphere-centric terms have been used for a long time. Equinox means equal night, and so on the equinoxes day and night are of equal length: 12 hours each. The other extreme points are when the Sun reaches its most northern and most southern points, and these are called the solstices. The northern solstice, when the sun is at its most northern point, is the day when the sun is at its highest point at midday in the northern hemisphere, and at its lowest point at midday in the southern hemisphere. That is called the first day of summer in the North, and the first day of winter in the South. Conversely, the southern solstice is when the Sun reaches its highest point at midday in the southern hemisphere and its lowest point at midday in the northern hemisphere. This is the first day of summer in the South and the first day of winter in the North.

The Sun appears to move on the celestial sphere.

The Sun appears to rise and set at different locations:
Winter: toward the South
Summer: toward the North
Fall and Spring: due East and West

Another consequence of the tilt of Earth's axis and the apparent motion of the Sun on the celestial sphere is that the Sun appears to rise and set at different locations along the horizon throughout the year. On the equinoxes, the first day of fall or the first day of spring, the Sun rises due east and sets due west. In the northern hemisphere, on the first day of winter, when the Sun is at its lowest point in the sky, the Sun rises in the southeast and sets in the southwest, making a very short arc across the sky. On the first day of summer, when the Sun is at its highest point in the sky, it rises in the northeast and sets in the northwest, making a very long arc across the sky. In the southern hemisphere, these directions are the same, except that rising in the southeast and setting in the southwest causes the Sun to make a long arc in the sky (that would be southern summer), and rising in the northeast and setting in the northwest causes the Sun to make a short arc in the sky, for southern winter.

ConceptCheck

If Earth’s axis was not tilted, but rather was straight up and down compared to the path of Earth’s orbit, would observers at Earth’s north pole still observe periods in which the Sun never rises and the Sun never sets?
How long does the Sun take to move from being next to a bright star all the way around the celestial sphere and back to that same bright star?
How often each year does an observer standing on Earth’s equator experience no shadow during the noon-time Sun?
Approximately how many days are there between the northern solstice and the March equinox?

OK, so let's see how you understand these concepts. As always, please don’t move on until you are clear on these ideas. You can review the previous slides as well as section 1-4 in the textbook. Answers are at the back of the chapter, but try not to look there until you’ve fully thought it out for yourself and reviewed the material.

The Moon is lit by sunlight.

Just like the Earth, half of the Moon is lit by sunlight.
The Moon does not produce its own light.

This image of the Earth and Moon was taken by the Galileo spacecraft.

Now let's move on to the motion and appearance of the Moon in the sky. Just like the Earth, half of the Moon is lit by sunlight at all times. The Moon does not produce its own light, so we see it only by reflected light. This is a very cool image taken by the Galileo spacecraft as it was heading toward Jupiter. It was the first image taken of the Earth and Moon together.

Understanding the Moon’s Phases

The phase of the Moon is a result

of our point of view.

Now, to understand the Moon's phases, we first need to think about the fact that the phase of the Moon is a result of our point of view. Nothing about the Moon itself is changing. A “new” moon is one that we actually cannot see in the sky, because from our point of view the far side is in daylight and the side facing us is experiencing night. Since the Moon makes no light of its own, we cannot see the part of the Moon experiencing night. A "full" Moon occurs when, from our point of view, the side facing us is in full daylight. First quarter and third quarter Moons occur when half of the side facing us is in daylight. First quarter is when the right-hand side is lit up from our point of view; third quarter is when the left-hand side is lit up from our point of view. When I learned this, I remembered the order by thinking that a first-quarter Moon looks like a capital D, a full Moon looks like a capital O, and a third quarter Moon looks like a capital C. So, in order they spell out DOC.

The “pictures” of the Moon show

what you would see from Earth

when the Moon is in that location.

There are actually more phases of the Moon than the four mentioned on the previous slide. When the moon is past the new phase, but not yet to first quarter, it is called a waxing crescent. Think about dipping a wick in candle wax… the candle keeps getting bigger. When the Moon is past first quarter, but is not yet full, it is called a waxing gibbous moon. After the Moon has passed the full stage, but is not yet to third quarter, it is a waning gibbous moon. After third quarter, as the Moon is heading towards another new moon, it is called a waning crescent moon.

If you look at this diagram the drawings of the Moon on the inside circle show that the side of the Moon facing the Sun is lit, while the photographs of the Moon on the outside circle show what it would look like from our perspective on Earth. The phases are entirely a result of the changing angle between the Moon and the Sun as seen from Earth. The times written inside the inner circle represent when that phase is highest in the sky. So, a new moon is highest in the sky at noon (and of course, so is the Sun, so we do not see the Moon). A first quarter moon is highest in the sky at about 6 PM or sunset, a full moon is highest in the sky at midnight, and a third quarter moon is highest in the sky at around 6 AM or sunrise. This means that the Moon is visible in the daytime almost as much as it is visible in the nighttime sky (almost, because a new moon isn’t actually visible). The Moon looks faint in the daytime, because of the brightness of the sky, but it is still lit up by sunlight and still quite visible, you just have to know to look for it.

There is a different phenomena that allows us to faintly see the unlit part of the nearside of the Moon sometimes. That phenomena is called Earthshine, and it is what happens when sunlight reflects off of the Earth, travels to the Moon, and reflects back off the nighttime side of the Moon, returning to Earth. That reflected light lets us see the otherwise dark part of the Moon, having made an extra journey to the Moon and back to our eyes.

The Moon’s Synchronous Rotation

The Moon makes one orbit around Earth,

and spins one time on its axis, in the exact same amount of time.

We always see the same side of the Moon―the other side is not the “dark side,” but the “far side.”

There is something special about the Moon's orbit around the Earth, that is also true for most of the other moons in the solar system, and that is that it orbits in “synchronous rotation.” That is, in the time that it takes to orbit the Earth, it spins exactly once on its axis. As a result, we always see the same side of the Moon from the Earth. That is called the near side, while the side we cannot see from Earth is called the far side. It is not called the dark side (except in a Pink Floyd song) because all parts of the Moon experience the same amount of daylight and night (about 2 weeks at a time for each). We did not see the far side of the Moon until the first spacecraft went around it.

You can test out synchronous rotation for yourself. Have a friend stand in the middle of the room, while you face them. Have another friend stand on the outside. Walk around the friend in the center, keeping your face pointed toward them at all times. Ask them what they saw. They will have seen the front of you the whole time. Now ask the friend on the outside what they saw. They will have seen you make one complete rotation (they will have seen all sides of you). So, the friend in the middle is the Earth, and only sees one face of the Moon (you). The friend on the outside is the Sun, and sees all sides of the Moon.

Sidereal and Synodic Months

The Moon orbits the Earth as Earth orbits the Sun, so the Moon has to “catch up.”

The Moon returns to its place on the background stars: sidereal month.
The Moon completes a full cycle of phases: synodic month.

One last bit about the Moon's orbit: the word “month” comes from “moonth,” and is about the length of time it takes the Moon to orbit the Earth. But there are two kinds of months, because as the Moon orbits the Earth, the Earth is orbiting the Sun. So, in the time it takes the Moon to orbit the Earth, the Moon actually has to move a little bit more to get back to the same relative position between the Earth and the Sun. Let's look at this diagram to explain this. At step 1, the Moon is exactly in between the Earth and the Sun, and that is the new moon phase. If we on Earth are looking at the Moon, we are looking in the direction of a particular constellation (ignoring the fact, of course, that we would not actually see the constellation because looking at a new moon implies looking at the Sun). At point 3 in the diagram, the Moon is again in the direction of the same constellation (the Moon's position is indicated by the line coming from the text box). This is called a sidereal month. However, notice that at this point the Moon is not new again yet, so it needs to move a little bit more to get back on the line between the Earth and the Sun to be a new moon. The time between new moon and new moon is one synodic month. Note, that it is also a synodic month from full moon to full moon, first quarter to first quarter, or any other phase.

ConceptCheck

If an observer on Earth sees just a tiny sliver of the crescent moon, how much of the Moon’s total surface is being illuminated by the Sun?
If the Moon appears in its waxing crescent phase, how will it appear in two weeks?
If astronauts landed on the Moon near the center of the visible surface at full moon, how many Earth days would pass before the astronauts experienced darkness on the Moon?
If Earth was orbiting the Sun much faster than it is now, would the length of time between full moons increase, decrease, or stay the same?

And here we are at another concept check. These concepts can take some time to understand, so make sure you take the time to read, ask questions, and act it out. One way to do this is to hold a ball in a dark room with one light source. You are the Earth, the ball is the Moon, and the light source is the Sun. Spin the ball around your head and watch the changing patterns of shadow and light.

Eclipses occur when the Sun, Moon, and Earth are perfectly aligned.

The Moon’s orbital plane is just a little off of the ecliptic.

Now let's talk about a very cool thing that happens occasionally as a result of the alignment between the Sun, Moon, and Earth: lunar and solar eclipses. In order for these to occur, the alignment must be perfect, and so they happen only at certain times (certain very predictable times). The Moon's orbital plane is slightly off of the ecliptic (which is the plane of Earth's orbit). As a result, the Moon is usually either slightly above or slightly below the ecliptic. The two points where the plane of the Moon’s orbit and the plane of the Earth's orbit intersect are called nodes. An imaginary line connecting the nodes is called (surprise, surprise) the line of nodes.

The Sun, Moon, and Earth rarely line up.

In order for the Sun, Moon, and Earth to be in perfect alignment, the Moon must be on the line of nodes during a new moon or a full moon. This is a relatively rare event, as you can see in this diagram, but when it does occur, eclipses are possible.

Lunar Eclipses

When the Moon is opposite the Sun, it can travel through the Earth’s shadow.

The Earth’s shadow is complete in the center and partial on the edge.

When the Moon is opposite the Sun (in other words, at full moon), it can travel through the Earth's shadow (but it has to be along the line of nodes, otherwise it would be too high or too low). The Earth's shadow is actually not completely dark. As sunlight passes through the Earth's atmosphere, it refracts or bends inward. The redder colors of the sunlight refract more than the bluer colors, and so the Earth's shadow gets filled with that red light. So, when the Moon passes into the Earth's shadow, it gets hit with that red light, which then reflects back to our eyes on Earth. The very center of the Earth's shadow, called the umbra, is darker than the outer part of the shadow, called the penumbra. Therefore, when the alignment allows the Moon to pass through the very center of the shadow, the eclipse will be darker than if it just passes through the penumbra.

Total Lunar Eclipse,
January 20, 2000

This time lapse image shows the total lunar eclipse of January 20, 2000. Time flows in this image from right to left, so the far right photograph shows the very beginnings of the Moon entering the penumbra of the Earth's shadow. The center image is at totality, when the Moon is completely bathed in the reddish light of the Earth's shadow. Note that lunar eclipses can only occur at full moon, and again, only when the Moon is on the line of nodes. A lunar eclipse is visible from anywhere on the side of the Earth facing the Moon at that time.

Total Solar Eclipses

The Moon totally covers the face of the Sun.
From inside the darkest part of the Moon’s shadow.
Those inside of the Moon’s partial shadow see a partial eclipse.

On the other hand, solar eclipses occur only at new moon (and again, only when the Moon is on the line of nodes). What happens in this case is that the perfect alignment allows the shadow of the Moon to fall on the Earth. Those who are on Earth in the shadow path will see the eclipse, which they will see as the Moon completely covering the face of the Sun. People just outside the shadow path will see a partial eclipse, in which the Moon only partially covers the Sun. The inset image is an actual photograph taken from the International Space Station of the Moon's shadow on the Earth.

Spectacular, Rare Total Eclipses

Here are some photographs of total solar eclipses. For the length of totality (which can last anywhere from seconds to a maximum of about seven and a half minutes), the sky darkens, stars and planets become visible, birds stop chirping because they think it’s nighttime, and the faint outer atmosphere of the Sun (the corona) becomes visible. This brief period of totality is the ONLY time that it is safe to look directly at the Sun with unprotected eyes. People travel all over the world to catch these rare and beautiful events.

Annular Solar Eclipses

When the Moon is at its farthest position, the cone of its shadow doesn’t reach Earth.
The Moon appears to be too small to cover the Sun.

The Moon slightly changes its distance from the Earth, as its orbit is not perfectly circular. If a solar eclipse occurs when the Moon is at or near apogee (its furthest point from the Earth), then the Moon will appear very slightly smaller than the Sun, and will therefore not completely cover the face of the Sun. Because the Moon is slightly further away, the tip of its shadow doesn't quite reach the Earth. So instead of a total solar eclipse, people along the shadow path will see an annular solar eclipse. At maximum eclipse, a thin ring (or annulus) of the Sun will appear around the Moon. It is NOT safe to view an annular solar eclipse without proper eye protection (sunglasses not included). Page 26 in your textbook has a map of eclipse paths for total solar eclipses through the year 2020, so check it out to see whether you might be able to see one. This is seriously not an event to miss if you are near it.

ConceptCheck

Why don’t lunar eclipses occur each time the Moon reaches full moon phase?
Why does the eclipsing Moon spend more time in the penumbral shadow than in the umbral shadow?
Why can total lunar eclipses be seen by people all over the world, whereas total solar eclipses can only be seen from a very limited geographic location?
If you had a chance to observe a total solar eclipse and a total lunar eclipse, in general, how much longer would you expect one type to last than the other?

And here is the final concept check for this chapter. Pause the audio to read and think about these questions.

The material in this chapter, in particular, knowing what causes the seasons, the phases of the moon, and eclipses, are the subject of the most common misconceptions about astronomy. So, if you understand the material in this chapter well, you will be doing better than most people in this regard. Once you understand this chapter, ask your friends and family about these things. Not for the purpose of making fun of them for not knowing, but to teach them. When you make the effort to teach these concepts to someone else, you will understand them better. The process of explaining to someone else can help point out things that you’re not yet clear about, and you can go back and learn them better. Reading and hearing about something is one level of learning, but to achieve true understanding, you need to be able to teach someone else. See you next week!