In the earliest recorded history of humans, we see evidence of their wonder and amazement at the Sun, the Moon, and the stars. Look at the time-lapse video below of a sunrise, for example. Witnessing this event up until about 400 years ago, respected scientists and astronomers concluded that the Earth stood still and that the Sun, stars, and other planets revolved around it. Even today, with incontrovertible evidence proving the Earth revolves around the Sun – not the other way around, it is difficult to view the sunrise time-lapse video below or watch a real sunrise or sunset without “feeling” that the Sun is moving and that we are standing still on a stationary Earth.
Lessons from the History of Science for LabLearner Students
Sometimes in the history of science, we find that most or all of human society viewed the world around them and interpreted the occurrence of natural events entirely wrong. This is because, in the absence of data, and certainly in the absence of science, reasonable people and experts alike can only guess at reasons and explanations of physical phenomena. Once additional data is collected and digested, scientists must change their minds and propose new or altered explanations, eventually arriving at reality, at the truth. This is a lesson all LabLearner students can learn from. Every time you make a hypothesis or prediction and then find contrary evidence in the lab, you must reevaluate your views and opinions. This has always been the case in science and always will be. This line of thinking naturally leads us to conclude that even things that we believe today, things that we are certain are correct, things that we read online or in textbooks, may be wrong. That is why good scientists and even good science students must be skeptical and challenge even the most widely-accepted facts and explanations they are presented! That’s science.
For example, today most people take it for granted that the Earth is round, spins rapidly on its axis, and orbits the Sun, while the Moon orbits the Earth.
Understanding the Universe
Until relatively recently during human history, our human-centric view of the Universe was that everything revolved around the Earth. This conception of the Univers was proposed by the brilliant mathematician and astronomer Ptolemy, who lived in the second century AD. Ptolemy placed the Earth at the very center of the Univers and for many centuries was able to explain observations from earth. Notice how the Sun, like the Moon and other planets (Uranus and Neptune were not yet observed at the time), revolve around the Earth. The Ptolemaic Model of the Universe had a stronghold on scientific thought for many centuries, guiding both Islamic and Western European astronomers and philosophers as they accepted it dogmatically.
It was not until over a thousand years later that the Polish astronomer and mathematician, Nicolaus Copernicus (1473-1543), recognized that the planets revolve around the Sun. The Copernican heliocentric (ancient Greek ήλιος (helios) “sun”) put the Sun at the center of the Universe, rather than the Earth. The Copernican heliocentric model of the Universe is shown here as a reproduction of a graphic from his book, De Revolutionibus Orbium Coelestium (On the Revolutions of the Heavenly Spheres) published the year of his death. In the Copernican model, the Sun is at the center and the Earth occupies the third orbit in distance from it following Mercury and Venus, respectively. He shows that the Moon revolves around the Earth and then the other know planets Mars, Jupiter, and Saturn revolve around the Sun as well. Copernicus depicts each planetary orbit as a perfect sphere. Finally, beyond Saturn, just as in the Ptolemy model, Copernicus places all of the stars in an outer ring, the Stellarum Fixarum, or stationary sphere of immobile stars.
The Copernican heliocentric model is basically true in its most essential features. It is not entirely accurate, but close. Part of the issue is that, as you may have noticed, in the case of both the Ptolemaic and Copernican models, the models are referred to as the “Universe”. What they called the Universe, we now call the Solar System. This is because today we know that our Solar System is only one of the trillions of others.
In addition to the significance of the publication of Copernicus’ heliocentric model, Copernicus is also often credited as one of the founders of modern science. He used observation, careful measurements, and exhaustive calculations to support his model. Interestingly, the same year (1543) as the publication of the heliocentric model, the anatomist, Andreas Vesalius, published in Basel, Switzerland, his now-famous work, De Humani Corporis Fabrica (On the fabric of the human body). Vesalius applied careful observation and painstaking dissection and detailed drawings of the human body. Vesalius’ work revolutionized anatomical science and medicine. While there was still a long way to go, Vesalius and Copernicus are often thought of as two of the first truly modern scientists.
Returning to the Copernican heliocentric model of the Universe, even though it is correct in many important aspects, it was entirely unproven and, at the time, unprovable. Coupled with its discrepancy with what appeared to be obvious to everyone observing the skies for a thousand years, it was not accepted when first published. For example, according to Copernicus, the Earth rotates on its own axis at incredible speed as it flies through space in its rotation around the Sun. This is contrary to human experience. Why, it was asked, if the Earth was spinning at high speed, did humans not simply fly off from its surface? Why can’t we feel such movement? These and other objections were left unanswered with the death of Copernicus. First, the Italian scientist Galileo Galilei (1565-1642), some 50 years after Copernicus’ death, used the newly discovered telescope to finally contribute experimental data in support of the heliocentric model at the expense of considerable friction with the Church.
Finally, when Isaac Newton studied and explained the force of gravity, which finally provided an acceptable reason why people were not thrown into space from the rapidly-spinning Earth, essentially all of the scientific arguments in favor of the heliocentric model alined and the model has been accepted as an accurate interpretation of indisputable fact ever since.
By the time of Newton, and largely because of Newton, modern science was firmly established. As time passed, opposition to the Copernican model melted away as both scientific and theological arguments against it were dropped and our understanding of the Universe and humanity’s place in it have formed the foundation of the science we know today. Nonetheless, LabLearner students should remember this short walk through the history of science because, as stated earlier, we must always remember that new ideas are often rejected when they are first proposed. Just like it did for Copernicus, Gallio, Newton, and many others, the tools and methods of science, as well as an inquisitive and open mind, will continue to push our knowledge forward – and you can certainly be a part of an even more exciting future.
Scientific Misconceptions Involving Space
Even though Isaac Newton explained the motion of the planets rather clearly over 300 years ago, the majority of people today do not understand what causes the orbits of planets, night and day, the phases of the moon, or the seasons. When asked, many people report misconceptions such as the Earth’s shadow causes the phases of the Moon or that the seasons occur because of the changing distance from the Sun of the Earth during its annual elliptical orbit around it. The goal of this CELL is to provide you with the correct conceptions about planetary motion through models and simulations.
Seasons of the Year: Misconception and Facts
One of the great names in the history of astronomy is Johannes Kepler (1571-1630). Kepler was a contemporary of Galileo and accepted the Copernican model of a heliocentric Universe. However, in attempting to explain various astronomical measurements, he proposed that, unlike the perfectly circular planetary orbits of the Copernican Universe, the orbit of the planets are elliptical.
Today we know that the orbi of the planets, including Earth, are much closer to the Copernican circular orbit rather than the Kepler ellipse, even though not perfectly circular.
Nonetheless, Kepler’s reputation and excellent mathematics resulted in the elliptical orbit remaining a reasonable view of planetary motion for many years, particularly in the popular imagination. As a result, many diagrams of the Solar System have been depicted in science textbooks for many generations after Kepler’s death. And once an image is embedded into the popular imagination, it may appear in non-scientific publications and other work for years, often leading to highly-accepted and reproduced misconceptions. One such misconception is that the seasons of the year on Earth is caused by how close our planet is to the Sun during its annual rotation around it. Thus, in the figure below, we see how this misconception can be drawn to lead to entirely wrong conclusions.
Even though this misconception of the seasons of the year is incorrect, just look at how reasonable it seems. According to this view, the Earth is significantly closer to the Sun in the Summer than in the Winter. This would naturally lead one to believe that is the reason it is hotter in the summertime. Distance from the Sun does, of course, have a great deal to do with why Mercury is so much hotter than the Earth and Mars and the outer planets are so much colder than the Earth. Returning to the misconception, in Spring and Fall, the Earth is intermediate in distance from the Sun and therefore intermediate in temperature. Everything makes so much sense. But now consider only two facts that are required to entirely discard this misconception.
Fact 1: The Earth’s orbit around the Sun is nowhere near the elliptical form proposed by Kepler and ingrained in the popular imagination. In fact, the Earth is actually furthest from the Sun (but not by much) in July.
Fact 2: When it is Summer and hot in the Northern hemisphere, it is Winter and cold in the southern hemisphere. Thus, regardless of the actual path our planet takes around the Sun, the misconception above provides absolutely no explanation for this indisputable fact.
From this brief discussion, we can clearly see that any explanation of any physical phenomena, any conception we have must stand the scrutiny of a test of the facts! That’s why scientists spend so much time and effort collecting facts and carefully analyzing them. This is a very important lesson for you to learn.
So why is it hotter in the Summer than Winter, Spring, and Fall? And why is it hot in the southern hemisphere when it’s cold in the northern hemisphere?
Think of nearly every desk globe you have ever seen – in the classroom, your bedroom, or the library. Now, look at the example of the globe shown here. The Earth is tilted. The degree to which it is tilted is about 23.5o. That is not an insignificant tilt. You will learn much more about the seasons of the year in relation to the tilt of the Earth on its axis in Investigation 1. As you will see in your lab experiments, the tilt of the Earth on its axis, coupled with its counterclockwise orbit around the Sun, provides all of the necessary facts to explain the observed seasonal temperature fluctuations around the entire world.
Relative Sizes: Misconception and Facts
When viewed from the Earth, the relative size of the Moon and Sun are strikingly similar – they appear to be the same size to an Earth-bound observer. However, in fact, the Moon is approximately 400 times smaller than the Sun. How can this be? How can the Sun be so much larger than the Earth and yet appear the same size to us? The answer to this puzzle is the same as for why stars that are thousands of times larger than our Sun appear only as tiny points of light in the night sky when viewed from Earth – they are much, much further from us than the Sun.
At the top of the illustration above, the relative distance from Earth to Moon and Moon to Sun is shown to scale. Look how close the Moon is to the Earth than the Sun. In fact, the Moon is 400 times further from the Sun than it is from the Earth. Thus, the Moon is 400 times smaller than the Sun and 400 times closer. The net effect of this coincidence is that Sun and Moon appear to be almost exactly the same size when viewed from Earth (bottom of the illustration above). This size comparison is most easily appreciated at the time of a complete solar eclipse. In a solar eclipse, the Moon comes directly between the Earth and Sun. The sky grows dark and the only part we can see of the Sun is a part of its glowing atmosphere, called the corona. The actual surface of the Sun, called the photosphere is almost exactly covered by the Moon. At all other times, the surface (photosphere) of the Sun so overpowers our view that we cannot see the fainter corona.
Lunar Cycle: Misconception and Facts
The lunar cycle is the sequence of images we see of the Moon during a complete rotation around the Earth of approximately 28-days (~27.3-days). The crescent-shaped moon typical in children’s books portrays one such image. What causes the different “shapes” of the Moon on different nights of the month?
A common misconception of the lunar cycle is that it occurs due to the Earth’s shadow on the Moon. In fact, a now widely repeated finding that many Harvard graduates and some of their non-science professors apparently believe that the phases of the Moon are caused by the shadow of the Earth cast on its surface has become legendary. But there are other misconceptions about the phases of the Moon, many others. Here are a few of the most common examples taken from a study of students in the 9- through 16-year-old range:
- Clouds cover the part of the Moon we cannot see.
- Planets cast shadows on the part of the Moon we cannot see.
- The shadow of the Sun falls on the Moon, blocking our view of it all.
- The shadow of the Earth falls on the Moon, blocking our view (the Harvard misconception that is now called “the eclipse explanation”)
In your experiments in Investigation 2, you will see for yourself the correct explanation for the phases of the Moon, also called the lunar cycle.
The Moon, like the Earth, is approximately a sphere. If you shine a bright light on a sphere (you will do this in the lab) exactly half of the sphere will be in full light and half will be in full darkness (if you are in a dark enough room). However, when the Moon is beside the Earth, we only see part of the side of the Moon that is illuminated. Further, since the Moon is orbiting around the Earth, the pattern of light we see from the Earth continually changes as well. The phases of the Moon are simply the cycle of illuminations we see as the lunar cycle progresses.
In the image below, the most frequently referred to lunar phases are labeled. In the top illustration, as well as in the timelapse video below it, the Sun would be far off in the distance to the right of the picture. We start at the New Moon: the First Quarter Moon occurs about 7 days after the New Moon. The Full Moon occurs about 7 days after that, with the Third Quarter Moon following 7 days later. Finally, 7 days after the Third Quarter Moon (sometimes called the Last Quarter Moon), the lunar cycle is completed and a New Moon occurs again.
Notice that between the quarter positions, there are additional phases labeled above. For example, a Waxing Crescent occurs between a New Moon and the First Quarter Moon. A Waning Gibbous Moon occurs between the Full Moon and the Third Quarter Moon, and so on. The animated illustration above shows what we would see from the Earth on a nightly basis as the Moon cycles through its phases. This timelapse is a good way for you to appreciate that the images of the Moon do not abruptly jump from one phase to the next, but rather morph smoothly from one to the other as the Moon orbits the Earth.
When you look at the Moon (try it tonight), you may get confused at the direction in the lunar cycle. Here is a simple trick to quickly orientate yourself regardless of the stage of the cycle:
If the Moon has more light on its right side than its left (it might be slightly tipped but you can usually still identify a left and right), it is heading from a New Moon to a Full Moon. On the other hand, if the Moon has more light on its left side than its right side, it is heading from a Full Moon to a New Moon.
The Moon is Not Out During the Day: Misconception and Facts
The Moon is most typically associated with the nighttime – with horror stories about werewolves, vampires, and even Hansel and Gretel finding their way back home from the woods following white pebbles in the moonlight. At night, against a dark background, it is sometimes the only natural light, or at least the brightest natural light, when our side of the Earth is turned away from the Sun. However, while it is much more noticeable at night than during the day, the Moon is also out during sunlight hours.
Perhaps you have seen the Moon against the blue sky on a clear day. This is no mystery and in fact, you see the Moon during the day for exactly the same reason that you see it at night – it reflects the Sun’s light off its surface. It is certainly harder to see during the day and appears much fainter because it doesn’t contrast against the dark night sky. But keep your eyes open, you’ll see it. And once you have seen it, you’ll likely notice it more often.
- Fun Facts: Calendars and Sundials
- More Fun Facts: Planetary Motion
- Learn the Lingo
- Get Focused
Calendars and Sundials
From the very beginnings of culture, humans have linked space and time. At the most simplistic level, day and night are inseparably united by light and dark, which is linked directly to the rotation of the Earth on its axis. The Moon has a cycle that repeated roughly every 28 days. Seasons repeat in a predictable sequence and time.
Because time and space are so closely linked, one can predict the precise position of any star in the Universe or planet in our Solar System any minute of the day, any minute of the day of any day of the year, and any day of the year of any year for thousands of years both forward and backward in time. Yes, time and space are very tightly linked indeed.
Well before the knowledge that the Earth is round or that it revolves around the Sun rather than the other way around, humans used the movement of objects in space to measure time. The longest and shortest day of the year (the summer and winter solstice, approximately June 21 and December 22, respectively) could be followed by 3,000 BC at Stonehenge, England. On these special days, the Sun rises and sets at precise locations relative to carefully placed stone columns and lintels (the stone blocks laid horizontally across the vertical columns), and a large Heel Stone (see video below).
Other yearly astronomical dates could also be determined by sunset or sunrise relative to other stones in Stonehenge’s circular arrangement. Thus, at least one of the functions of Stonehenge was for use for tracking times of the year, making it one of the world’s oldest calendars. This massive structure may have had religious significance as well. In any case, the functions of Stonehedge must have been very important to our Neolithic ancestors, since some of the stones are over 27,000 kilograms, about five times the mass of an elephant. On the other hand, the calendar function of Stonehenge still works today, some 5,000 years later!
Hours of the Day: Sundials
Sundials have been used to tell daytime hours since several hundred years BC. The stem that sticks out of the sundial and casts its shadow on the numbers is called a gnomon. The entire sundial is orientated so that the gnomon and twelve-noon position (XII in Roman numerals often used on sundials) point to the north. On the portable sundial shown in the time-lapse video below, you can see the small built-in compass required to point the instrument in the correct direction. The video begins at around 9 in the morning and ends around 3:30 in the afternoon. Notices the glare on the dial at noon (XII) as the Sun passes directly overhead. Also, note that the very expression “the Sun passes directly overhead” refers back to the notion that the Earth stands still and the Sun moves above us!
MORE FUN FACTS
Planetary Motion: Orbits
In addition to understanding the causes of night and day, the seasons of the year, and the lunar cycle, we should also discuss the causes of the unique orbital motion of the Earth, Moon, and other planets.
Investigations Three and Four are designed to help you to comprehend the causes of planetary motion (see the figure to the right). Circular (e.g., planetary) motion occurs because of two Forces. Firstly, an object (e.g., planet) is already moving with a certain speed or momentum in a particular direction (i.e., constant velocity). Secondly, a linear force (e.g., gravity) acts on the object (e.g., planet). The force of gravity causes the Earth to accelerate (speed up) in a straight line towards the Sun. This occurs due to Newton’s second law known as the law of acceleration, which states that the force on an object is linearly proportional to the mass of the object and its acceleration:
The Earth has been orbiting around the sun for over 4.5 billion years. The reason that the Earth and Sun do not collide is because the Earth has a linear motion in a straight line parallel to the Sun. It is this combination of the Earth’s linear momentum and the acceleration toward the Sun (perpendicular to the linear motion) which leads to a curved trajectory, and ultimately an orbit, around the Sun.
To see that this linear motion is important in circular motion, you will perform experiments in the lab that simulate what would happen if the force of gravity was suddenly turned off. If this occurs, the Earth would move in a straight line (at a tangent from its orbit) at a constant speed. The Earth would not continue to move in a curved path. This is simulated in Investigation Three by letting go of the rope or hand which represents the gravitational force of the Sun. Also, in Investigation Four, when the string is let go (gravity is turned off) the ping pong ball travels in a straight line through the flour. This observation is explained by Newton’s first law, the law of inertia, which states that an object will remain at rest or move with a constant velocity (i.e., constant speed in a straight line) unless acted upon by an external force.
This model of the Earth and Sun which includes only two factors, the force of gravity and linear motion, explains the motion of all the planets. In fact, the orbit of the Moon around the Earth is also explained by the Moon’s linear motion and the force of gravity between the Earth and the Moon.
The success of Newton’s laws is that they not only explain planetary motion, but also the motion of objects on Earth. If you drop a basketball it accelerates towards the ground until it collides with the ground. If you roll the basketball off a picnic table, however, it then has linear motion parallel to the force of gravity so that it moves forward as it accelerates towards the ground. What happens if you roll the ball faster and harder off the table? Yes, the ball covers a larger horizontal distance due to its velocity (linear speed) before impacting the ground. The combined forces cause it to take a curved path before reaching the ground. Now imagine that you could roll the ball fast enough so that its fall towards the ground exactly matches the curvature of the Earth. At this point, the ball will continue to orbit the Earth (neglecting other forces). This is exactly what happens to an artificial satellite in orbit around the Earth!
By recognizing that the Earth is approximately spherical, rotating while revolving around the Sun, and the Moon also approximates a sphere that rotates around the Earth, we can begin to understand concepts behind the calendar. We can understand a year as the time it takes for the Earth to complete one orbit of the Sun. A day occurs as the time for the Earth to rotate on its axis once, with daytime occurring while we face the Sun and night being the time we face away from the Sun. Therefore half the world experiences night, while the other half experiences day. The lunar cycle is approximately 28 days which is similar to the length of a calendar month.
This CELL will help you to understand the causes behind our everyday experience of night and day, the seasons, and the phases of the moon. It also explains the two factors causing planetary motion, which are gravity and linear motion. Through understanding planetary motion, which is an example of circular motion, you will be introduced to Newton’s first and second laws of motion.
LEARN THE LABLEARNER LINGO
The following list includes Key Terms that are introduced within the Backgrounds of the CELL. These terms should be used, as appropriate, by teachers and students during everyday classroom discourse.
Note: Additional words may be bolded within the Background(s). These words are not Key Terms and are strictly emphasized for exposure at this time.
- Rotation: the turning of a planet on its axis
- Orbit: the pathway of an object around another, such as the Moon around the Earth or the planets around the Sun
- Revolution: the movement of a planet or Moon around another body, such as the Sun or a planet
- Gravitational force: the force that exists between all objects because of their mass. The force of gravity acts to pull objects together.
- Newton’s First Law of Motion: a scientific principle first described by Sir Isaac Newton, which states that an object in a state of uniform motion stays in that state unless acted upon by an outside force.
- Force: a kind of a push or a pull on an object. A force causes an object to accelerate in the direction of the force.
The Focus Questions in each Investigation are designed to help teachers and students focus on the important concepts. By the end of the CELL, you should be able to answer the following questions:
- What causes the changes in day and night on the Earth?
- What causes the change in seasons in the Northern and Southern Hemispheres?
- Based on your model, why do you think we observe different phases of the Moon from the Earth?
- What causes the orbit of the planets around the Sun and the Moon around the Earth?
Note: The Focus Question for Investigation Three and Four is the same.
- What causes the orbit of the planets around the Sun and the Moon around the Earth? Why are they important in maintaining the orbit of the planets and the Moon?