As we have all observed, astronauts on the space station experience what appears to be zero gravity. You might think this is because the space station is far from Earth, and the force of gravity diminishes with the square of the distance, according to Newton’s Law of Universal Gravitation. Thus, being far from Earth would result in low gravity. However, this is not the case. The reason for the microgravity on the space station is not due to its distance from Earth, but rather because the station, and everything inside it, is in “free fall.” Although the space station is only about 250 miles from the Earth’s surface and still experiences about 90% of Earth’s gravity, the astronauts inside experience microgravity. This is because the space station, the astronauts, and all objects inside are falling towards Earth at the same rate, creating an environment where there is relatively no gravity. It’s a phenomenon very similar to the sensation of weightlessness you experience on a roller coaster, when it accelerates towards the earth at around 9.8 m/s²
Retrograde motion is a fascinating celestial phenomenon that has puzzled astronomers and stargazers for thousands of years. Imagine you were stargazing across multiple nights and traced the planets as they danced across the sky, but suddenly, one of the planets stops, reverses direction, and then after a while, resumes its original motion with the stars. This phenomenon is called retrograde motion.
When the Greeks first developed their geocentric models of the solar system, retrograde motion posed a big problem. If the Earth was truly at the center of the universe, the planets would always move eastward with the stars. However, the fact that the planets stopped, reversed course to move westward, then stopped and resumed their eastward motion had the Greeks questioning the accuracy of their models. To this point, the Greeks proposed a complex system where all the planets other than Earth follow circular orbits within circular orbits. This became known as the Ptolemaic system, formulated by the astronomer and mathematician Ptolemly around 150 CE. Although we know now that the Ptolemaic model was not correct in its explanation of planetary motion, Ptolemy’s tables derived from his model were so effective at predicting positions in the night sky that they were used for over 1,500 years!
It wasn’t until the 16th century that Copernicus proposed a heliocentric model to try and account for the slight errors that resulted from using Ptolemaic’s model. Copernicus’ model was much simpler and stated that every planet orbited the sun in a perfect circle. Although this model accurately described the retrograde motion as an optical illusion, it still had inaccuracies as Copernicus did not realize that planetary orbits are actually eccentric.
Overall, retrograde motion was once a baffling phenomenon to astronomers that had a pivotal effect in reshaping the general consensus that the Earth is not truly at the center of the universe and instead orbits the Sun as all the other planets do in our Solar System!
Have you ever wondered why spaceships do not rely on fuel once they are in outer space or why it is easy to throw a light object far but difficult to throw a heavy object far? At the end of the day, the answers to these questions boil down to Newton’s laws of motion.
THE FIRST LAW
Newton discovered that an object in motion will remain at the same velocity and that an object at rest will remain at rest unless it is acted on by a force. Imagine if this law were not true, your parked car in a parking lot could spontaneously drive off without you. This also answers why spaceships no longer need fuel in outer space. Once out of Earth’s atmosphere, which causes drag and friction, the spaceship maintains the velocity it is at since no forces act on it.
THE SECOND LAW
The force applied to an object is equal to the product of its mass and acceleration. Therefore, a lighter object can be thrown further since it has a smaller mass and therefore a greater acceleration. Mass and acceleration are inversely proportional in this law.
THE THIRD LAW
All forces have an equal and opposite reaction force. This means that for any force applied in a certain direction, there is a force of equal magnitude being applied in the opposite direction. In fact, you encounter this law in your daily life. When you sit on a chair, not only do you exert a downward force on the chair, but the chair exerts an upward force of equal magnitude on you. This is why a water bottle can sit on a desk and a pan can rest on a stove!
Have you ever wondered how we predict the rise and fall of tides? Tides are driven by the gravitational forces of the Earth, Moon, and Sun, and has been a relevant subject to astronomers for hundreds of years. In a general sense, we are able to predict the timing of high vs. low and neap vs. spring tides based on the relative location of the Earth, Moon, and Sun. However, we can go further…
Through modern technology, we are able to actually predict what tidal patterns would be before they happen in specific locations. The NOAA (National Oceanic and Atmospheric Administration) has 3,000 locations along coasts all over the United States where they have collected and analyzed data to predict the severity and variation in tides. The NOAA Tide Predictions interface allows users to select a specific tide station and generate tide predictions for the current day and the following day. Users can customize the predictions by adjusting parameters such as the begin date, time range (daily, weekly, or monthly), time zone, and units of measurement (feet or meters)
Meanwhile, tide-predicting machines (pictured below), marvels of the late 19th and early 20th centuries, were earlier tools for forecasting tidal patterns. The first tide-predicting machine, created in 1872–73, and later improved with two larger versions in 1876 and 1879, was developed by Sir William Thomson. These machines mechanized the laborious computations involved in tide prediction.
The force of the moon’s gravity on earth causes tides. Due to the difference between the strength on the moon’s force on different parts of the earth, a tidal force is created. This tidal force, could also be referred to as a “stretching force,” as it creates two tidal bulges, with one being larger on the side of earth that faces the moon (Bennett et. al, pg. 128).
Not only is there a tidal effect on the moon, there is a tidal effect on the sun. The gravitational force between the sun and the earth is much stronger than the force between the earth and the moon, due to the sun’s large mass. However, because the distance between the earth and the sun is so great, the pull between the different sides of the earth is quite minimal. The tide force caused by the sun is less than half of the force caused by the moon (Bennett et. al, pg. 129).
When the tidal forces of the sun and moon work in tandem, we get spring tides, which are pronounced. We see this at new moon and full moon. When the tidal forces of the sun and moon counteract each other, we get neap tides, which are relatively small. We see this at first-quarter moon and third-quarter moon (Bennett et. al, pg. 129).
There are just so many limitations, on a physics level, that prevent us from observing the Universe around us. Telescopes are hindered by light pollution and atmospheric disturbances on Earth, preventing them from peering into extreme distances. In space itself, obstructions such as gas clouds block our view of more distant, and perhaps interesting, objects. The size of the observable Universe, and its accelerating expansion, puts a hard cap on how far we can see. We are limited by the size of our telescopes — to view the supermassive blackhole at the center of the galaxy M87, one requires a telescope with the diameter of the Earth itself.
To each of these problems, though, humanity has responded with unbelievable innovation. Clever software and remote locations have reduced hindrances to observation on Earth. Even more impressive is our launching of our telescopes into space — most famously Hubble, and most recently, JWST (which sits an an unbelievable 1,000,000 miles away). To peer through gas clouds, our telescopes view different wave lengths, like gas-penetrating infrared. And to view extremely distant objects like M87, we use a series of telescopes all over the world, sci-fi like accuracy of time measurement, and innovative software to create our first picture, and undeniable evidence, of a black hole.
Black Hole in M87, Event Horizon TelescopeMain mirror assembly from the front with primary mirrors attached, NASA November 2016Hubble Deep Field, NASA
The Universe has provided us with countless obstacles to unlocking its secrets. But, so far, scientists have improvised, adapted, and overcome.
Before enrolling in this course, I had given no thought as to what light truly was. To me, light simply stemmed from the light switch on my wall as I flicked it on and off. However, there is so much more to light than what first meets the eye (no pun intended). According to the The Cosmic Perspective by Jeffrey O. Bennett, Megan O. Donahue, Nicholas Schneider, and Mark Voit, Newton correctly inferred that light was made up of tiny particles, though he didn’t know quite what. The first component of light that I would like to highlight are waves. There are many types of waves, including radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, and gamma rays. All of these waves have different properties, and some are visible to the naked eye, while many are not. Waves are often characterized by their peaks (uppermost parts) and troughs (lowermost parts). However, although waves move outward, the particles within them move up and down. Another term important to the conversation of waves is wavelength. Wavelengths describe how long a specific wave is by illustrating the distance between peaks and troughs. Additionally, frequency is characterized by the number of peaks passing by any given point each second. Then, when we multiply wavelength by frequency, we get speed. The speed of waves is the distance that the waves traveled in a specific amount of time.
Now that you have a decent groundwork for what light is, we can begin our discussion of the electromagnetic spectrum. The range of colors that we can see with our eyes is only a tiny part of all of the light in the universe…or, in other words, the spectrum of light. Hence, the use of the term: electromagnetic spectrum. Though it is difficult to fully grasp, there is light beyond the rainbow that we are unable to see. The form of light in which we are able to see is dubbed visible light. Virtually, these are the colors of the rainbow. Colors between about 400-700 nanometers make up the colors of the rainbow. Light with wavelengths of just beyond 700 nanometers (what we see as red) are called infrared light. Just beyond infrared light are microwaves, and then we move to radio waves, which are the longest wavelength-light. Then, moving along the spectrum in the other direction, if a light is just shorter than 400 nanometers, it is considered ultraviolet light. Ultraviolet light can be extremely damaging, causing sunburn and even skin cancer. Light with shorter wavelengths are x-rays and the shortest wavelength-light are gamma rays. I hope that this helped to lay the foundation of light and the electromagnetic spectrum for you!
Many people have heard about Stonehenge, one of the world’s most famous monuments. But did you know that the circle of stones was actually an astronomical device? Archeoastronomists have debated what the original purpose of Stonehenge was, but many believe that it was used to mark solar and lunar alignments, including eclipses, solstices, and equinoxes. Many other ancient cultures built structures for astronomical purposes. Another famous example is the Templo Mayor, built by the Aztecs in modern-day Mexico City. This structure was built so that the Sun would rise right in between the two temples on the equinoxes. A third famous structure is the Sun Dagger, which is found on the Fajada Butte in New Mexico. This special structure, made up of three slabs of rock leaning against a cliff, show different patterns of light based on the time of year. For example, sunlight shines through the rocks and produces a single “dagger” on sunlight only on the summer solstice. These structures were a way for ancient civilizations to mark special dates, such as solstices and equinoxes, as well as keep track of the seasons. It’s interesting to see the different structures each ancient civilization thought of to measure the same astronomical events. Unfortunately, many of these ancient structures have either shifted or been destroyed and no longer serve their original purpose. They are still cool to learn about though!
Do you guys remember when we were learning about historical astronomical sites in class? I remember one of the sites catching my eye because it looked incredibly familiar. It was the one in Korea called Cheonseongdae in Gyeongju, South Korea. I once visited this while on a trip with family and friends when I was much younger but seeing it again sparked so many memories. Learning about how old it is amazed me because its so well preserved.
This astronomical site is considered to be one of the earliest in East Asia (maybe the entire world). It is believed to have been constructed in 647 C. E. This observatory may have been used to make astronomical observations to plan agriculture. There is a lot of disagreement around Cheonseongdae because there is no clear documentation about how it was once used. Some believe that this site is simply an altar to pay tribute to the god of agriculture. This is like the Stone Henge of Asia.
Chapter 4 of the textbook explained how the Moon and the Sun affect ocean tides. We learned that the timing and height of tides at a given location depends on its latitude, the orientation of the coastline, and the depth and shape of any channel the tide has to flow through. The book gave an example of a location with an unusual tidal pattern; the incoming tide at the abbey of Mont Saint-Michel in Normandy, France.
The difference between low and high tide at Mont Saint-Michel is up to 15 meters which makes it one of the highest tides in Europe! The tide rises faster than people can swim, but many kayakers enjoy the strong current!
If you were planning a trip to Mont Saint-Michel to see the impressive tidal bore, you would want to consider a few factors. Since it is a spring tide, you would want to visit during the New Moon or Full Moon (the strongest tide is usually 36 to 48 hours after). You could also use this tide forecast to pick the best day, time, and viewing site! Would you travel to Mont Saint-Michel or any other famous tidal site to see the tidal bore in person?
