As we’ve discussed in class, the Moon revolves about the Earth in synchronous rotation, meaning it rotates such that the same side of the Moon always faces the Earth. Not only does the Moon do this, but all moons in our solar system do as well. (The information for this blog post is sourced here.)
This is different from geosynchronous orbit, which means the orbiting body stays above the same point of the Earth at all times when orbiting. Many of our satellites are placed into geosynchronous orbit, for example. However, given the Moon’s orbital period of 29.5 days compared to the Earth’s 24-hour rotation, the Moon is clearly not in geosynchronous orbit.
Instead, the Moon rotates at the same rate that it orbits Earth. The theory behind this has to do with the formation of the Moon: namely the impact theory, which posits that some body struck the Earth and broke off to become the Moon. At the time of formation, the new “Moon” was spinning incredibly fast.
However, Earth is so much larger and more massive than the Moon that at all times, the side of the Moon facing Earth would be deformed towards the Earth, as shown in the figure below. This deformation, always facing the Earth, counters the angular momentum of the Moon’s spin, slowing it down until it reaches an equilibrium. This is the point at which the same side of the Moon is always facing the Earth, and synchronous rotation has been achieved.
Diagram showing the effect of gravitational deformation on the Moon’s spin over time.
Posted inClass|Taggedastro2110, blog2, tides|Comments Off on The “Why?” behind Synchronous Rotation
Most people have already heard of the Doppler Effect, an interesting phenomena of sound that alters the pitch of moving objects depending on their direction. The Doppler Effect functions similarly by affecting the shifts of light wavelengths. When, for example, a planet is moving toward us, its light waves will be clumped together and appear as shorter wavelengths. We call this blueshift, as shorter wavelength of light are bluer on the visible spectrum. If a planet is moving away from us, then its light wavelengths will be more spaced out and appear longer. This is redshift, as longer wavelengths of light are redder on the visible spectrum.
We measure Doppler Shifts by comparing how much measured spectral lines have moved compared to their reference spectral lines at rest. It’s important to note, however, that we will not observe a wavelength shift if the object is moving tangentially to us and, therefore, not toward or away from us. Similarly, if an object is not moving directly away from us, then the Doppler Shift will only reveal part of the object’s speed toward or away from us (only one vector component) and not its speed in the other wayward direction (another vector component. Regardless, by using a measured wavelength shift we can then determine an object’s radial velocity.
What I find fascinating about the Doppler Shift is how it appears on rotating planets. As a planet spins, light on one edge will be blueshifted (since its spinning toward us), light on the other edge will be redshifted (since its spinning away from us), and light in between will not be shifted.
One of the biggest current paradigms in the science of astronomy is the certainty that the speed of light is constant in a vacuum. However, new research shows that this might not entirely be accurate which could have massive consequences for cosmology.
Thanks to the use of lasers here on the earth, the speed of light has been measured very accurately at a speed of 299,792,458 m/s. Current scientific knowledge has this placed as the ultimate speed limit in the universe. Two scientific papers propose that the speed of light varies due to the very nature of spacetime. They propose that with the assumption that the vacuum of space is full of basic quantum particles like quarks, they speed of light is affected by them. Depending on their charge, these quarks would absorb and emit photons, effecting the speed of light by one millionth of one billionth of a second for every square meter of vacuum.
This study also purports that the charge of the quark, which varies, would also vary the effect of it on the speed of light. Since the incidence of these quantum particles would be random, the speed of light could, over billions of light years, be random. Although the second paper proposes subatomic particles’ interaction with electromagnetic fields, rather than individual photons, the proposal is the same. Although this is firmly in the hypothesis portion of the scientific process, this would have drastic implications on cosmology and the cosmic distance ladder. Predictions on the age of the universe could be drastically off and estimates on the size of the universe could similarly be off.
Celestial Bodies throughout the universe are extraordinarily far away, yet despite this we are able to describe these objects with precision. The reason this is possible is because the radiation of these objects. We can determine a bodies composition through the spectra of light they emit/reflect; however today’s focus is determining a star’s temperature, which requires understanding black body and by extension thermal radiation.
Radiation itself is just the distribution of electromagnetic waves, so thermal radiation must then have some relation to light. William Herschel can be cited as the first person to find this connection though measuring the temperature of light at different frequencies. From this it was deduced that thermal radiation was something that was carried by light, which Balfour Stewart used that to describe how a black surface would absorb the most thermal radiation.
With this knowledge Gustav Kirchhoff theorized that if in thermal equilibrium, a surface that can potentially absorb all radiation can emit them as well. This theory showed that in the ideal situation absorption and emission ratio are a function of only temperature/the thermal energy. Later on, Planck was able to produce a formula and graph that the amount of possible emitted spectra at various temperatures, otherwise known as a black body curve.
Now the word ideal was used a lot when describing black bodies, that’s because a perfect version does not really exist. The closest example in the universe would be a black hole (Which is displayed in the above image), however despite this, for many celestial bodies using a black body curve still yields a good approximation towards the exact thermal energy emission.
Above is the image of a black body curve which as you can see covers most of the emission spectrum of the sun, showing the curve is a good approximation. Furthermore, the most prevalent wavelength is green given by the curve, which is the color of the sun spectral class. Side-note: Stars can be any range of colors, however we ourselves cannot perceive green stars due to how our red, blue and green rods have a lot of overlap at that frequency, so “green stars” appear white. Side-Side-note: green is also not a color possible incandescent color in black body radiation.
Despite that this means that the apparent color of a star can be a good approximation of the temperature, where a hotter star constitutes a bluer color, and a cooler star red. It’s such that if one were to apply a red filter to a cool star it would look dimmer. Doing that just so happens to be known as a Color Index, measuring the difference in magnitude of the star under an Ultraviolet, Blue and visible light filter.
Color Indices were then used in 1911 to form the Hertzsprung Russel Diagram, as the axis vs absolute magnitude, and became the metric in which modern spectral are defined.
Black body radiation and how light and thermal energy thus are related are core to figuring out exactly very minute details about bodies, from temperature to classification. Showing that physics reigns true at all sizes and distances.
The Hubble Space Telescope was one of the most impactful inventions in furthering our understanding of the universe. Launched in 1990, this telescope provided us with detailed information of our solar system and universe. Some of the incredible discoveries the Hubble has made are determining the age of the universe and determining changes in many different celestial bodies in our solar system. Throughout its time in deployment the Hubble has made over a million other observations including the birth and death of stars and galaxies billions of light years away. The importance of the Hubble comes from it being in the earth’s atmosphere which allows it to get a much better view of the universe than a telescope on earth. It operates by using two mirrors to collect and focus light, the light travels down the telescope and hits the primary concave mirror which than reflects and travels back to the front of the telescope.
(Diagram showing Ptolemy’s early explanation of retrograde motion)
Understanding retrograde can get a little confusing. Before taking astronomy, I’d only ever heard the term used by the astrology girlies. I thought maybe it was another “constellations are not real” scenario- but retrograde is real!
Early astronomer Ptolemy (c. AD 100), as he observed the night sky, noticed that a planet in the sky appeared to move backwards. He explained this “backwards” motion using “epicycles”- smaller circles moving along a larger circle- to explain retrograde motion. In the past, when a geocentric model ruled astronomy and religion ruled societies, individuals believed that the heavens (space) had to consist of perfect circles, as God’s creations were always perfect. This may have had a role to play in Ptolemy’s model, which also consisted of perfect circles.
Today, we know that retrograde is not actually the backwards motion of planets, but rather, created by the differing number of days it takes each planet to revolve around our sun. Think of it as Earth lapping the other planets on the race track that is orbit! Pretty cool.
For any of you who have seen the movie adaption of, “A Wrinkle In Time” may remember the scene where two of the protagonists hide in a tree stump in order to be thrown over a wall.
Meg, one of these protagonists, has parents who work in quantum theory, so she has picked up knowledge in physics throughout the years at home. She was the one who directed her friend to hide in the tree stump because she knew that the tornado was giving speed to objects that fell into it, calling it, “The Slingshot Maneuver”.
From the very first time I watched this film and learned of this trick, it immediately seemed like a physical conundrum: how something can escape an attractive entity with more speed than it entered with. It actually wasn’t until tonight that I fully grasped the concept with the help of some articles and Neil DeGrasse Tyson.
I found the graphics online of the gravitational slingshot to be rather ambiguous and misleading. The following one, however, is an exception. So as I explain the mechanics operating behind the curtains of the “slingshot maneuver “or “gravity assists” as I’ll be referring to them from this point foward, reference the diagram I’ve embedded below:
So gravity assists have actually been somewhat of a popular yet unsung hero of space travel and research. In this article from NASA, you can read about how space probes have historically made use of multiple during their treks to the exterior solar system for their practically free acceleration.
But how does a body falling into a planet gain speed? Even if said object is later able to escape the heart of the planet’s gravitational pull, all of that energy gained while advancing toward it will be stripped from the body upon departure.
The conservation of energy does still hold during this process, so it is very correct to assert that there is no net change in speed of the probe. But you’re forgetting that this is not the only motion at play in these conditions— planets revolve around the sun.
In order for the planet to have its gravitational influence, the probe has to catch up to its speed, relative to the sun (we use the Sun as a rest frame, since compared it doesn’t have motion like the satellites of our solar system). If the probe weren’t able to match the planet’s speed, it would gain too much distance from the planet for a gravity assist to take place.
So in approaching the planet from behind (remember this detail) the probe gains the velocity of the planet. And through doing this, it saps some of the planet’s angular momentum, although the effects of this are negligible since the planet is immense in mass.
And while the planet will continue in its elliptical path of orbit, the probe will not; it will continue in the same direction it was moving with the planet when it matched its speed. If you look at the diagram above, the path of the prove moves in an overall hyperbolic fashion with the eccentricity increasing or decreasing contingent on the angle in which the probe enters in reference to the planet’s direction of motion.
And it is through this minor purloin of momentum that our exploratory satellites reach speeds and distances in space that were never even thinkable prior.
In science fiction novels and movies, we occasionally see a planet or a moon being teared into pieces due to it being to close to a star or a larger planet. In the newly premiered Chinese sci-fi movie The Wandering Earth II, our moon potentially gets torn into chunks as it moves closer to the Earth and passes its Roche limit. You might wonder, what exactly is this limit and why would it tear objects apart?
We know that tidal forces exist due to the differences in gravity on the near and far sides of a planet or a satellite. We also know that tidal forces become more extreme as an object gets closer to the source of the tidal effect (due to the object’s radius occupying a larger portion of the distance between the object and the source). The Roche limit, first calculated by French astronomer Édouard Roche, is the distance to a (larger) celestial body when an approaching (smaller) celestial body disintegrates due to extreme tidal effects that exceed the self-gravity that holds the smaller object together.
A completely rigid object would maintain its shape up to the point of the Roche limit, while a more fluid object will tend to get elongated due to tidal forces as it approaches its Roche limit, and this elongation further increases the tidal effects and rips the object apart. The Roche limit depends on the ratio of the density (or mass) of the two objects, and the calculation of this limit for rigid bodies is shown below.
One example of a celestial body being torn apart in the Solar System is the comet Shoemaker-Levy 9, which unfortunately traveled too close to Jupiter, got past its Roche limit, and was broken into over 20 pieces and eventually bombarded the cloudy surface of Jupiter (See figure below).
Within the Roche limit of a planet, chunks of rock and ice will not tend to coalesce to form moons, which is why rings of planets generally lie within this radius, as asteroids and moons that enter this radius disintegrate into small pieces. In contrast, larger moons of planets generally orbit beyond this radius to stay in one piece.
There are several exceptions, though. Saturn’s moon Pan, a ring shepherd in Saturn’s Encke division, and Jupiter’s moons Metis and Adrastea all lie within their Roche limits, since forces other than their self-gravity holds themselves together. Another exception is the minor planet 50000 Quaoar in the Kuiper Belt which has a ring far beyond its Roche limit. Astronomers are still investigating why Quaoar’s ring did not amalgamate into a satellite.
Before any GPS or easy to use maps, explorers were completely reliant on the stars and their hunches to determine their location during their travels. In the Northern Hemisphere, it was much easier to determine latitude because of the conveniently located star Polaris just above the northern celestial pole. Using the Sun is also a possibility for navigation, but more precise measurements of dates and the path of the ecliptic are required for an accurate determination. Navigators have used tools like the sextant featured above since at least the 1700s in order to measure precisely their position in the oceans. The Ancient Greeks created similar items like the astrolabe which was very important and useful with hundreds of different astronomical uses varying from navigation to timekeeping. In modern times, global positioning systems have satellites orbiting the Earth act as “artificial stars” which are able to accurately send radio signals to travelers below.
There are many different types of telescopes, each designed for a specific purpose. Here are some of the most common types:
Refracting Telescope: This type of telescope uses lenses to refract (bend) light and form an image. They are often called “refractors” and are easily recognized by their long, narrow tubes.
Reflecting Telescope: This type of telescope uses mirrors to reflect light and form an image. They are often called “reflectors” and are usually shorter and wider than refractors.
Catadioptric Telescope: This type of telescope combines elements of both refracting and reflecting telescopes, using lenses and mirrors to form an image. They are often more compact than other types of telescopes and are popular for both amateur and professional astronomers.
Radio Telescope: This type of telescope is specifically designed to detect radio waves from space. They are often much larger than optical telescopes and come in a variety of shapes and sizes, including dish-shaped and cylindrical.
Space Telescope: This type of telescope is placed in orbit around the Earth and is used to observe the universe in different parts of the electromagnetic spectrum. The Hubble Space Telescope is one of the most well-known space telescopes.
Solar Telescope: This type of telescope is specifically designed to observe the Sun. They typically use special filters to block out the Sun’s bright light and allow safe observation.
Interferometric Telescope: This type of telescope uses multiple smaller telescopes working together as an array to simulate a larger telescope. They can provide high-resolution images and are commonly used in radio astronomy.
Conclusion
Each type of telescope has its own strengths and weaknesses, and the choice of which one to use depends on the specific observing goals and requirements of the astronomer.
