These are my complete notes for the Introduction and Basics of Classical Astronomy.
I color-coded my notes according to their meaning - All numbered notes (which I call rules) are red, and include examples and the basis for understanding a topic. Definitions are written in green, and other important information (such as large-scale drawings that are better visualized than explained) was written in blue. All of this information is preserved on this page, with logical flow and breaks. I use ascii line drawings sparingly - If I can convey information or a graph using an image online, I will do so.
All of the knowledge present in these notes are filtered through my personal explanations for them, the result of my attempts to understand and study them from my classes. In the unlikely event there are any egregious errors, contact me at jdlacabe@gmail.com.
Introduction to Classical Astronomy
I. Introduction/General Terms.
Astronomy: The study of the objects that use beyond planet Earth and the processes by which these objects interact with eachother. Organization of a history of the universe, from the Big Bang to the present. "Science is the progress report of the progress report probing of the secrets of the universe" - Charles Bailyn.
Light-Speed: The distance which light travels within one year. This is 9.46 × 10¹² kilometers per year. Light travels through the universe at this constant. The stars outside are only sending their light at lightspeed, so if one is 20 lightyears away, Earth will only see the stars as it looked 20 years previously. Therefore, we can learn about what happened billions of years ago, at the dawn of the universe, by looking at stars farther away.
Astronomical Units: The distance between the Sun and the Earth. Light travels from the sun over 8 minutes to the Earth. 1 year is the length of time it takes the Earth to rotate around the Sun. Our velocity with respect to Earth is zero, but with respect to the Sun, we're moving 110,000 kilometers per second due to the Earth's orbit. This is Relative Motion & Velocity, which is mathematically elaborated on in Classical Physics.
A. Rule 1. There are eight planets that orbit around the sun along with bunches of other bodies like moons, dwarf planets, and whatever. These make up the solar system we live in. Planets are celestial bodies of significant size that orbits a star. If the planet consistently produces its own light, it is a star.
The Sun being our local star, and stars themselves are basically just enormous balls of bglowing gas which generate energy by internal nuclear reactions.
A. Rule 2. All of the stars visible out in the sky are a part of the Mily Way Galaxy - there are hundreds of billions of more stars in our galaxy, and hundreds of billions of galaxies in the Universe. The Milky Way Galaxy is like a giant disk with a small ball in the middle. The space between the stars is actually not completely empty: there is a sparse distribution of gas intermixed with tiny particles called interstellar dust. The gas and dust will collect into huge clouds in the galaxy, which will form the raw material for future star generations. This interstellar dust is extremely sparse, however, so sparse still serves as a great vacuum. Built-up dust is the space smog, blocking the view of more distant regions of the galaxy to Earth.
A. Rule 3. Because of experimental/observational evidence proving that there is some unknown material exerting gravitational force upon space objects, we know there is some "Dark Matter" that cannot be directly observed interfering with everything.
A. Rule 4. While many solar systems only have on star (like ours), many systems are double or triple systems, with 2, 3, or more stars revolving around each other. Several places in the galaxy have so many stars they are called star clusters.
A. Rule 5. All stars die after running out of fuel, because stars can only produce energy as they do while they have some fuel source. The Sun, for example, uses nuclear fusion to create fuel, converting hydrogen into helium, which releases energy in the form of light and heat. When stars die, they explode and their star dust is fed back into the universe for reuse.
Constellation: The 88 patches of sky which humans have deliminated to simplify observations. For example: the Saggitarius Dwarf Galaxy is in the direction of the Saggitarius constellation.
A. Rule 6. The Milky Way Galaxy has many orbiting satellite galaxies that will eventually be incorporated/subsumed by the Milky Way Galaxy itself, such as the Magellanic Clouds and the Sagittarius Dwarf Galaxy. The nearest non-satellite galaxy to the Milky Way is the Andromeda Galaxy (of the Andromeda Constellation), and these two galaxies, along with 50 others nearvy, are called the Local Group, our local galaxy cluster. Most galaxies occur in cluster.
A. Rule 7. Some galaxy clusters themselves from into larger groups, called superclusters. The Local Group is part of the Virgo Supercluster, 110 million lightyears across. Farther away, there are quasars, the especially brilliant centers of galaxies which glow from an extremely energetic process. Normally, at the distance where we see quasars, galaxies are too dim to see, but because quasars have an energy produced by a gas heated to a temperature of millions of degrees as it gravitates toward a massive black hole and swirls around it, quasars are bright enough to be the most distant objects we can see in the expanses of space. The quasars are 10 billion plus light-years away, and thus showing us 10 billion years into the past. Beyond these quasars, astronomers detected the feeble afterglow of the Big Bang, detected from all directions in space.
A. Rule 8. While the universe is very large, it is also very sparse. In the interstellar gas of the galaxy, there is on average one atom per cubic centimeter, while in intergalactic space (much sparser than our galaxy), that number drops to one atom per cubic meter of space.
A. Rule 9. Summary of the Atomic Level of Matter:
The smallest part of any matter (anything more complex than mono-atomic structures) are the molecules, particles that are the smallest part of the matter that retains its chemical properties. Water, for example, has the molecule H2O, three atoms bonded together. Molecules are always built of atoms, the smallest division of an element that can be identified as that element.
There are over 100 elements that occur in nature, but most are rare, and only a few occur with any real frequency, known as the “Cosmically Abundant” elements. Listed in descending frequency, they are as follows:
- Hydrogen
- Helium
- Carbon
- Nitrogen
- Oxygen
- Neon
- Magnesium
- Silicon
- Sulfur
- Iron
Zenith: The point in the 'space dome' directly above you, from your perspective.
Horizon: The point where the space dome meets the Earth. In a flat land, the horizon would be seens as a circle around you.
Celestial Sphere: The ancient Greek view of the world as a sphere, of which the outermost shell had stars embedded like sky decorations.
Axis: The central line of the Earth going from the North Pole through the Earth to the South Pole. The Earth rotates on its axis counterclockwise, from the perspective of the Northern direction.
Celestial Pole: If we extend the Earth's axis into space, we find that there is now a north and south celestial pole that the stars rotate around:
Celestial Equator: As an extension of the celestial poles, we can imgine Earth's equator stretching off into space.
A. Rule 10. Way back when, everyone thought the universe revolved around the Earth, known as the geocentric model of the universe. This is ignorant and wrong. This idea was developed for more reasons than sheer human-centrism, however: the sun was about 1° to the east each day, relative to the stars. It takes 1 year for the sun to make a complete circle. This path is elliptic - the Sub rises 4 minutes later each day with respect to the stars. During the day, sunlight is scattered by the molecules of our atmosphere, filling our sky with light and hiding the stars above the horizon. The elliptic is not along the celestial equator (see below), but rather inclined to it at an angle of 23.5°, because Earth's axis of rotation is tilted by 23.5°. Other planets are so tilted that they orbit the sun on their side. This tilt is why the sun moves north and south in the sky as the seasons change.
A. Rule 11. North of the equator, the north celestial pole will appear above the northern horizon at an angle equal to one's latitude - In San Francisco, 38°N, the north celestial pole is 38° above the northern horizon. The south celestial pole is thus 38° below the southern horizon, and furthermore at 38° South the southern celestial pole will be 38° above the horizon. The part of the sky however many degrees from the pole (in this case, 38°) will seem to never set, known as the circumpolar zone. There is a particular star right at the Earth's north celesetial pole that seems to move minimally during the daily rotations of the heavens: this is Polaris, the North Star.
A. Rule 12. Apart from the sun, the other planets change their positions slightly everyday. They have their own rotational patterns, rising and setting and what not. The Greeks differentiated these bodies from the rest of the "fixed stars" by calling them planets, or wanderers. The moon has the fastest planetary motion, since it moves 12° per day. All of the planets have their paths close to the elliptic path of the sun, because the paths of the planets around the sun are all almost in exactly the same plane, like circles on a paper along with the Earth. Each of the major celestial bodies are found with an 18°-wide belt, centered on the Sun's elliptic path, known as the zodiac. The motions of the planets, as seen from Earth, are somewhat obfuscated by the movement of Earth, making the process rather complex. Constellation: The groupings of stars in the night sky that different cultures saw different meaning in, with patterns like Orion the Hunter and the Big & Little Dippers.
While we have divided the sky into 88 sections, there are tons of less-bright night-sky objects that are not included in the constellations.
Asterism: A star pattern within a constellation or spanning multiple constellations, such as the big Dipper as a part of Ursa Major.
