These are my complete notes for the Introduction and Basics of Classical Astronomy, serving as a preliminary guide to many of the terms and concepts that will be discussed in future sections.
I color-coded my notes according to their meaning - for a complete reference for each type of note, see here (also available in the sidebar). All of the knowledge present in these notes has been filtered through my personal explanations for them, the result of my attempts to understand and study them from my classes and online courses. 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 universe, and the processes by which the objects within it interact with eachother.
# Lightspeed: The distance which light travels within one year, e.g. 9.46 × 10¹² kilometers. Light travels through the Universe at this constant speed: The light from stars is travelling at lightspeed, so if a star is 20 lightyears away from Earth, a viewer on Earth will only see the stars as they looked 20 years previously. Therefore, information can be discerned about what happened billions of years ago, at the dawn of the Universe, by looking at stars farther away.
Light travelling from the sun takes over 8 minutes to the Earth.
# Astronomical Unit: The average distance between the Sun and the Earth, defined as 149,597,870,700 meters.
# A. Rule 1. There are eight planets that orbit around the Sun, along with many other bodies like moons, dwarf planets, and others. These compose the 'Solar System'. Planets are celestial bodies of significant size that orbit a star. If the planet consistently produces its own light, it is a star. Though there is man-made light on Earth, anything involving life doesn't count.
The Sun is the local star of the solar system (the 'primary' of the Earth), and stars themselves are basically just enormous balls of glowing gas which generate energy through internal nuclear reactions.
# A. Rule 2. All of the stars visible to the naked eye in the night sky are part of the Mily Way Galaxy - there are hundreds of billions of stars in this galaxy, and hundreds of billions of galaxies in the Universe.
The space between the stars is not completely empty: there is a sparse distribution of gas intermixed with tiny particles called 'interstellar dust'. Progressively, the gas and dust collect into huge clouds in each galaxy, which form the raw material for future generations of stars. This interstellar dust is extremely sparse, however, and can thus still serve as a great vacuum. Built-up dust is akin to space smog, blocking the view of more distant regions of the galaxy to Earth.
# A. Rule 3. Because of experimental/observational evidence indicating that there is some unknown material exerting gravitational force upon objects in space, it is believed there is some "Dark Matter" that cannot be directly observed, interfering with everything and frustrating the current 'laws' of Physics.
# A. Rule 4. While many solar systems only have one 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 known as star clusters.
# A. Rule 5. All stars die after running out of "fuel", because stars can only produce energy 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 groupings of stars in the night sky that different cultures assigned different meanings - patterns, such as Orion the Hunter and the Big Dipper, are among these groupings.
While there are 88 patches of sky which humans have deliminated (additionally serving to simplify observations), there are many less bright night-sky objects that are not included in the constellations.
Many galaxies/celestial objects are named after the constellation in which they reside - 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 nearby, are collectively known as the Local Group, the 'local' galaxy cluster. Most galaxies are found in clusters.
# A. Rule 7. Some galaxy clusters themselves form into larger groups, known as 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 one could 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 (produced as it gravitates toward a massive black hole and swirls around it), quasars are bright enough to be the most distant objects that can be seen in the expanses of space.
The quasars are 10 billion plus light-years away, and thus show 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 the Milky Way 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 that retains its chemical properties (anything more complex than mono-atomic structures) are the molecules. 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
Elements are defined by the number of protons in its atoms. The distance between the nucleus to the electrons is 100K times that of the diameter of the nucleus itself. Thus, solid matter is mostly empty space, and atoms are much emptier than the solar system, if they were to be placed on the same scale.
# 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 form a 360° circle around the observer.
# 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 the Earth's axis were to be imagined as extended into space, it would be found that there now is a north and south celestial pole in the space dome that the stars rotate around, seen from (almost) any point on Earth:
# Celestial Equator: As an extension of the celestial poles, the Earth's equator can be imagined as stretching off into space in a circular fashion, bisecting the Universe.
# 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 Sun rises 4 minutes later each day with respect to the stars. During the day, sunlight is scattered by the molecules of Earth's atmosphere, filling the sky with light and hiding the stars above the horizon.
The elliptic is not along the celestial equator (see definition above), but is 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 northward and southward in the sky as the seasons change.
# A. Rule 11. When 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 celestial 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' (in relation to the visibility of the Sun from a particular point on the object) over specific periods of time. 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 to understand from the Earth as the reference point.
# Asterism: A star pattern within a constellation (or spanning multiple constellations), such as the big Dipper as a part of Ursa Major. Think of them like sub-constellations.
