AS 101 - Lecture 01 - 2026-09-14

Overview

This first lecture covered how the course runs: no generative AI, how you're assessed, the textbook and the key dates. It then introduced the ideas the rest of the course builds on. The universe is made of space, time and matter, and everything in it is moving. Science works by building models that get updated or thrown out when they fail, and those models are written in mathematics. The lecture then covered SI units, scientific notation, and fundamental versus derived units. It finished with two new astronomy units of distance, the light-year and the astronomical unit, and the idea that looking at distant objects means looking into the past.

Key concepts

• Astronomy as the root science. Physics, chemistry and materials science all branched out of astronomy. It started when people looked up and asked "where did we come from, where are we?" Asking a question, finding a way to answer it and calling the answer a theory is the scientific method, which is the "heart and soul" of the course.
• Hypothesis vs. theory. An idea starts as a hypothesis before it becomes a theory. Examples of competing ideas about the universe's future: the Big Rip (the expanding universe snaps "like a rubber") and the Big Freeze.
• Topics coming this term. History of astronomy, the night sky, constellations, seasons, lunar phases and eclipses, Newton's law of gravity, the Sun, telescopes (Hubble and JWST), the birth and death of stars (white dwarfs, neutron stars, black holes), wormholes, the Milky Way and exoplanets.
• The Sun's future. The Sun is not eternal. In about 4.5 billion years it will die and become a white dwarf, not a black hole. Not every dead star becomes a black hole.
• Telescopes. JWST cost about $10 billion and sits 1.5 million km from Earth. Hubble gave the first solid evidence that the universe is not static but expanding.
• Exoplanets. Planets that orbit stars other than the Sun. (The lecturer's wording came out as "not going around any other stars," which is a slip.)
• Galaxies. The Milky Way is our galaxy, and Andromeda is another one. A spiral is not a circle: a spiral is open with "wings" or "fingers," while a circle is closed. Not all galaxies are spirals.
• Space, time and matter. These are the three parts of the universe. They depend on each other, and none makes sense without the other two: no matter means nothing to put in space, and no space means nowhere to put matter. Dark matter still counts as matter.
• Everything is moving. Earth spins at about 1,600 km/h and orbits the Sun at about 30 km/s. The solar system also moves around the centre of the Milky Way. Because of this you never return to the same point in space.
• Relativity. The study of describing motion relative to something else. Einstein did not start it: Galileo, then Lorentz, then Einstein developed it.
• Circles and cycles. A circle is the set of points at a constant distance from a fixed centre. Early astronomers noticed that day and night, the seasons, eclipses and lunar phases all repeat in cycles.
• Branches of physics. Classical mechanics, quantum mechanics, optics, and electricity and magnetism.
• Units and the SI system. Units are standardised so that 1 m means the same thing everywhere. This course uses SI units only: no miles, feet or pounds.
• SI base units. Length in metres (m), mass in kilograms (kg), time in seconds (s), temperature in kelvin (K), electric current in amperes, luminous intensity in candelas, and amount of substance in moles. Length, mass and time are the most important, and temperature will be covered later.
• Reasonableness of results. Always check that an answer makes physical sense.
• Scientific notation. A way to write very large or very small numbers compactly so they are easier to work with, e.g. $6.02\times {10}^{23}$ instead of writing out 23 zeros.
• Prefixes. milli = ${10}^{-3}$, nano = ${10}^{-9}$ (a nanometre is smaller than the width of a hair), angstrom = ${10}^{-10}$ m. Kilo, mega and giga also came up.
• Million, billion, trillion. ${10}^6$, ${10}^9$ and ${10}^{12}$. To feel the difference: a million seconds is about 12 days, a billion seconds is about 32 years, and a trillion seconds is about 31,000 years.
• Fundamental vs. derived units. Fundamental units are the base units, such as metres. Derived units are built from them: area (m²), speed (distance ÷ time) and density (kg/m³).
• Models in science. Models are human-made tools for studying complex systems. They are not reality: cutting open an atom would not show a little nucleus with electrons circling it. "Nature does not have an obligation to reveal itself to you." When a model fails, it is upgraded or discarded.
  • Aristotle and Ptolemy: the geocentric model, with Earth at the centre.
  • Copernicus and Galileo: the Sun at the centre.
  • Kepler: the Sun is slightly off the centre of the orbits.
  • Newton settled it.
  • Today's picture: the electron is not a solid particle but behaves as a wave of energy. The universe has no centre and no known shape, which raises the question "expanding into what?"
• Nature is not random. Random means following no rule, like a number someone picks between 10 and 27. Nature does follow rules, and Galileo said they are "written in the beautiful language of mathematics." That is why science developed algebra and calculus and tries to express every theory as a mathematical expression (e.g. $E=m{c}^2$) that gives numbers.
• Significant figures and rounding. How trustworthy your numbers are. This was assigned as self-study (see Admin).
• Light-year (ly). The distance light travels in one year. It is a unit of distance, not time.
• Astronomical unit (AU). The average distance between Earth and the Sun. It is smaller than a light-year. It has to be an average because Earth's orbit is an ellipse ("the shape of an egg"), not a perfect circle, so the distance keeps changing.
• Looking back in time. Light takes time to reach us, so we see objects as they were when the light left them. We see the Sun as it was about 8 min 20 s ago, and a star 100 ly away as it was 100 years ago. A light-year distance therefore tells you both how far away something is and how old its light is. Seeing the past like this is possible, but travelling back in time is not. (Forward time travel was mentioned as possible, not explained.)
• Light from the Sun's core. Light takes more than 100,000 years to get from the core to the surface, then about 8 minutes to reach us.
• Your age. Your age is the number of times you've gone around the Sun.

Formulas & models

Circumference and area of a circle. This is the level of algebra the course expects.

Use this to find how long light takes to cover a distance. $c$ always means the speed of light.

The speed of light.

One light-year: the speed of light multiplied by the number of seconds in a year.

The average Earth–Sun distance. His example: if the two distances were 10 and 12, the AU would be $(10+12)/2=11$.

An example of a derived unit, alongside area [m²] and speed [m/s].

The prefixes you should know.

Energy equals mass times the speed of light squared. It was mentioned as an example of a theory written as a clean mathematical law and won't be used in calculations yet.

Worked examples

• Reasonableness check. Driving at 100 km/h for 5 hours: $5\times 100=500$ km. If a calculation says you went 7,000 km in 5 hours, it can't be right, because even a plane tops out around 1,600 km/h.
• Earth–Moon distance. About 384,000 km, which he rounded to about 300,000 km to point out it is roughly the same number as the speed of light in km/s. His conclusion: light reaches the Moon in about 1 second. (His own summary table later gave 1.3 s, which fits the actual 384,000 km.)
• Sunlight travel time. $t={\displaystyle \frac{1\text{ AU}}{c}}\approx 500$ s $\approx$ 8 min 20 s. (The AU value wasn't read out. About $1.5\times {10}^{11}$ m is the figure that gives 500 s.)
• Light travel times from his closing summary:
  • Moon to Earth: 1.3 s
  • Sun to Earth: 8.3 min
  • Sun to Pluto: 5 h 40 min (Pluto is no longer classed as a planet)
  • Solar system to the nearest star, Alpha Centauri: 4.3 years
  • Across the Milky Way: 100,000 years
  • To the Andromeda Galaxy: "2.5" (the lecture ended mid-sentence; the figure is 2.5 million years)
• Nearest star. A star about 4 ly away is seen as it was about 4 years ago. He called it "Andromeda star," but the group of three stars he described is the Alpha Centauri system from his table. Andromeda is a galaxy.
• Why a trillion is so much bigger. A million seconds is about 12 days, a billion seconds about 32 years, and a trillion seconds about 31,000 years.

Flagged for exams

• "Why do we use scientific notation?" He said "This is one of the questions that will always come in your midterm or final exam." Answer: it expresses very large or very small numbers in a simple form that is easier to calculate with.
• A light-year measures distance, not time. He repeated this several times: "though I call it year, it is a distance traveled by light in 1 year. It's distance, not time." His trap example: "this assignment will take a light-year" makes no sense.
• Light-years, AU and parsecs are all distances. The parsec comes later.
• The AU is the average Earth–Sun distance. "Keep in mind that new word is very important." The reason: Earth's orbit is elliptical, not circular.
• A light-year distance tells you two things: how far away, and how old the light is. Looking at the Sun or a star means looking at the past.
• Sunlight takes about 500 s (8 min 20 s) to reach us. Light from the Sun's core takes more than 100,000 years to reach the surface.
• "Models are not real, keep in mind." Models are human creations, they fail, and then they are upgraded or discarded. Know the order geocentric (Aristotle, Ptolemy) → heliocentric (Copernicus, Galileo) → Kepler → Newton.
• Space, time and matter all depend on each other. None makes sense without the other two.
• Know nano (${10}^{-9}$) and angstrom (${10}^{-10}$). "You should know at least this one." You don't need to memorise the other prefixes.
• Fundamental vs. derived units. Examples: m vs. m², m/s, kg/m³.
• The SI base units for length, mass and time (m, kg, s), plus kelvin for temperature.
• Numbers he gave. Earth rotates at about 1,600 km/h, orbits at about 30 km/s, and 1 ly ≈ $9.46\times {10}^{15}$ m.
• Exam format.
  • Almost all questions are multiple choice or true/false.
  • No written answers and no numerical problem-solving.
  • "I am not interested in you memorizing something." For a question on something like Newton's laws, you'll get the law or a data sheet and need to interpret it.
• Surprise quizzes can ask anything basic, e.g. "what is the code or title of this course?"

Admin & deadlines

• Lectures. Mon/Wed, 5:30–6:50 pm, LH 3094. He said "come back on Wednesday at 9:30," but the syllabus says 5:30 pm.

• Instructor. Dr. T. Stephen Sathiaraj, N 2084A, [email protected]. Office hours Mon/Wed 3–5 pm, by appointment only.

• Generative AI is not allowed (ChatGPT, Gemini, Copilot, etc.). Using it counts as academic misconduct.

• Textbook.

  • Required: ASTRO, 3rd Canadian Edition (Seeds, Backman, Ghose, Milosevic-Zdjelar, Read; Cengage). Digital access is about $75 from the bookstore.
  • Used or older copies are fine; he said the 3rd or 2nd edition works. Avoid the 4th edition, which is the US version in imperial units.
  • The free OpenStax Astronomy 2e is an optional alternative.

• Reading this week. Week 1 is The Scale of the Cosmos, Chapter 1.

• Slides are posted on MyLS after every second lecture.

• Self-study. Go through the significant figures and rounding slides (5–6 slides, high-school level). He said they'd be posted "today or tomorrow."

• Grading.

  Component	Weight
  10 in-class quizzes	10%
  2 assignments (MyLS)	20%
  Midterm	30%
  Final exam	40%

• Quizzes (10%).

  • Surprise quizzes, roughly one a week, on either Monday or Wednesday at a random time.
  • Password-locked, so you have to be in the room.
  • About 5 minutes for 3–5 simple questions. You get the mark just for attempting, regardless of the answer.
  • The syllabus says you must vote in every poll that class for full marks.
  • Miss one and you lose the mark: no late submissions or make-ups.
  • The first one could come as soon as this Wednesday, Sep 16. He used that as an example, not a firm date.

• Bonus points. Answering his questions during lecture earns a point; "see me after" class to claim it.

• Assignments (20%).

  • 50 multiple-choice/true-false questions each, take-home on MyLS.
  • Each is open for one week, but once you start you have 1 hour to finish.
  • Assignment 1: opens Sep 30, due Oct 7.
  • Assignment 2: opens Nov 30, due Dec 7.
  • A late assignment is considered only with an Illness Self-Declaration or other documentation. Travel is not an excuse.

• Midterm (30%). Monday, October 19, 5:30–6:50 pm, LH 3094. In person, on paper (the syllabus says it's written with Gradescope), multiple choice and true/false.

  • A midterm missed for a valid reason (illness, accident with documentation) may be rescheduled once. Submit the request within 72 hours with documentation. Travel is not a valid reason.
  • The midterm's weight will not be shifted to the final under any circumstances; this is also university policy. "Please stop asking me."

• Final exam (40%). In person, on paper. Scheduled by the university in the Dec 12–23 exam period, date to be announced.

• Key dates. Reading week is Oct 13–16. The last day of classes is Dec 9.

• Coming up. The parsec, a third distance unit, will be introduced later, probably after the midterm.

Transcribed automatically from the lecture recording. Course info used: AS 101 - Syllabus - Fall 2026.pdf. Audio archived at /mnt/porsche/configs/lectures/archive/2026-09/AS 101 - 2026-09-14 - Lecture 01.opus.