Physics 320
Astrophysics I:  Lecture #1
Prof. Dale E. Gary

Introduction to the Solar System

A: What is the Solar System?

Among otherwise well-educated people, it is common to hear the terms Solar System, Galaxy, and Universe interchanged.  For instance, you might hear "Jupiter is the biggest planet in the Galaxy," or the question, "How many stars are there in our solar system?"  These may seem silly to those who know a bit about the subject, but even knowledgeable people are not really sure what constitutes the solar system, what objects are part of it and what objects are not--in short, what constitutes the boundary between our solar system and the rest of the galaxy.  This lecture will hopefully give you a feeling for what is part of the solar system and what is not, and will also give you some idea of the size scale of the solar system.

As the name implies, the solar system has something to do with the Sun, or Sol.  (Incidentally, the words Sun, Moon, Earth, Mars, etc., should be capitalized since they are proper names.)  The Sun dominates and controls the solar system, mainly by its gravitational influence (keeping the planets in their orbits), but of course its light, heat, and other forms of energy are important also.  We will learn of another important way that the Sun dominates its surroundings--through its magnetic field.

Most people would agree that the solar system is made up of

and probably would include

However, there are also The subjects of this course will include all of these things, and we will learn about them from the point of view of physics.  The language of physics is mathematics.  To quote from Galileo:
Philosophy is written in this grand book, the universe, which stands continually
open to our gaze.  But the book cannot be understood unless one first learns to
comprehend the language and to read the alphabet in which it is composed.  It
is written in the language of mathematics, and its characters are triangles,
circles, and other geometric figures, without which it is humanly impossible to
understand a single word of it; without these, one wanders about in a dark
labyrinth.                           Galileo in The Assayer.

We will be using the somewhat more advanced language of algebra, trigonometry, calculus, and even a bit of simple differential equations.  I hope and expect that you have learned this language well, although we will go over some of it in review as we go along.  If you find yourself in a "dark labyrinth," come and see me for help.

B: The Sun and Planets

Let us list the major components of the solar system, the Sun and planets, in order of their distance from the Sun.

C. What we have Learned

As a result of this lecture, you should have a much better idea of the scale and size of the solar system.  For the next few weeks we will be discussing the motions of planets and other solar system bodies, both as seen from our vantage point on the Earth and as would be seen from a fixed point in space.  When we discuss planetary orbits, we will get into some rather heavy mathematics and physics, but keep in mind that we are talking about something really very simple--the motions of these little "seeds and nuts" in a vast volume of empty space, under the influence of a far-reaching, but rather weak central force, the force of gravity.

Homework: Last year (24 August, 2016), the discovery of a new planet was announced around the star Proxima Centauri. This is exciting news, because this is the nearest star to the Sun, and the planet was found to be in the "habitable zone" where life may be possible. Use the information at the Proxima Centauri b Wikipedia page to answer these questions, using the scaling of the Thousand-Yard Model:

    1. How big (inches) would be the parent star, Proxima Centauri? What nut or fruit might be used to represent the star?
    2. How big (inches) would be the planet, Proxima Centauri b? What nut or seed might be used to represent the planet?

    3. How many paces away would the planet be from the star?

    4. The star, Proxima Centauri, is part of a triple-star system, with Alpha Centauri A and B far away, but gravitationally bound (maybe).Using the information at the Alpha Centauri Wikipedia page, how far away (in the Thousand-Yard Model scale) is Proxima from Alpha A,B.