Sun, Moon, and Stars: Astronomy for Beginners · Part II, Ch. I · Part 2: Part II: A closer look at the Solar System
13. More about the Solar System: ellipses and a corkscrew path
Goal: Your child can draw an ellipse the book's way, explain that the Sun sits at one focus of each orbit, and describe the Earth's corkscrew path as the Sun carries it through space.
You'll need
- The book, open to Part II, Chapter I
- A sheet of paper on a corkboard, cardboard or a folded towel
- Two pins or thumbtacks
- A loop of thread or thin string (tie the ends), and a pencil
- A Slinky or a spring, if you have one
What's in the book
Part II begins by looking at the whole Solar System again, far fuller than people once thought. It explains the Earth's third motion (traveling through space with the Sun), the difference between a circle and an ellipse, how to draw both with pins and thread, and how the planets tug on one another.
The lesson, step by step
Read together
Read the chapter aloud. At the part about the little boy walking round the mast of a moving ship, stop and act it out: walk in a circle around a chair while someone slowly pushes the chair across the room on a rug, or just picture it.
Draw a circle and an ellipse
Follow the book exactly. One pin and a loop of thread make a circle. Then:
To draw an ellipse you must fix two pins.
Draw ovals with the pins close together, then farther apart. Finally draw a 'comet orbit' with the pins five or six inches apart, as the book suggests.
Where is the Sun?
SayIn a planet's orbit, the Sun sits at one of the pins, not in the middle. So a planet is a little nearer the Sun at one time of year.
The book says the Earth is about three million miles nearer in our northern winter than in summer, and that this hardly matters to how warm we feel.
The corkscrew
Stretch a Slinky slowly upward, or draw a long spiral. Say: The Earth goes round and round the Sun, but the Sun is moving too, carrying us along. So our true path through space is like a corkscrew.
Planets pulling on planets
The book explains that Jupiter and Saturn speed each other up and slow each other down as they pass, and that this is how astronomers weigh the planets: like two children pulling against each other to see who is stronger.
What we know now
The book wonders whether there is life on other worlds. Scientists are still searching, on Mars and on icy moons such as Europa and Enceladus, which have hidden oceans under their ice. So far, life has been found only on Earth.
Space between the planets is not quite empty, as the book guesses: it holds dust, gas and a thin wind of particles streaming out from the Sun.
Tell it back
Ask your child to explain, using the ship and the mast, how the Earth can go round the Sun and travel through space at the same time.
Check questions
What is the difference between a circle and an ellipse?
How many foci does an ellipse have?
When is the Earth nearest the Sun?
Afterwards
Draw a planet orbit (pins about half an inch apart) and a comet orbit (pins five or six inches apart) on the same sheet, and color the Sun at one focus of each.
Tip
Keep the thread tight against the pencil the whole way round; that is the trick to a smooth ellipse.
Answers to the book's problems
A circle is the same distance from its center all the way round and has one center (focus); an ellipse is an oval with two foci. Two. In the northern winter (early January).
Worked out for this site from the scan. If a number in your copy differs, trust the book.