Sun, Moon, and Stars: Astronomy for Beginners · Part I, Ch. IX, part 2 · Part 1: Part I: Our family in the sky, and its neighbors
11. The speed of light, and light that is years old
Goal: Your child can say how fast light travels, how long sunlight takes to reach us, and what it means to see light that left a star years ago.
You'll need
- The book, open to the last pages of Part I, Chapter IX
- A watch or timer
- A calculator (optional)
What's in the book
The end of the chapter measures the star distances with light: it races 186,300 miles a second, takes about eight minutes from the Sun, over four years from Alpha Centauri, and nearly nine years from Sirius. The light of fainter stars may have started thousands of years ago.
The lesson, step by step
Read together
Read from 'Do you know how fast light travels' to the end of the chapter.
Light takes time to travel.
Eight minutes from the Sun
Start the timer and sit quietly together for one minute, picturing a ray of sunlight racing toward the Earth. Say: It would have to keep going for about eight more minutes to reach us.
The book says light could go about eight times round the Earth in one second. Count 'one-thousand-one' together and picture that.
Light from the past
SayWhen you look at Sirius, the light hitting your eye left it nearly nine years ago. How old were you then?
Work out together what year the light left Sirius. (Take away nine from this year.)
What we know now
Sunlight takes about 8 minutes and 20 seconds to reach us, a little less than the book's eight and three-quarters.
Astronomers now measure star distances in light-years: how far light goes in a year. Alpha Centauri is about 4.4 light-years away, and a small companion star of it, Proxima Centauri, is the nearest star of all, at about 4.2.
The book's 'more than seven years' for 61 Cygni is too short: its light takes about eleven years. Sirius is about 8.6, just as the book says, 'nearly nine'.
The farthest galaxies we can see sent their light billions of years ago.
Tell it back
Ask your child to explain, in their own words, why looking at a star is a little like looking back in time.
Check questions
How fast does light travel, according to the book?
Without air, what color would the daytime sky be?
How long does light take to reach us from Alpha Centauri?
Notice it this week
In winter, look for Sirius, the brightest star of the night, low in the south below Orion. Say hello to light that left it before you started school.
Answers to the book's problems
186,300 miles a second. Black (the book says the sky would be black, with the stars showing). Four years and four months, by the book's figure (about 4.4 years).
Worked out for this site from the scan. If a number in your copy differs, trust the book.