Ray's New Practical Arithmetic · Arts. 81–82 · Unit 8: Operations with Compound Numbers
46. Longitude and time
Goal: Your child changes a difference of longitude into a difference of time and back, and finds the clock time at a place east or west.
You'll need
- The book
- A slate, small whiteboard or scrap paper
- A notebook and pencil
What's in the book
Why 15 degrees of longitude make 1 hour of time, the table for comparing longitude and time, rules for changing one to the other, and problems about the time in different cities. Answers are printed.
The lesson, step by step
Warm-up
Ask: How many hours does the earth take to turn once? (24.) 360 ÷ 24? (15.)
The idea
Read:
The circumference of the earth, like other circles, is divided into 360 equal parts, called degrees of longitude.
15° of longitude = 1 hour of time.
Use a ball or globe and a lamp to show the sun reaching places in turn.
Longitude to time
Read:
Divide the longitude by 15, according to the rule for Division of Compound Numbers, and mark the quotient hr. min. sec.
Together:
The difference of longitude between two places is 30°: what is their difference of time?
(2 hr.)
East is later
Read:
It is noon (12 o'clock), at any place when the sun is on the meridian of that place.
Hence, a place has later or earlier time than another, according as it is east or west of it.
Together:
When it is 11 o'clock A. M. at New York, what is the time in longitude 30° east of New York?
(1 P.M.)
Independent practice
Your child does problems 3 to 12 of these two articles.
Check and wrap-up
Answers are printed. Note for you: in 1877 every town kept its own sun time; standard time zones came later, which is why Ray's problems give odd minutes and seconds between cities.