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← Same Sun, True Size

Same Sun, True Size

Three short lessons on the globe your child is learning: why noon comes later as you go west, why a flat map makes Greenland look as big as Africa, and why one side of a mountain is a desert.

Spark & Whetstone · sparkwhetstone.org/learn/same-sun-true-size · CC BY-NC-SA 4.0. Answers are at the end, so you can try each question first.

Lesson 1. Noon moves west

The Earth turns once a day, a full 360 degrees in about 24 hours. That is 15 degrees every hour.

So the Sun reaches its highest point, true noon, at different times in different places. For every degree you go west, noon arrives about four minutes later. Time zones round this off into hour-wide bands so a town shares one clock.

Question. Town B is 15 degrees of longitude west of town A. When is true noon in town B?

  1. About an hour after town A.
  2. About an hour before town A.
  3. At the same moment.

Say it back. In your own words: why does noon come later further west?

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Lesson 2. The stretched map

Every flat map of a round Earth has to stretch something. The Mercator map, made in 1569 for sailors, keeps directions true, so a straight line on it is a steady compass heading. The price is size: land gets stretched more and more towards the poles.

On a Mercator map, Greenland looks about as big as Africa. In fact Africa is roughly 30 million square kilometres and Greenland about 2.2 million, so Africa is around fourteen times bigger.

Question. Why does Greenland look so big on a Mercator map?

  1. The map stretches land more the closer it is to the poles.
  2. Greenland is about the size of Africa.
  3. The map shrinks everything near the equator to fit.

Say it back. Say it back: what does the Mercator map keep true, and what does it give up?

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Lesson 3. The dry side of the mountain

When moist air blows into a mountain range, it is pushed upwards and cools. Cool air holds less water, so the moisture falls as rain or snow on the side facing the wind.

By the time the air comes down the far side it has lost most of its water, and it warms as it sinks. That side is dry: a rain shadow. California's Sierra Nevada is wet on its western side, and the dry lands of Nevada lie in its shadow to the east.

Question. The wind blows from the west over a tall mountain range. Which side is likely to be driest?

  1. The western side.
  2. The eastern side.
  3. Both sides equally.

Say it back. In your own words: how does a mountain make a desert on one side?

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At the table: The orange and the torch

Players: a grown-up and a child, 6 or older. You'll need: an orange, a pen and a torch (or a lamp) in a dim room.

  1. Draw a dot on the orange for your town and another a little to the left (west).
  2. Shine the torch from one side and turn the orange slowly from west to east.
  3. Watch which dot reaches the bright middle first. That is the first to have noon.
  4. Peel the orange and try to lay the peel flat. See what has to tear or stretch.

Answers

  1. About an hour after town A. — 15 degrees is one hour of turning, and the Sun reaches the western town later, so noon there is about an hour after town A.
  2. The map stretches land more the closer it is to the poles. — To keep directions true, the map stretches areas towards the poles. Greenland is far north, so it is stretched a lot.
  3. The eastern side. — Air rising on the western side drops its rain there, so the eastern side, downwind, is in the rain shadow.