“That is in our geography,” Gerda interrupted.  “The path which the earth takes in its trip around the sun is called its orbit.  The axis is a straight line that passes through the center of the earth, from the North Pole to the South Pole.”

“That is right,” said her father; “and if old Mother Earth went whirling round and round with her axis perpendicular to her orbit, we should have twelve hours of daylight and twelve hours of darkness all over the earth every day in the year.”

“I suppose she gets dizzy, spinning around so fast, and finds it hard to stand straight up and down,” suggested Gerda.

“No doubt of it,” answered her father gravely.  “At least she has tipped over, so that in summer the North Pole is turned toward the sun, but in winter it is turned away from the sun.”

“Let me show you how I think it is,” said Gerda eagerly.  She was always skillful at drawing pictures, and now she took the paper and pencil which her father gave her, and talked as she worked.  “This is the sun and this is the earth’s orbit,” and she drew a circle in the center with a great path around it.

“This is Mother Earth in the summer with the sun shining on her head at the North Pole,” and a grandmotherly-looking figure in a Raettvik costume was quickly hung up on the line of the orbit, her head tipped toward the sun.

“Here she is again in winter, with the sun shining on her feet at the South Pole,” and Gerda drew the figure on the opposite side of the orbit with her head tipped away from the sun.

“That is exactly how it is,” said her father.  “But do you understand that, when she is slowly moving round the sun, she is always tipped in the same direction, with the North Pole pointing toward the north star; so there comes a time, twice a year, when her head and her feet are both equally distant from the sun, which shines on both alike?”

“No,” said Gerda.  “When does that happen?”

“It happens in March and September, when Mother Earth has travelled just half the distance between summer and winter.”

“Oh, I see!  This is where she would be;” and Gerda made two dots on the orbit, each half-way between the two grandmothers.

“Good,” said her father.  “Now when she is in that position, day and night, all over the earth, are each twelve hours long.  We call them the ‘Equinoxes.’  It is a Latin word which means ‘equal nights.’”

“In March and September do we have a day when it is twelve hours from sunrise to sunset, and twelve hours from sunset to sunrise?” questioned Gerda.

“Yes, and it is the same all over the earth the very same day,” repeated Lieutenant Ekman.  “If you will look in the almanac when you go home, you will see just which day it is.”

Gerda studied her drawing for a few minutes in silence.  “I think I understand it now,” she said at last.

“It is easy to understand after a little study,” her father told her; “but everyone has to see it for himself, just like the midnight sun.