"Now, rotate the orange, holding the pencil exactly upright. You will see that the moving surface brings its shadowed side into the sunlight. This rotation gives us our alternating night and day. "Still holding the pencil upright, begin now slowly carrying it with the orange around the edge of the table. You will realize, if you think for a moment, that, _with the pencil held exactly vertical_, it makes no difference whether the orange is on one side of the table or the other. The sunlight on its surface is exactly the same in every position around the table. Under this condition, therefore, we would have uniformly alternating days and nights of equal length; and _no change of season_. You can see the most intense light would always be at the equator, and the least intense, down to perpetual twilight, at the Poles. Thus it would always be midsummer at the equator, temperate to the north and south equally, and winter equally and always at both Poles. "But this, of course, was not our condition. The axis of our earth was not vertically upright, as I have asked you first to picture it. Conceive now that you hold the orange and pencil again to your right at the table edge. Instead now of having the pencil point directly upward, slant it off _to the right_--_away from the sun_--toward the edge of your ceiling where it joins the wall, for instance. To be more exact, you are to tilt it over until it is about one-quarter of the way to a horizontal position. Mathematically, this is twenty-three and a half degrees from the vertical. "The top of the pencil--the North Pole--is now tilted away from the sun--the bottom is tilted toward the sun. You will realize now that the sunlit half of the orange is not from Pole to Pole. The light extends beyond and around the South Pole to the other side--and the light _does not reach the North Pole at all_. "Rotate the orange with the pencil held at that tilted angle. There are points at and near the South Pole which do not leave the light; and points at and near the North Pole are always dark. That is our _normal_ condition in December. In the northern hemisphere we call it winter; in the southern hemisphere they call it summer. "Now move your orange around the edge of the table, halfway around until you are on the other side. If you have kept the pencil tilted at that same angle toward your ceiling corner, you will find now that its top is pointing _toward the sun_. All the conditions on the orange's surface are reversed. That is June; summer in the North, winter in the South.