World Builders™                                                                        Session One  --  Astronomy 


What Causes Seasons?


Seasons are determined by the way that your planet orbits its sun.  Find out how.

A planet spins about an axis like a bead on a toothpick.

The axis is an imaginary line that goes through both of the planet's poles. The planet rotates around this line.

The planet also revolves around the sun in a path called its orbit. The orbit is elliptical (see Kepler's First Law). Imagine the orbit as a flat piece of paper with the planet moving along its outside. This is called the plane of the planet's orbit.

The axis may be tilted in relationship to the orbital plane. The axis may be straight up and down or tipped over to one side. The position of the axis influences the climate and weather of the planet. Let's take a few minutes to think about this.


 Like the earth, this planet has an axis that is tilted. Note the effect that this has on the planet.

  • In the first position, it is summer at the planet's north pole. The north pole is getting sunlight with no darkness at night because it is tilted towards the sun. the south pole is having winter, as it is in darkness all the time.

  • In the second position, the planet is having its autumn equinox -- the day when the daylight and darkness last for the same length of time. If this is the earth, every part of the earth is getting twelve hours of light and twelve hours of darkness.

  • In the third position, the north pole is now tipped away from the sun. The north pole is now in darkness, while the lighted south pole is experiencing the very long summer days.

  • In the fourth position, the planet is experiencing the spring equinox. Again, the length of the day and the length of the night are equal everywhere on the planet.

However, the temperatures are not even all over the planet, and, at the poles, even the long days of summer are not so warm as days in the tropics. Why is this?

Look at this diagram carefully.


See how the dark yellow rays of the sun strike the earth. The earth is curved here, so that the rays spread out a long way over the slanted surface.

In the central portion, the rays of the sun strike the earth more directly. This means that each little part of the earth here gets more direct light. Therefore, it also gets more energy and more heat.

Notice how, at the lighted pole, the sunlight is also spread out. Even though the days are long and the nights very short, the weather does not get really hot here because the sunbeams are spread out over a greater area of ground that in the hot tropical regions. The sunlight here is simply not as intense as it is closer to the center of the planet.

                                      Now let's look at some other possibilities.

 With this planet, the axis is straight up and down. As this planet travels around its sun, every day is the same length. The planet is rotating at a constant speed, and all parts of the planet receive light.

However, the temperature is not even all over this world. Study the diagram above again.

This planet will receive direct light at its equator, and the weather will be hot from receiving all that energy.

However, up toward the poles the curvature of the planet will again cause the rays of the sun to be more spread out. Parts of this planet near the poles will be cooler than the temperatures at the equator.

An exception to the No Seasons rule will occur if the planet has an elliptical orbit.  Then the planet would be nearer the sun on part of its journey and farther from it at other times.  On this planet the temperatures would be warm when the planet is near the sun and colder when the planet is farther away.  The whole planet would be warmer or cooler at the same time.  What does this mean? 

Plants would adapt to the cooler time as they do on earth, by shutting down during inhospitable times (due to extreme heat, drought, or cold) and  by making seeds.  Hibernation would work for animals, but migration might or might not.  This would depend on the temperature range,

This planet looks as if it is lying down on its orbit, just rolling along. What will the weather be like here?

(1) At one end of the orbit, half of the planet will be in sunlight all the time. The pole will be right in the middle of this sunlit part, and will be a hot place! Temperatures will decrease in concentric circles as measurements are taken farther and farther from the pole, and as the light covers more curved parts of the planet. The other side of the planet will be freezing in the dark.

(2)Midway along the orbit, where earth experiences equinoxes, the planet's axis will be parallel to the orbital path. The planet will be turning like a roast on a spit. Every part of the planet will get equal hours of light and darkness every day. The hottest part will be along the equator, midway between the poles, because that part of the planet will receive the most direct solar rays.

(3) This position, opposite (1) will be exactly like (1) except that the other pole will now be hot. The pole that was hot before is now in freezing darkness.

(4) This position will be exactly like (2). Days and nights will be equal, and every part of the planet will receive light every day.

If you want to do a planet with a day like this, use a ball and maybe a flashlight to make a model. Talk with Dr Viau about this. It is very different from what we know.

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© 1998, 2003. Elizabeth Anne Viau. All rights reserved. This material may be used by individuals for instructional purposes but not sold. Please inform the author if you use it at