### Seasons

I'm going to spring this on you

Introduction

For planets in elliptical orbits like Mars, are the seasons all the same length?

Material

Styrofoam ball perhaps 10 cm in diameter
Skewer long enough to piece the ball
Light source.

To Do and Notice

Push the skewer through the center of the ball so the ball rotates smoothly about the skewer as its axis of rotation.
An axle of rotation if you wish.
Place a light source just above the top of a table.

Consider the ball as a planet orbiting the light source just above the table.

Consider circular orbits in which the planet rotates many times each year. The planet orbits counterclockwise as viewed from above.
If you wrap your right hand around the equator of the ball so that the fingers point in the direction of rotation then your thumb points in the direction of the northern hemisphere.

Consider a planet with its axis of rotation in the plane of its orbit, horizontal in our model. (This is very close to Uranus)
Are there seasons? How do the seasons depend on the hemisphere?
Are there places that never see a sunrise during one full day of rotation?
When are the equinoxes?
When are the solstices?

Consider a planet with an axis of rotation tipped 45 degrees from the vertical, 45 degrees from the line perpendicular to the plane of its orbit. (this is like the Earth)
Are there seasons? How do the seasons depend on the hemisphere?
Are there places that never see a sunrise during one full day of rotation?
When are the equinoxes?
When are the solstices?

Consider a planet with its axis of rotation vertical, perpendicular to the table.
Are there seasons? How do the seasons depend on the hemisphere?
Are there places that never see a sunrise during one full day of rotation?
When are the equinoxes?
When are the solstices?

Now consider planets in elliptical orbits.

Consider a planet with its axis of rotation vertical, perpendicular to the table.
Are there seasons? How do the seasons depend on the hemisphere?
Are there places that never see a sunrise during one full day of rotation?
When are the equinoxes?
When are the solstices?

Consider a planet with its axis of rotation in the plane of its orbit, horizontal in our model.
The axis of rotation in the northern hemisphere points toward the sun at perihelion, the closest point in the orbit to the sun.
Are there seasons? How do the seasons depend on the hemisphere?
Are there places that never see a sunrise during one full day of rotation?
When are the equinoxes? (Write a recipe for another group so that they can find the equinoxes. Can you tell them how to find the spring equinox?)
When are the solstices?

Consider a planet with an axis of rotation tipped 45 degrees from the vertical, 45 degrees from the line perpendicular to the plane of its orbit.
At perihelion the northern hemisphere axis of rotation points closest to the sun. (This is like Mars)
Are there seasons? How do the seasons depend on the hemisphere?
Are there places that never see a sunrise during one full day of rotation?
When are the equinoxes?
When are the solstices?

What's Going On?

Seasons are usually defined by the tilt of the axis of a planet. The distance from the sun in the case of an elliptical orbit influences the strength of the temperature changes in each hemisphere.

For planets in nearly circular orbits

A planet with its axis perpendicular to the plane of its orbit has no seasons. Every day is equinox.

A planet with its axis in the plane of its orbit, like neptune, spends half of its orbit with one pole in darkness and the other half with that pole in continual sunshine.
The equinox occurs when the plane of the equator passes through the sun.

A planet with its axis tipped at some angle from the perpendicular to its orbit, 23.5 degrees for Earth, has one point where the sun is highest in the sky for one pole, that is the summer solstice. The axis points closest to the sun.
There will be two equinoxes when the sun passes through the plane of the equator. The equinox when one hemisphere passes from winter to summer is the spring equinox.

For planets with elliptical orbits and an axis that is perpendicular to the plane of the orbit the seasons are caused by the changing distance to the sun. Summer is at Perihelion, closest to the sun, and winter is aphelion. Every day is an equinox.

Mars is in an eliptical orbit with an axial tilt of 25 degrees from the perpendicular to its orbit. The seasons are defined by the axial tilt. Northern hemisphere winter solstice accurs when Mars is near perihelion. This makes northern hemisphere winters milder than southern hemisphere winters.

Because Mars has an elliptical orbit and the equinoxes are defined by when the sun passes through the plane of the equator, the seasons have unequal lengths on Mars.

Going Further

The day on Mars is called a sol.

There are 668 sols in a Martian year.

Northern hemisphere:

Spring 193 sols
Summer 178 sols
Fall 142 sols
Winter 153 sols

 Scientific Explorations by Paul Doherty © 2015 20 May 2015