Up to 32 students

Two to Three 45-60 minutes


Like a top unwinding, the Earth has a “wobble”. This wobble shifts which star over polar north at which we are pointed. We are currently pointed in the direction of Polaris. 5000 years ago we were pointed at Thuban and in about 25,771.5 years we will be pointing at Vega. This is a shift of the Earth’s revolution it is called the precession of the equinox. An equinox is when day and night last for equal time for the entire planet. Currently our equinoxes occur in the spring, March 21st, and the fall, September 20-21st. Through the progression of time the point on our orbit in which this occurs happens in different spots.

Currently we are tilted at an angle of about 23.5 degrees, this angle shifts as well. Our planet has in the past changed its tilt between 22° 38’ to 24° 21’, therefore affecting the seasons by changing the concentrations of incoming light. As our tilt is lessened the incoming solar energy is more evenly spread though out the globe making the seasons less extreme. Should we have no tilt there would be no seasonal changes. As the Earth makes it way to a greater degree of tilt our seasons become more extreme, winters harsher and reaching lower latitudes and summers hotter for the higher latitudes. The obliquity of the ecliptic is not a fixed quantity but changing over time in a cycle with a period of 100,000,000 years. It is a very slow effect known as nutation.

The Earth’s revolution around the sun is elliptical. However the degree of eccentricity in which we are currently under is a near perfect circle. The degree of eccentricity does shift over a 100 k.y. timescale. The eccentricity of the Earth's orbit is currently about 0.0167. Over thousands of years, the eccentricity of the Earth's orbit varies from nearly 0.0034 to almost 0.058 as a result of gravitational attractions among the planets

All of this wobbling and tilting changes the amount of incoming solar energy received by the sun. These changes can greatly affect our climates throughout the global, throwing us into an ice age or taking us out of one. These are commonly referred to as the Milankovich Cycles.

Learning Objectives:

1. Understand the effects of the Milankovich Cycles, eccentricity, obliquity and wobble of the Earth.

2. Understand that their is a predictable pattern in the changes to these shifts in orbit and revolution.

3. Understand that the current orbit is a near circle and that the changes in seasons have to do with direct and indirect sunlight not distance from the sun.

Guiding Questions:

Do you think that if the Earth changed its tilt it would affect how extreme the seasons are, why or why not? How do you think a change in eccentricity of the orbit would affect life, weather or climate on Earth?



Internet connection


Drawing Paper or Graph Paper

Thumb Tacks or Tape Paper sized Pieces of Cardboard String 30’ Rope (optional)

Two securely Mounted Poles (optional)

Globe (substitute large balls)



Extension cord

Activity 1: Ellipses of Earth

Discuss ideas about the changes in Earth’s eccentricity with the class. Clarify any misconceptions as per aphelion and perihelion having an affect on seasonal changes at our current situation.

1. Have students map out a scale orbit on a piece of blank drawing paper or graph paper. This can be done by tying a string into a loop, placing two thumb tacks at the two foci points, using the loop of string to draw out the ellipse of our orbit.

Note: If the ellipse is to the correct ratio it should be a near perfect circle. This will help students understand that the distance of 1 million miles overall makes little difference to the amount of incoming energy.

2. Have students map out the extreme eccentricity of the Earth’s orbit by shifting only one of the foci points.

Activity 2: Think, Pair, Share

Have students discuss the changes in Earth’s eccentricity and address questions like the ones listed below:

Could the changes shown affect the incoming energy when it is at the extremes? Would Earth move through the tighter curves of the ellipse faster and therefore not affecting the incoming energy as much as it would if it moved through the ellipse more slowly?

Optional: Discuss and replicate the precession of the equinox for the class.


Activity 3:Enactment of Eccentricity (optional)

If space is available in a gymnasium of an outdoor space; have students act as the Earth in orbit. One lone pole is good for a representation of out current orbit, secure a loop with the 30’ rope around the pole, have students jog around the pole at a constant speed while holding the rope taught – like that of drawing out the ellipse on paper. Shift the activity to the two poles set up, here the student should feel the change in speed when they reach the far point from the central foci as they revolve around this set up.

Conclusion and Wrap Up:

Have students add to their LINK.

Vocabulary to note:







Activity 4: Direct, Indirect and Tilt

Discuss concepts in axial tilt with class. Place an un-shaded lamp in the center of the room and turn off all other lights. Walk through the current seasons at our current tilt. Emphasize each solstice and equinox and discuss the amount of direct and in direct light at each stop, pointing out specific locations (northern and southern locations) at each point helps student get a better grasp on direct and indirect light.

Shift tilt the globe to a greater degree (more than the 24.5 to emphasize the effects of tilt) and follow the steps above, point out the same locations to see if anything has changed.

Shift the globe to 90 degrees to emphasize the lessened angle, follow the same steps above, point out that there are no seasonal changes throughout the globe.

Activity 5: Online Exploration

Have students visit NASA and Jet Propulsion Lab web sites for videos and other information on Earth’s eccentricity.

Ask students to write a brief summary on the effects of Earth’s changes in tilt.

Activity 6: Wild Wobbling

Demonstrate the changes in the wobble, to add to the affect place three “stars”, Polaris, Vega and Thuban on the wall and ceiling (placing stars at extreme positions helps to show the models wobble better).

At this point you may want to demonstrate all three phenomena together; this does take a lot of coordination so practice the process first.

Activity 7: Online Exploration

Have students visit any of the following sites for videos and other information on Earth’s obliquity. Ask students to write a brief summary on the effects of Earth’s obliquity.

Conclusion and Wrap Up:

Have students add to their LINK.

Vocabulary to note:

Axial tilt


New York State Scope and Sequence:

Intermediate and High School Science Standards

Physical Setting:

Key Idea 1.1d, 1.1f - 1.1g

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