## Introduction: Earth's Elliptical Orbit Marble Model

Need a science project that is quick to make and easy enough to involve the kids? Want to teach the kids about the first and second laws of planetary motion? Read on to see how we made this model of the Earth's orbit around the Sun!

## Supplies

Any large square scrap wood piece you have lying around (mine was 2ft by 2ft)

Pencil

Ruler

String

Pushpins

Paint, various acrylic

Styrofoam sphere cut in half

Router with U-channel cutting bit

Marble that fits in the channel

## Step 1: Draw the Elliptical Orbit

The gravitational pull of the Sun is responsible for the Earth's orbit around the Sun. Earth's orbit is not a perfect circle either-- it is elliptical, or slightly oval-shaped. This means there is one point in the orbit where Earth is closest to the Sun, and another where Earth is farthest from the Sun. In describing the nature of elliptical orbits, scientists use a factor known as "eccentricity", which is expressed in the form of a number between zero and one. If a planet's eccentricity is close to zero, then the ellipse is nearly a circle. If it is close to one, the ellipse is long and slender. Earth's orbit has an eccentricity of less than 0.02, which means that it is very close to being circular.

Our orbit ended up very "oval" looking and you can adjust this for your model, but I personally like that it drives home the point that Earth does not have a circular orbit around the Sun.

This is a great time to involve your kids by making them draw the orbit. In drawing the ellipse, you pick two points that are equidistant from the center of the board. These are the ellipse's two focus points. The word foci (pronounced 'foe-sigh') is the plural of 'focus'. The foci always lie on the major (longest) axis, spaced equally each side of the center. Put your pushpins in these two spots and wrap a large loop of string around them. Put your pencil in the loop of the string and stretch it to its limit and start drawing a large oval around the center of the board, keeping the string loop taunt at all times.

## Step 2: Carve Out the Marble's Channel

I used a router to carve out a U-shaped channel following the pencil line on my board. Go slow and use appropriate safety measures such as ear protection, safety goggles, and clamps to hold the board in place. Don't worry too much if there ends up wobblies in the line, this is not the time to stress about perfection!

## Step 3: Paint the Board

Get one of the kids to paint the board black. Have them flick "stars" onto the board by watering down white paint and stroking their thumb across the bristles. Do this outside where random "stars" won't ruin your floors!

## Step 4: Add the Sun and Finishing Details

Paint the styrofoam hemisphere to resemble the Sun and glue it to one of the two focus points used to make the elliptical orbit. This is Kepler's first law of planetary motion: all planets move about the Sun in elliptical orbits, having the Sun as one of the foci.

Paint the months of the year around the orbit, making sure to put them in the correct spot based on where the Sun is located. Notice how the length of each calendar month varies around the orbit, i.e. December and January have a bigger portion of the arc of the orbit because the Earth travels fastest around that part of the ellipse that is closest to the Sun. The months of June and July have a smaller portion of the arc of the orbit because the Earth travels slowest around that part of the ellipse farthest away from the Sun. This is Kepler's second law of planetary motion: a line between the sun and the planet sweeps equal areas in equal times. Thus, the speed of the planet increases as it nears the sun and decreases as it recedes from the sun.

Add more interesting facts around the model in paint pens.

Now you can talk to your kids about how Earth's elliptical orbit has nothing to do with our seasons. Many people believe that the temperature changes because the Earth is closer to the Sun in summer and farther from the Sun in winter. In fact, the Earth is farthest from the Sun in July and is closest to the Sun in January! Earth comes closest to the Sun every year around January 3 (the "perihelion"). It is farthest from the Sun every year around July 4 (the "aphelion"). The difference in distance between Earth's nearest point to the Sun in January and the farthest point from the Sun in July is 5 million kilometers.

The four seasons are actually determined by the fact that the Earth is tilted 23.4° on its vertical axis, which is referred to as "axial tilt." This quirk in our orbit determines the solstices – the point in the orbit of maximum axial tilt toward or away from the Sun – and the equinoxes, when the direction of the tilt and the direction to the Sun are perpendicular. So when the northern hemisphere is tilted away from the Sun, it experiences winter while the southern hemisphere experiences summer. Six months later, when the northern hemisphere is tilted towards the Sun, the seasonal order is reversed. In the northern hemisphere, the winter solstice occurs around December 21st, the summer solstice is near June 21st, spring equinox is around March 20th, and autumnal equinox is about September 23rd.

## Step 5: Invent Silly Challenges With Your Kids

We had fun seeing who could flick the Earth marble around the Sun and land it in their birth month!

Participated in the
Space Challenge