Solar System Model - the Orrery

The Parts. 
 
Step 1. Put the seasonal labels on the corners in order: Winter, Spring, Summer, Fall. Put the two small pegs into the holes in the base. Then find the parts shown and carefully put them together as pictured. 
 
Step 2. Put the dowel with the Sun ball into the rounded end of the arm and tightly into the hole in the slotted wheel that you put onto the base in Step 1. 
 
Step 3. At the other end of the arm, put a large wheel on the peg sticking out the end of the arm (put the o ring on the outside to hold it on) and put the smaller wheel with the dowel in it up through the hole next to the large wheel. 
 
Step 4. Loop the rubberband around the slotted wheels on the bottom as shown. Note there is a twist in the rubber band 
 
Step 5. Put the last large wheel on the dowel so it rests on the other large wheel. Carefully place the earth/moon assembly on the top of the dowel and make it secure. Move the Orrery arm counterclockwise to rotate the moon around the earth. 
 
Step 5a Detail. 
 
Step 5b Detail. 
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The Orrery

Isaac Newton (1642 - 1727) worked out the mathematics of the movement of the planets. His contemporary, Charles Boyle, the Earl of Orrery, commissioned George Graham to build a mechanical model of Newton's ideas in 1700. Orrery's name stuck.

A Learning Tool

When Eli Whitney went to Yale in 1788, science as a subject was still new. The first Orrery purchased by Yale arrived from Germany broken. Whitney fixed it, a hint of the practical mechanical skills that would make him famous.

Models

Models can help us answer questions. With this model, you can demonstrate the cycles of the Moon, the timing of the tides, the relative positions of the Sun and the Moon during eclipses, and the changing of the seasons. It accurately shows that the Moon is about 1/4 the size of the Earth. But all models have limitations. This Orrery would have to be much more complicated to more accurately demonstrate many other relationships. For example:

True Scale

Our model's Earth is 1" in diameter. Our model's Moon is 1/4" in diameter. Reality: The Moon is 30 Earth diameters away from from the Earth. So our model should show the moon 30" away from our Earth. It shows it about 3" away. Reality: The Sun is about 109 times larger than the Earth. So our model Earth should travel around a 109" diameter sun (about 9 feet wide.) Our model sun is only 1.5" in diameter. Reality: 12,000 Earth diameters (12,000 inches in our model!) would accurately show the distance of the Earth to the Sun. Our model would need an arm with a circular radius of 1000 feet. Ours arm's circular radius is only about 10". Certainly you can guess why we had to do this.

What moves?

The model reminds us that Earth travels around the Sun. But in reality, does it always seem that way? We all often say, The sun rises in the morning. But the Sun does not move. The Earth turns. The Model shows that the Earth turns toward the Sun in the morning.

Direction of Travel

When we wake up in the morning we see the sun first in the East. So our Earth is turning on it's axis towards the East. Looking down on the North Pole, we call that direction counter clockwise. The Earth also travels around the sun in a counter clockwise direction. And the Moon travels around the Earth in a counter clockwise direction.

Telling Time

One turn of the Earth around its axis = a day. One trip of the Moon around the Earth = about a month. A revolution of the Earth around the Sun = about a year. Explaining Changes

Lunar Phases

First, shine the light on the earth. New Moon Position the Moon between the Earth and the Sun. The face of the Moon that we see from the earth is dark. A New Moon. Waxing Moon Move the Moon counter clockwise. The sliver of light that we see projected on the Earth grows larger. Waxing means growing. Full Moon Position the Moon so that its whole face is illuminated. That's a Full Moon. Waning Moon Move the Moon counter clockwise again. The sliver of light we see grows smaller. Waning means emptying.

Tides

The Moon is close enough to the Earth that its gravity pulls on the planet. That gravity pulls on the oceans which create the flows we call tides. That gravity also creates tides on the side of the Earth opposite the Moon. The Moon moves as the Earth turns each day. So the times of the tides change each day.

Seasons

The Earth is tilted on it's axis 221.2 degrees. Winter When the Earth is tilted away from the sun, the arc of light that reaches the Northern Hemisphere is short. The shortest day (The Solstice) is the first day of Winter. Spring When the Earth tilts neither to or away from the Earth, the Sun will light half the earth. Days grow longer. Equal days and nights (the Spring Equinox) begins the season we call Spring. Summe When the Earth is tilted toward the sun, the arc of light that reaches the Northern Hemisphere is long. The longest day (The Solstice) is the first day of Summer. Fall When the Earth tilts neither to or away from the Earth, the Sun will light half the earth. Days grow shorter. Equal days and nights (the Fall Equinox) begins the season we call Fall.

Eclipses

Solar Eclipse The Moon is 400 times smaller than the Sun. The Sun is about 400 times farther away from Earth than the Moon is.* So they appear to us about the same diameter. Occasionally a Full Moon's path will cross in front of the Sun. Some people on Earth may see the Moon block the Sun's light briefly. We call those few minutes a Solar Eclipse. *This Orrery shows the position of the Moon in the eclipse, but not the proportions that make it work. Remember the Scale issue. Lunar Eclipse The shadow of the Earth crossing the Moon is called a Lunar Eclipse. This must happen when the Moon is a Full Moon. It does not happen every Full Moon because the Moon travels on a tilted orbit** There can be two or more Lunar Eclipses in a year. **This Orrery can show the position of the Moon in an eclipse, but not the proportions or tilt that determine the timing. ,

1. The Earth travels around the Sun in approximately 365 ¼ days. How long does the Moon take to revolve around the earth?
2. The same side of the Moon is always facing the Earth. Does the Moon spin on it axis? How fast, if at all?
3. The base of the orrery has four sides. Could those sides represent the four seasons?
4. Are the relative sizes of the moon, the planets and the sun accurate?
5. Are the distances accurate?
6. A day is 24 hours long. Does this fact mean that the earth rotates exactly 360 degrees in 24 hours?
7. Textbooks usually describe the periods of rotation and revolution for the planets and the moon, but rarely do they identify the direction of movement. Can you make observations or plan experiments to determine which way the moon orbits the Earth? .... which way the Earth obits the Sun?
8. Is there "day" and "night" (a period of light followed by a period of dark) on the moon? How long is a "Moon day?"
9. The tides are predominately caused by the motion of the Moon. The Earth rotates approximately once every 24 hours. Yet, if a high tide is at 9am one day, on the next day there will be a high tide at about 10am. How could this be?
10. The arms that support Earth, Mercury and Venus are fixed length. Is this accurate?
11. What causes the earth to have different seasons? Is this characteristic accurately modeled with this Orrery?
12. From antiquity the circle has always had 360 degrees. What did one degree represent?
13. In summer daylight lasts longer. In winter nighttime is longer. Is this true in the Southern Hemisphere as well? Is there any place on Earth where the amount of daylight does’t change throughout the year?
14. In the United States the Sun rises in the East and sets in the West. In Australia are those directions reversed?
15. Venus, and especially Mercury, either set in the West soon after dusk or rise in the East just before dawn. They never appear in the night sky directly over head. Why is that?
16. When there is a "new moon," will the lit section of the moon be on the East or West section of the moon.
17. What are the positions of the Earth and the Moon when there is an eclipse of the Sun?
18. What are the positions of the Earth and the Moon when there is an eclipse of the Moon?
19. Where are the Earth and the Moon when there is a Full Moon? If you are on the Moon where are their positions when there is a "Full Earth?"
20. Why are shadows longer in the winter than in the summer?
21. The earth in this model is ¾ inch in diameter. If the Sun were modeled to this same scale, how big would it be?

, The NASA Website Lunar Eclipses Explore the solar system. This applet shows the current positions of the planets. Solar System Live is an interactive Orrery on the Web Solar System Simulator Explore the Planets