Sun Moon and Earth System

This is a big electromechanical, working model of the sun, moon and earth system. It shows four motions -

Various phenomena that occur due to the above motions can be shown using this model -

Not everyone is able to visualize the above mentioned phenomena or understand them quickly. It might take some days to properly understand them but with this model I can explain them in just 30 minutes. The explanation is not with a chalk and board but with a 3D model existing in solid reality. You can feel it, move around it, view it from different angles, go near the sun, moon or the earth to see how the other bodies would look from that point.

A full view of the model

1. Solar Eclipse

A Solar Eclipse happens due to the sun, moon and the earth coming more or less in a straight line, and the moon being in the middle of the other two. The following image shows the position of a new moon day. You can see the shadow of the moon fallen on the earth.

Fig 1.1


Fig 1.2

A close view of the shadow casted by moon on the earth's surface. You can clearly make out a circular dark shadow called Umbra and a peripheral faint shadow called Penumbra

2. Lunar Eclipse

Fig 2.1
The moon is about to go into the earth's shadow. You can see the sunlight falling on the moon
Fig 2.2
The moon is in the earth's shadow, causing a Total Lunar eclipse

3. Solar and Lunar eclipses don't occur on every new moon and full moon day

Since the moon's orbit is tilted to the earth's orbital plane, it doesn't cause a solar eclipse or a lunar eclipse to happen every full moon or new moon day. In the following two images note that the moon's orbit is inclined and no eclipse occurs due to the misalignment between the three bodies.

Fig 3.1

The moon is far above the orbital plane of the earth, and hence the shadow of the earth passes below the moon. There is no Lunar Eclipse on this full moon day.
Fig 3.2

The moon far below the orbital plane of the earth, and hence the shadow of the moon passes below the earth. There is no Solar Eclipse on this new moon day.

4. Equinox and Solstice points

Below are 4 views of the earth at 4 different points seperated by 90 ° in its orbit. Set you page such that the 4 earths in the 4 images below are visible simultaneously on the screen. Now visualize the sun is in the middle of the screen, near the joining point of the 4 images, and try to find out the regions of the earth which will be illuminated. Also, try figuring out when the north pole experiences a day and when it experiences a night.

These four points in the earth's orbit are named as indicated.

Autumnal Equinox
Summer Solstice

Winter Solstice

Vernal Equinox

5. Precession Motion

The Precession motion is the third motion of the earth, due to which the earth moves like a top. Imagine rotating a top. Initially it rotates with its axis perpendicular to the ground. But as its energy decreases, its axis starts tilting. It continues to rotate with its axis also rotating and pointing in different directions as it does so.

The earth's precession takes and an overwhelming 25800 years. It means the axis will complete one rotation and point in the same direction after 25800 years. In between this period it will trace an imaginary circle in the sky.

Fig 5.1
Here the axis of the earth, if produced upwards, is pointing to the left.
Fig 5.2
The axis of the earth, if produced upwards, is pointing to the right.


Some Historic Information

In February 2001, the model was taken to IIT Kanpur for the Astro modeling competition in the Techkriti 2001. festival. It bagged first prize at all India level. The director of the Nehru planetarium, Delhi had come there for giving a seminar and was very impressed by the model. She even asked to make a similar model, bigger in size, to be put up in the planetarium.

This model has been demonstrated in various exhibitions. Thousands of people have seen it. A number of schools have asked for a similar model to be made for them. I am not yet able to fulfill their request, though.

In August 2003, after working on it for more than six months, I was successful in making a new version of the model. It is more compact, weighs much less and is portable than its predecessor.

It has a small, handheld control panel to control the motors that drive the four distinct motions shown. The huge, clumsy aluminium drafter has been eliminated and so the sun is supported from the bottom. Unlike the previous model, the moon is also supported from the bottom. No pullies are used as they were in the previous model. This one uses only gears. A lot of things are totally hidden so you might not have a clue as to how things work. It has 2 homemade slip rings to supply power from the bottom stationary part to the upper moving parts.

The making of slip rings was a great breakthrough. I searched a lot in Mumbai for manufacturers of the slip rings for the required specifications, but couldn't find one. I searched the internet and found manufacturers from US and UK, none from India matching my requirements. So I decided to make one. It was hard but finally I got it. Albeit a crude slip ring, it works for me.

The four motors have a regulated power supply along with speed control. All the electronic circuits used in the model were designed and implemented by me. Later, I intend to interface this model with a computer and run it in synchronization with my 3D simulation software called Universe. It will also have an infrared remote control.

This model has been demonstrated to the Director of the Nehru Planetarium, Mumbai. He has greatly appreciated it.

A snap taken at an exhibition arranged by Khagol Mandal somewhere in the year 2000. It has me explaining the model to the curious onlookers which include Dr. Jayant Naralikar.