# Practical exercise 2: Model the motion of Earth around the Sun

## Aids:

• the models of the Sun and Earth, or also the model of Moon (see section a), a pin with a coloured round head, strong source of light, calculator, charts

## Instructions:

1. Place the models of the Sun and Earth on the table. Put the pin with the coloured head into the model of the Earth in the spot where the Slovak Republic is. The pin will mark the location of the observer.

2. Place the strong source of light in front of the Sun’s model in such a way that it lightens the Earth. By rotating the Earth around its axis (pay attention to the correct direction of the rotation: the country rotates from west to east) demonstrate day and night alternation in the location of the observer. At the same time watch how day and night alter in other places of Earth.

You can now solve the following questions and practical exercises:

• Find a location on Earth where there is no day and night alternation in the given position of Earth.

• Compare the length of a night in the spot of the observer and at the equatorial.

3. Demonstrate the motion of Earth around the Sun on its circular trajectory. Put the Sun in the middle of the desk. Put the Earth in the distance of about 15 cm from the Sun. Demonstrate the motion of Earth around the Sun on its orbital trajectory (neglect Earth’s rotation). Pay attention to the correct position of the Earth’ axis while orbiting the Sun (the position of Earth’s axis does not change in relation to distant stars).

Now do the following exercises:

• Place the Earth in a position to have summer in the northern hemisphere. By rotating the Earth about its axis (and adding the source of light into the experiment as in Exercise 2) demonstrate that the length of night in the spot where our country is located is longer than at the equatorial. Around the North Pole there is a polar day and around the South Pole is a polar night. What season is in this position in the southern hemisphere?

• Place the Earth in a position to have winter in the northern hemisphere. By rotating the Earth about its axis (and adding the source of light into the experiment as in Exercise 2) demonstrate that the length of night in the spot where our country is located is shorter than at the equatorial. Around the North Pole there is a polar night and around the South Pole is a polar day. What season is in this position in the southern hemisphere?

4. Demonstrate how the situation changes if we do not neglect the trajectory ecliptic of Earth around the Sun. Place the Sun in the middle of the desk again. With the help of charts calculate the minimal and maximal distance of Earth from the Sun and mark it on the desk with a chalk. Place the Earth into minimal distance from the Sun. Pay attention to the correct position of Earth’s axis (Earth is in the minimal distance from the Sun in winter, actually early January). Demonstrate the motion of Earth around the Sun again.

Now do the following exercises:

• Based on the experiment, can you explain why the inclination of Earth’s axis is more important for season change in our latitude than the trajectory ecliptic?

• Compare winter in the northern and southern hemispheres (as well as summer in the northern and southern hemispheres) in relation to the trajectory ecliptic (do not consider ocean and air currents).

• What conditions would have to be met to achieve no season change in every place on Earth, that means individual days would be the same all year round?

• Think what influence has atmosphere on the change of temperature in particular places on Earth’s surface. How would seasons be different from the current ones if atmosphere was much thinner than it is? And how would they be different in case of a much denser atmosphere?

5. Place the Sun and Earth on the desk again. Place there also the Moon. Assume that the Moon rotates exactly on the plane where Earth orbits the Sun. How many times does Moon orbit Earth in one year? With a classmate’s help now demonstrate the motion of Earth around the Sun while maintaining the correct ratio of rotations’ periods of Moon around Earth.

You can also do the following exercises:

• Place the Earth and the Moon in such a position that an observer in the Slovak Republic can see full moon. What part of day will there be at that time in our country?

• Place the Earth and the Moon in such a position that an observer in the Slovak Republic can see new moon. What part of day will there be at that time in our country?

• Place the Earth and the Moon in such a position that an observer in the Slovak Republic can see the first and then the last (third) quarter.

6. Calculate the sizes of Earth’s and Sun’s models so that they are in the same ratio as their distance. The models are placed on the desk 15 cm from each other.

• What spherical object would have its size calculated this way and could be a correct model in terms of size instead of a table tennis ball? What object would represent Earth?

• Assume that the Sun is modeled by a table tennis ball with the diameter 4 cm. In what distance should the model of Earth be located? An object of what size would represent it in the correct ratio?

7. Move the model of Earth around the Sun and at the same time demonstrate the rotation of Earth about its axis.

• Define all latitudes in the which the Sun is observed in the sky at least once a year towards the south, it means as it is in our latitude.

(source: https://www.nasa.gov/multimedia/imagegallery/image_feature_329.html)