Venus has fascinated observers since before the dawn of recorded history. Venus is the brightest object in our sky after the Sun and the Moon. Because of its brightness, the Romans named it for their goddess of love and beauty.

The first spacecraft to visit Venus was Mariner II in 1962. Since then more than 20 spacecraft have visited Venus including Venera 7, launched by the Soviet Union and the first spacecraft to actually land on another planet. Each of these missions contributes more to our knowledge of Venus and of our solar system in general.

In this module, you will study the motion of Venus and Earth. You will discover that Venus' rotation is retrograde, meaning that it rotates in a direction opposite from Earth and most of the other major planets. You'll also discover some interesting facts about a Venus year and a Venus day.

## Orbit and a Planetary Year

In this activity, you will examine the orbits of Venus and Earth. A planet's orbit refers to its motion along its path around the Sun. The amount of time that it takes a planet to complete one full orbit is called a year for that planet. In this Gizmo™, the Sun is at the center of the screen. The inner planet is Venus. (Mercury is not shown in the simulation.) Earth is the outer planet and is orbited by the Moon. The plane of the computer screen corresponds to the plane of the ecliptic, which is the plane containing the orbits of all eight major planets. You are observing the scene from ecliptic north, which means you are looking down on the North Pole on Earth. Ecliptic south, and the South Pole, are behind the computer screen.

1. Click Play () to start the simulation.
1. In which direction (clockwise or counterclockwise) does Venus orbit the Sun? In which direction does Earth orbit the Sun?
2. In which direction (clockwise or counterclockwise) does the Moon orbit Earth?
3. If you were to view this scene from ecliptic south, would your answers to the preceding questions be the same? If not, how would they differ?
2. Click Reset () to return the simulation to its original state. Click Play to start the simulation.
1. Watch Venus as it completes one full orbit about the Sun. (You can adjust the speed of the simulation using the Speed slider if you like.) When Venus reaches its original location, to the right of the Sun in the Gizmo, click Pause (). Look at the clock on the DESCRIPTION pane. This clock reports the time since the simulation started in Earth days, hours and minutes. How long did it take Venus to complete one full orbit about the Sun? What is the duration of a Venus year (in Earth days)?
2. Click Play to resume the simulation. When Earth reaches its original location, to the right of the Sun, click Pause again. How long did it take Earth to complete one full orbit about the Sun? What is the duration of an Earth year? You should have found that an Earth year is longer than a year on Venus. The further a planet is from the sun, the longer its year is.

## Planetary Rotation and the Length of a Day

Rotation refers to the planet's spin on its axis. The amount of time that it takes for a planet to undergo one complete rotation around its axis is called the rotational period of that planet. The rotational period on a planet is related to the length of a day on that planet (although they are not exactly equal). In this activity, you will examine the difference between the rotational period of Venus and the length of one day on Venus.

1. Click Reset and then click Play. Adjust the Speed if you like so that you can easily observe the rotations of Earth and Venus around their axes.
1. In which direction (clockwise or counterclockwise) does Earth rotate about its axis?
2. Carefully watch the motion of the person standing on Venus. In which direction does Venus rotate about its axis?
3. In our solar system, most of the planets rotate counterclockwise when viewed from ecliptic north. Those that rotate in the opposite direction are said to be in retrograde rotation. Which planet shown in the Gizmo is in retrograde rotation?
2. Reset the Gizmo once again. Notice that the person standing on the surface of Venus extends directly toward the right edge of the screen.
1. Can the person shown standing on Venus see the Sun? What time of day is it for that person (morning, noon, evening, or midnight)?
2. Click Play. Observe the person as Venus rotates about its axis. When the person once again points toward the right edge of the screen, click Pause. Look at the clock. What is the approximate rotational period of Venus?
3. Is it exactly midnight for the person on Venus at the point at which you paused the Gizmo? Explain why or why not.
3. Click Reset. Notice that, once again, the person standing on the surface of Venus extends directly toward the right edge of the screen. Because the person is on the side of the planet that is directly opposite the Sun, it is currently midnight at the person's location. Click Play. When it is midnight for the person again (when he is on the side of Venus directly opposite the Sun), click Pause.
1. The amount of time between midnight and the next occurrence of midnight on a planet is the length of one day on that planet. About how long is a day on Venus, in Earth days?
2. Compare the rotational period of Venus with the length of its day. Explain why they are different.
3. Challenge: The rotational period of Earth (23 hours and 56 minutes) is shorter than a day on Earth (exactly 24 hours). However, the rotational period of Venus is much longer than a day on Venus. Why is this the case?
4. Click Reset once again. Click Play and watch the motion of Venus.
1. Watch the person as Venus orbits the sun while rotating on its axis. Use the Gizmo clock to estimate what time the person on Venus experiences sunrise, noon, and sunset.
2. Imagine standing on Venus (assuming you could survive there) and watching the setting sun. How would the sunset on Venus be different from the sunset on Earth?
5. Answer the questions below to compare a day and a year, on Earth and on Venus.
1. How many hours long is an Earth day? How many Earth days are in an Earth year?
2. About how many Earth days are in a Venus day? About how many Earth days are in a Venus year?
3. About how many Venus days make up a Venus year?