Phases of Venus: Changes in Shape and Size

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Author: Janice VanCleave

Galileo observed that, like the Moon, Venus had phases. His sightings led Galileo to question the established idea that everything in the universe orbited Earth. The cycle of phases of Venus noted by Galileo did not fit the pre-Copernican idea that all celestial bodies orbit Earth. However, they did fit with Copernicus's idea that Earth, Venus, and the other planets orbited the Sun.

In this project, you will model the phases of Venus as a morning and evening star. You will determine the position of Venus along its orbit by observing its phases. You will find its elongation, diameter, magnitude, and illumination. You will also learn about the planet's reflective surface.

Getting Started

Purpose: To model the phases of Venus, the morning star.


  • lamp
  • pencil
  • 4-inch (10-cm) Styrofoam ball


  1. Set the lamp on a table and remove its shade.
  2. Insert the point of the pencil into the Styrofoam ball.
  3. Darken the room.
  4. Holding the pencil, position the ball in front of but below the bulb of the lamp.
  5. At a slight angle, slowly move the ball counterclockwise halfway around the light, stopping when the ball is behind and above the bulb ofthe light (see Figure 17.1). As the ball moves, note changes in the shape of its lighted side.

Phases of Venus: Changes in Shape and Size


In front of and behind the lamp, the lighted side of the ball is not visible. As the ball moves, the lighted part increases in size and shape.


Venus's path about the Sun is inclined only about 3° , but due to the planet's distance from the Sun, it generally is above or below the Sun's disk at inferior and superior conjunction. The angle of Venus's orbit is exaggerated in this experiment. In this investigation, the ball represents Venus, the light is the Sun, and you are an observer on Earth. When Venus is at inferior conjunction, between the Sun and Earth, it cannot be seen. The illuminated side of the planet faces away from Earth. Also in this position, the Sun's glare prevents the planet from being seen. As Venus moves, more of its illuminated side faces Earth. The apparent form of the sunlit surface of Venus is called a phase. During the phases of growing illumination, Venus is said to be waxing. When the planet is almost fully sunlit, it disappears due to the brightness of the Sun's light. It then moves beyond the Sun. In this position, when the Sun lies between Venus and Earth, Venus is at superior conjunction. At conjunction, the Sun and Venus are on the same longitude lines but are not in a straight line as seen from Earth.

During its counterclockwise movement from inferior to superior conjunction, Venus appears to the west of the Sun as viewed from Earth. When Venus's elongation is great enough, Venus rises before the Sun. At this time, it rises before the Sun in the eastern sky and shines so brightly that it is misnamed the "morning star." After the Sun rises, the sunlight is so bright that Venus is no longer visible.

Try New Approaches

How do the phases change during the second half of Venus's orbit around the Sun? Repeat the experiment, starting at superior conjunction and moving the ball to inferior conjunction. During this part of Venus's orbit, illumination decreases and Venus is said to be waning. It appears to the east of the Sun as viewed from Earth and follows the Sun across the ecliptic. At this time, Venus can be seen above the western horizon in the evening after the Sun has set. In this position, Venus is called the "evening star." Science Fair Hint: Use a diagram to show the positions of Venus when it is a morning and an evening star.

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