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Author: Janice VanCleave

So You Want to Do a Project about Venus!

Let's Explore


To determine why Venus has phases.


  • lamp
  • pencil
  • 4-inch (l0-cm) Styrofoam ball
  • measuring tape
  • helper


  1. Set the lamp on a table and remove its shade. The light bulb will be referred to as the lamp.
  2. Insert about 1 inch (2.5 cm) of the pointed end of the pencil into the Styrofoam ball.
  3. Darken the room except for the light from the lamp.
  4. Holding the pencil, position the ball in front of but below the lamp.
  5. At a slight angle, slowly move the ball counterclockwise halfway around the lamp, stopping when the ball is behind and above the lamp.
  6. As you move the ball, make note of the changes in shape of the visible portion of the lighted side of the ball. Ask your helper to measure the width of the shadow cast on the ball when the ball is in front of, to the side of, and behind the lamp.


In front of the lamp, the lighted side of the ball is not visible. As the ball moves from the front to the back of the lamp, the visible lighted part of the ball increases in size.

Morning Star


Galileo Galilei (1564-1642), an Italian astronomer, observed that the planet Venus undergoes phases (changes in the size and shape of the lighted side of a celestial body visible to observers on Earth). He used this information to show that Venus orbits the Sun and does not orbit Earth as many believed at that time. He observed that when Venus is almost fully sunlit, it appears to move behind the Sun as viewed from Earth and is out of sight. If it orbited Earth, when fully sunlit it would be on the opposite side of the sky from the Sun, as the Moon, which orbits Earth, is fully lit when it is on the opposite side of the sky from the Sun.

In this investigation, the ball represents Venus and the lamp (lightbulb) the Sun. Venus is an inferior planet (planet whose orbit is closer to the Sun than Earth's). There are two inferior planets, Mercury and Venus. During Venus's orbit around the Sun, as seen from Earth, Venus moves in front of and behind the Sun. When Venus is between Earth and the Sun, Venus is said to be in inferior conjunction. Conjunction occurs when two celestial bodies appear in the sky one under the other, but not one in front of the other. Venus's apparent separation from the Sun as viewed from Earth is so small that during inferior conjunction as well as superior conjunction (position of an inferior planet when it is on the opposite side of the Sun from Earth), the light of the Sun is so bright that Venus is not visible. While it appears that Venus passes directly in front of or directly behind the Sun, this occurs rarely and most of the time they just appear to be close.

During inferior conjunction, Venus is not visible because the side of the planet reflecting the Sun's light is facing away from Earth. Also, the bright light of the Sun in this position prevents the planet from being seen. Venus moves in a counterclockwise direction around the Sun. As Venus moves from inferior to superior conjunction, more of its lighted side faces Earth, thus Venus is said to be in a waxing (growing in size) phase. As the planet approaches a superior conjunction, it is almost fully sunlit, then it disappears because of the Sun's light.

During this movement from inferior to superior conjunction, Venus would appear to the west of the Sun in the sky. When Venus is far enough away from the Sun to rise above the horizon before the Sun in the morning, the planet is seen in the eastern sky. At this time, Venus appears to shine like a star and is called the "morning star."

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