Northern Illinois University
Department of Physics
College of Liberal Arts and Sciences

Venus is our closest neighbor in the solar system, and is also the most earth-like in dimensions and composition. However, Venus is a thermal wasteland, a victim of its own atmosphere's ability to trap heat. That same atmosphere provides a beautiful viewing surface for the planet.

Venus is only observable from Earth in the early morning or the early evening when it is visible. This is because of our relationship to it in terms of the Sun. Since we occupy a larger radius orbit, we will always be looking in on it as it orbits the Sun. From Mars, the Earth would appear to be performing a similar feat. Therefore, Venus will always appear to be chasing the sun up or down. Since Venus always precedes or follows the Sun, a vast array of mythologies have centered around it. The planet has held many names, some of which may even be familiar to you. (Ishtar, Lucifer, etc.)

Because of Venus's fame and mystery, and its ease of location in the sky, it became a natural target for the first telescopes of the Renaissance. It was found that the planet, when viewed through even a low power telescope, was not a perfect sphere, but rather appeared in a sickle, as the moon did as it was going through its phases. The observation of the phases of Venus was important for showing that Venus circled the Sun, and for determining its position relative to the Earth. See notes in your text on Galileo for more details.

Due to its proximity, a large number of exploratory missions have been dispatched to the planet. A wealth of information is available, including surface maps and surface probes. The observational leg of this project will include multiple observations over a span of time to observe the subtle shift in the phase of Venus.

Procedure

1. Locate Venus in the telescope at Davis Hall.
2. With the TA's assistance, attach either a camera or the CCD to the scope.
3. Capture at least one image, or more if you choose. Try several different images in different scopes and powers of magnification. Choose which image gives the sharpest definition of the planet, or through which the phase is most visible. As always, sketching is a viable option.
4. With use of photography, or with the CCD images, the pictures may be enlarged so that phases may be more visible. Once you have collected data from a span of at least two months, you can procede to the calculations and interpretations below. Less data may not be as revealing of a change.

Observation of the Phases and Their Meaning

The phases of Venus which you observe say a lot about Venus's relationship to the Sun and Earth.

Therefore, there are two things to consider when interpreting the images of Venus.

First, consider the shape of the planet and how would we on Earth see that shape.

If you see a crescent, it means that we are seeing more of the dark side of the planet. Decide if this is due to our position relative to Venus.

Also consider how Venus would have to be oriented to the Sun for us to see, say, a gibbous phase. The included image should help in your interpretations.