Publications Topics:

Books

ASP Conference Series

Monograph Publications

IAU Publications

Books of Note

Purchase through the AstroShop

Journals

Publications of the ASP (PASP)

Magazines

Mercury Magazine

Archive

Guidelines for Authors

Order Mercury Issues

Mercury Advertising Rates

Newletters

The Universe in the Classroom

ASP E-mail Newsletters

Special Features

Astronomy Beat

Contact Us

Mercury, Jan/Feb 1996 Table of Contents

by James C. White II, Middle Tennessee State University

(c) 1996 Astronomical Society of the Pacific

Nearly four centuries ago, Galileo's simple observations of Earth's planetary sibling, Venus, buoyed the proponents of a Copernican universe. You can recreate those simple yet revolutionary observations.

This month's project: Observing Venus's changing phases

On a chilly, starry November evening in 1610, Galileo Galilei looked up from his telescope and admired the heavens above. The Italian astronomer had settled into a new house in Florence and had begun to observe Venus, which in November 1610 would have appeared as an evening star, bright in the western sky after dusk. Galileo was excited about his latest discovery, but wary of the intentions of other scientists; so wary, in fact, that he hid his discovery in a coded message to a politician and friend in Prague.

What was the discovery that Galileo felt he must guard? That Venus went through phases, just like our Moon. Galileo wrote: "At present she [Venus] is becoming sickle-shaped� her little horns will continue to become thinner, until she vanishes." But Venus soon reappeared as a thin crescent, just like the Moon does following its new phase, when it appears dark because we are seeing the side opposite the Sun. Over the course of months, Venus grew to full phase.

Galileo recognized that venusian phases could occur only if Venus orbited the Sun, rather than the Earth. He also noticed that Venus and Mercury never stray far from the Sun in the sky. Both planets appear to shift back and forth relative to the Sun, sometimes appearing as evening stars low in the western sky after sunset, other times as morning stars rising before dawn in the east. From these two observations, Galileo inferred that Venus must orbit closer to the Sun than does Earth.

Combined with his startling co-discovery (with German scientist Simon Marius) of four moons orbiting Jupiter, today referred to as the Galilean satellites, Galileo's detection of venusian phases shook the learned world. His observations corroborated the theories of Polish astronomer Nicholas Copernicus, who, on his deathbed in 1543, released a treatise declaring the Sun, not Earth, to be the center of the universe. Copernicus's Sun-centered model overturned the accepted, Earth-centered view of the heavens, developed by the Greco-Egyptian mathematician Claudius Ptolemaeus in the second century.

Because Galileo supported Copernicus's heretical views, he was forced by the Inquisitors in Rome to renounce his findings [see "A Non-Dialogue on the Two Great World Systems," November/December 1995, p. 10]. But the knowledge had already spread throughout Europe, and the universe would never be the same.

Observing Guidelines

If you don't have a telescope, contact your local astronomy club, tell them what you're interested in, and see whether one or more members is interested in joining you for the project. (See Amateur Astronomy Clubs for a worldwide listing of amateur clubs' World Wide Web sites.)

A 3-inch refractor or 4.5-inch reflector is a good choice for these observations. Once a week, set up your telescope in a spot that provides you a clear view of Venus, and record what you see. Photographs are perhaps the best way to chart the changing venusian phases as the planet speeds around the Sun. Drawings are also good -- that's what Galileo used -- but make sure your renditions are as accurate as possible. Through January and February, the fraction of Venus's illuminated side we see will decrease by about 20 percent; night-to-night changes will be small.

Whether you choose a photographic or sketched record, please make sure you also keep a written summary of your experiences. Include the time of your observations, your location, and the prevailing weather conditions (for example, "saw it through a break in the clouds"). Finally, try to describe what you see in words. Remember, Galileo relied on his written descriptions as well as his drawings. Putting into words what you see will exercise your skills as an observer and a communicator. And communicating what you discover may be as much fun as the discovery itself!

What to Do With the Observations

Incorporate your observations into a document with the following information: name of the project (such as "The Phases of Venus"), your name or the name of your group, details of the observing location, mailing address, telephone number, and email address, if available. We welcome reports from observers of all ages in all countries.

Please submit your completed report by Mar. 31, 1996 by email to 0002032694@mcima il.com or by regular mail to John Isles, Attn: Guest Observers, 1016 Westfield Drive, Jackson, Mich. 49203-3630. The selection committee will evaluate the reports and choose the Guest Observer for the July/August issue. If you are selected, we will request a photograph of you or your group to include in Mercury.

For additional information, visit our World Wide Web site. Send your comments and suggestions for future projects to James White at jwhite@physics.mtsu.e du or Department of Physics and Astronomy, Middle Tennessee State University, Murfreesboro, Tenn. 37132.

JAMES C. WHITE II is an assistant professor in the Department of Physics and Astronomy at Middle Tennessee State University in Murfreesboro. His astronomical research focuses on cataclysmic variables. White writes a monthly astronomy column carried by newspapers in Tennessee. His email address is jwhite@physics.mtsu.e du.

Illustration captions

The crescent Venus, as seen by the Hubble Space Telescope in ultraviolet light. Like the Moon, Venus goes through phases, and for the same reason: We see the planet from different vantage points as it and Earth orbit the Sun. When you look at Venus, you see sunlight reflected off the planet's nasty sulfuric-acid clouds. Image courtesy of Larry W. Esposito, University of Colorado, and NASA.