Saturn

Summary: In this lab you will observe Saturn and determine the diameter of the planet and its ring system. In doing so, you should gain some perspective of this planet's physical size and characteristics relative to the more familiar characteristics of Earth.

Note: Much of what you should do during lab time is to make observations and sketches and to discuss what you are doing with your lab partner and classmates. Any calculations can be done outside of lab time, based on the data you collect during lab. Similarly, you should discuss during lab time how to answer questions posed in this lab, but you should write down your answers outside of lab time.

Background: When, in 1610, Galileo pointed his rather crude telescope to observe Saturn, he noted that it appeared very flattened, as if it had "ears." However, with time, the optics used to make telescopes became more sophisticated, providing greater image clarity. In 1655, the Dutch astronomer Christian Huygens was able to resolve and recognize the rings of Saturn. Because of these spectacular rings, Saturn has enjoyed widespread popularity, and is always the favorite object for amateur viewers at telescopes. In the past twenty years, astronomers have discovered rings around Jupiter, Uranus, and Neptune, albeit not nearly as brilliant as those around Saturn.

The rings around Saturn, of which three are visible in small telescopes, are composed of ice and ice-covered particles with typical sizes ranging from that of a ping pong ball to that of a house. Each of these rings is made up of many smaller rings known as ringlets. The origin of the ring system remains a mystery. Some theories suggest that the icy particles are the remains of satellites of Saturn which were destroyed in violent collisions, perhaps with a passing large comet or steroid. Other theories maintain that the particles simply condensed out of the protoplanetary disk from which Saturn formed. These particles neither accreted to become part of the planet, nor came together to form a another moon.

The moons of Saturn also have given this planet/ring/moon system much notoriety. One good example is Iapetus, or the "two-faced" moon. The hemisphere on the leading side in Iapetus' orbital motion around Saturn is covered with dark, frosty soil the color of reddish tar, while the trailing side is covered with bright, white ice. Titan, Saturn's largest moon, was found by Voyager to have a hazy, reddish atmosphere. It has been shown that this haze is actually a photochemical smog produced by reactions of methane and other compounds when they are exposed to sunlight. Based on measurements of temperature and pressure on Titan's surface, researchers conclude that the surface may be largely covered by a cold ocean of liquid methane and liquid ethane estimated to be a kilometer deep. Still more complicated, gazoline-like compounds may form in the smog and rain out of the hazy clouds. One Voyager scientist had characterized Titan as "a bizarre, murky swamp." These are just two examples of the more than 17 moons orbiting Saturn. Titan is larger than our own Moon and is the brightest of Saturn's moons. The space probe Cassini is on its way to Saturn (it is currently about to pass by Jupiter). Once it reaches the distant planet, it will start orbiting it and study the planet and its satellites for several yaers. It will also drop a probe in the atmosphere of Titan, which will relay data about clouds, winds, temperature, composition of the atmopshere, pictures of the surface, etc. for several hours until it succumbs to the frigid temperatures on Titan.

1.Notes: Make notes describing the atmospheric conditions tonight. Is the sky clear, partly cloudy, high cirrus? Is the air moist? Is it windy? Think about how the weather may affect your view of Saturn. As always, not the date and time of your observations. Plot the position of Saturn on the long star chart (SC001). The position of the Sun along the ecliptic is determined by the time of year. On your chart, the ecliptic is marked with the days of teheyear so it is easy to plot the position of the Sun on the day of your observation.

2. Find Saturn: Using the 25 mm eyepiece, locate and center the telescope on Saturn. Make a sketch of the field of view near Saturn, including Saturn. Saturn is far enough away that any of its moons will look very similar to background stars in the field of view of your telescope. Mark the positions of any stars or moons in the field of view. Use the general template for this sketch.

Question: Can you tell which objects are moons?

Question: If not, how could you determine which objects (if any) are moons of Saturn?

3. Sketch Saturn: Switch to the 10 mm eyepiece. Refer to the following figures

for illustrations of some of the features that may be visible on Saturn, depending upon the telescope size and atmospheric conditions. Keeping in mind that your 8-inch Celestron inverts the image, make a sketch of Saturn, using the template provided by the TAs. In your sketch, identify Titan. Include as much detail as you can (but leave time for the Transit Time measurements in parts 4 & 5). With reasonable seeing conditions, Cassini's Division in the ring system should be visible as a thin, dark and very sharp line. However, you may have to look very closely for some time in order to see features in the rings. Note that the outer rings are brighter than the inner rings. You may be able to see the shadow of the planet on the rings and the shadow of the rings on the planet (more difficult). Remeber to indicate the orientation of your drawing (directions of celestial West and North. How can you determine that?). Compare your sketch to those of classmates. If others have seen features you haven't found, look again, perhaps through their telescope to see if you can find those other features. Why might some telescopes have a clearer view than others?

If you observe Saturn on more than one night, note how the moon Titan has moved along its 16-day orbit around the planet.

4. Diameter of Saturn: Using the Method of Transit Times, determine the apparent diameter of Saturn as well as that for the rings at the position of their greatest extent. During the Fall 2000 semester, the distance of Saturn will be about 8.18AU (1 AU = 1.496 X 10^8 km). You can find the declination of Saturn using the dial on the declination axis of the telescope. Calibrate the dial by checking its reading with the declination of a nearby star of know declination (see the list of stars in "Star hopping").

Transit Time for Saturn [seconds]:

Diameter of Saturn [seconds of arc]:

Diameter of Saturn [km]:

Transit Time for Rings [seconds]:

Diameter of Rings [seconds of arc]:

Diameter of Rings [km]:

5. Follow-up work to be done outside of lab: See Saturn at Home for additional questions to be answered based on your study of Saturn.

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