Observing Projects
You can find Uranus and Neptune with binoculars if you know where to look (a good star chart is essential), but Pluto is so dim that it can be a challenge to spot even with a 25-cm (10-inch) telescope. Each year, star charts that enable you to find these planets are printed in the issue of Sky & Telescope for the month in which each planet is first visible in the nighttime sky. You can also locate the outer planets using the Starry Night™ program if you have access.
Make arrangements to view Uranus through a telescope. The planet is best seen at or near opposition. Use a star chart at the telescope to find the planet. Are you certain that you have found Uranus? Can you see a disk? What is its color?
If you have access to a large telescope, make arrangements to view Neptune. Like Uranus, Neptune is best seen at or near opposition and can most easily be found using a star chart. Can you see a disk? What is its color?
If you have access to a large telescope (at least 25 cm in diameter), make arrangements to view Pluto. Using the star chart from Sky & Telescope referred to above, view the part of the sky where Pluto is expected to be seen and make a careful sketch of all of the stars that you see. Repeat this process on a later night. Can you identify the “star” that has moved?
Use the Starry Night™ program to observe Uranus. Open Favourites > Explorations > Uranus Satellites to see an image of Uranus and its five largest moons as they would have appeared through a telescope situated at the south pole of Earth in February 1986. Select Labels > Planets-Moons to turn labels to these moons on or off. With the Time Flow Rate at 5 minutes, click Play and observe the motion of the moons around the planet. (a) Describe how the satellites move, and relate your observations to Kepler’s third law. (b) Change the year in the toolbar to 2007 and observe the motion of the moons as time runs forward. How do the orbits look different than in (a)? Explain any differences.
Use the Starry Night™ program to examine the satellites of Uranus. Open Favourites > Explorations > Uranus and select View > Stars > Stars from the menu to remove the background stars from the view. Use the Elevation buttons in the Viewing Location section of the toolbar to change the distance from the planet to about 0.01 AU. You should now be able to see at least five satellites of the planet Uranus. Open the Find pane and expand the layer for Uranus. Click the checkboxes to the left of each of the listed moons to display their names and click the checkboxes to the right of the listed moons to display their orbits in the view. Use the location scroller to adjust the view so that the orbits of the moons appear edge-on. (a) Do all of the satellites appear to lie in this same plane? (Hint: Zoom in on Uranus if necessary to check your conclusion.) (b) How do you imagine that this plane relates to the plane of Uranus’s equator?
Use the Starry Night™ program to observe Neptune and two of its moons. Select Favourites > Explorations > Triton and Nereid. The view shows Neptune and its moons, Triton and Nereid, from a location in space hovering 0.01 AU above Neptune’s equator. The stars have been removed from the view. With the Time Flow Rate at 1 hour, click the Play button and observe the motion of these satellites around Neptune. You can use the location scroller to change your viewpoint and you can use the Elevation buttons to move closer to or farther from Neptune. (a) Does Triton orbit Neptune in the same direction as Neptune’s direction of rotation or in the opposite direction? (b) How will Triton’s orbital direction relative to the rotational direction of Neptune affect this moon’s ultimate fate? (c) Nereid is in a rather elongated orbit that takes it far from Neptune. Does Nereid orbit Neptune in the same direction as Neptune’s direction of rotation or in the opposite direction? In which direction (clockwise or counterclockwise) does Nereid orbit Neptune when viewed from above the north pole of the planet? (d) When it is closest to Neptune, does Nereid approach closer to Neptune than does Triton? If you decrease the elevation of the observing location above Neptune to about 500,000 km, you will see guides on Neptune indicating the planet’s poles, equator, and a meridian. Use the Time Flow controls and the position of the meridian on the planet to determine the approximate rotational period of Neptune. (e) What is the planet’s rotation period?
Use the Starry Night™ program to observe Pluto and Charon. Select Favourites > Explorations > Pluto. The view is centered on Pluto from about 42,000 km above its surface. One of Pluto’s moons, Charon, is also shown, along with its orbit. Note that the south pole stick on Charon lines up with the star Atria and that the red meridian and pole sticks on Pluto are aligned. Make a note of the time shown in the toolbar. With the Time Flow Rate at 30,000×, click the Play button and observe the motion of Charon around Pluto and the rotation of Pluto. Use the time controls to stop time once Charon has completed a full orbit with its south pole stick once again aligned with the star Atria. (a) What is the period of Charon’s orbit? (b) What is Pluto’s rotation period? (c) How do these two periods compare? Explain this result. (d) If Earth and the Moon were to have the same orbital and rotational relationship between them that Pluto and Charon have, which of the following statements would NOT be true?
Collaborative Exercise
Sir William Herschel, a British astronomer, discovered Uranus in 1781 and named it Georgium Sidus, after the reigning monarch, George III. What name might Uranus have been given in 1781 if an astronomer in your country had discovered it? Why? What if it had been discovered in your country in 1881? In 1991?