(1) Close, and yet so far.
(a) Which planet comes closest to Earth?
(b) How close?
(c) How many times farther is this closest planet than the distance to the Moon? So, take the ratio of the closest distance of the planet from Earth (in kilometers) to the distance of the Moon from the Earth (in kilometers).

(2) Knowing without touching. Even before we sent spacecraft to the planets, we knew a lot about them from telescopic observations. Temperature, composition, etc. you can get from the spectrum. Let's look at how we can figure out other things.. Give a short description of how it could be done--a couple of sentences is enough.  Write down any equations you need to use.

(a) How could you estimate the semi-major axis of the orbit of another planet in A.U.--not using a telescope--just watching it move in the sky, maybe over several months?
(b) How could you measure the size in km? You can measure the angular size (in degrees, arc minutes or arc seconds) with a telescope. Then use the distance found in part (a) above and convert the distance to km. The size in km can be determined from the distance in km and the angular size--HOW? (Hint: you will need some simple trigonometry - think right triangles).
(c) How could you determine the mass of another planet, making observations from Earth?
(d) From the above observations, how could you determine the density of the planet?
(e) Why is it useful to know the mean density of a planet?
(f) Some of the best telescope observations of Mercury are made during midday instead of after sunset or before sunrise. Why? Why are no observations of Mercury made at midnight?

(3) TPs vs. GPs. List 5 major differences between the terrestrial planets and the gas giant planets.

(4) Which Planet? Getting to know them

1. Which planets were discovered using telescopes? When?
2. In each case, which planet has the following - give quantities
1. the most circular orbit
2. highest albedo (lightest color)
3. lowest albedo (darkest)
4. the lowest surface gravity
5. the highest surface gravity
6. the shortest day (spins fastest)
7. the smallest tilt of spin axis
3. Venus has the greatest angular diameter as seen from Earth - if we divide Venus diameter (12,100 km) by the shortest distance it comes to Earth ( 1 AU - 0.723 AU = 0.277 AU), we find that Venus has an angular size of 0.0165 degrees. Compare this angular size of Venus with the angular diameter of the Moon (or Sun) which is 0.52 degrees - that is, how many disks of Venus would it take to stretch across the width of the Moon?
4. What is the orbital speed of Earth as it orbits the Sun (in km/sec or km/hour)?
5. Convert this speed to miles per hour (1 km ~ 0.6 miles)

(5) Learning from Planetary Missions (see Chapter S2 and/or WebCT Session 10)

(a) The Galileo spacecraft, while taking pictures of the asteroid Ida, discovered that Ida has a moon (called Dactyl). By measuring the motion of Dactyl we can use Newton's version of Kepler's 3rd Law to determine the mass of Ida (the orbitEE). We get a value of 5 x 10¹⁹ grams. By measuring the size and shape of Ida the volume of the asteroid was calculated to be 2 x 10¹⁹ cm³ .

(i) Divide the mass by the volume to get the density of Ida (in g / cm³ ).(For more about density see page 122 of the text)

(ii) What does this tell us about Ida's composition?

(b) Landing on the planets

(i) On the surface of which planets have spacecraft been landed?

(ii) Which planets have been explored by humans? Explain your answer.

(c) How many years did it take Voyager 2 to travel to Neptune? When did Voyager 2 leave Earth and when did it get to Neptune?