# ASTR1000 University of Colorado Distance and Density of The Planet Questions

(1) Close, and yet so far.

(2 pts.) (a) Which planet comes closest to Earth, at its closest approach?

(2 pts.) (b) How close is this planet at its closest approach?

(2 pts.) (c) How many times farther is this 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.

(2 pt.) (a) How could you determine the distance from a planet to the Sun?

(2 pt.) (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?

(3 pts.) (c) How could you determine the mass of another planet, making observations from Earth?

(1 pt.) (d) From the above observations, how could you determine the density of the planet?

(1 pt.) (e) Why is it useful to know the mean density of a planet?

(1 pt.) (f) Some of the best telescope observations of Mercury are made during midday instead of after sunset or before sunrise. Why? Why are none made at midnight?

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

(4) Which Planet?

(Remember: Pluto is not a planet!)

(1 pt.) (a) Which planets were discovered using telescopes? When?

Which planet has the following – give quantities

(1 pt.) (b) the most circular orbit

(1 pt.) (c) highest albedo (lightest color)

(1 pt.) (d) lowest albedo (darkest)

(1 pt.) (e) the lowest surface gravity

(1 pt.) (f) the highest surface gravity

(1 pt.) (g) the longest rotational period

(2 pts.) (h) the largest mean density

(1 pt.) (i) What is the orbital speed of Earth as it orbits the Sun (in km / sec)?

(2 pts.) (j) Convert this speed to miles per hour (1 km ~ 0.6 miles)

(5) Exploring the Moon – Imagine you are an astronaut exploring the Moon. Your lunar base is in Mare Imbrium, in the center of the near side of the Moon.

(2 pts.) (a) Describe where the Earth would appear in the sky and any changes in the Earth’s location and/or appearance with time.

(3 pts.) (b) Describe where the Sun would appear in the sky and any changes in the Sun’s location and/or appearance with time.

(1 pt.) (c) Would an ordinary compass work on the Moon? Explain.

(1 pt.) (d) Describe how you could use the Earth or Sun for navigation instead of a compass on the Moon.

(1 pt.) (e) You are selecting a lunar landing site. Where might astronauts land to seek evidence of the most recent volcanic activity?

(2 pts.) (f) If the ages of the Earth and Moon are identical, as believed, why are most rocks found on the Moon so much older than Earth rocks?

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