Astronomy 310 Assignment 1
The following assignment covers Chapters One to Three. Please answer the following questions and submit your work
using the assignment link. Show all your work in a clear and concise fashion, and include each step you take to reach
the answer. The way in which you obtain your answer is as important as the final answer, itself, and marks will be
given for showing all the relevant steps. Please refer to the appendix in the textbook (pages 392–395) to obtain any
needed planetary values, and to Table 22 on page 41 for any needed constants.
Submit your answers to your tutor for grading and feedback using the assignment link. You may submit your
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Note: Assignments must be submitted as .doc, .docx, or .pdf files.
Each question has its point value marked in bold, for a total of 85 marks on this assignment.
1. [12 marks total]
(a) [4] Use Kepler’s first law to derive explicit expressions for the perihelion and aphelion distances for a planet
in terms of the semimajor axis a, and the eccentricity e, for an elliptical orbit. Calculate these distances for
Mercury.
(b) [4] What are the perihelion and aphelion speeds of Mercury?
(c) [4] Calculate the maximum angular separation of Mercury as seen from Earth.
(d) [4] What is the angular size of Mercury in arcseconds when it is at this location?
2. [9 marks total] Problem 19 in Chapter Three of the text (page 72):
(a) [4] Consider two particles in circular orbits in Saturn’s rings 108 m from Saturn’s center. One is located 1
m farther from Saturn than the other. By Kepler’s laws, they have different periods and must occasionally
pass each other. How fast do they pass by each other? (Hint: You could compute Vcirc for each particle and
subtract, but the difference in orbits is only one part in 108
, so you would have to maintain accuracy to at least
eight decimal places, which is unrealistic. Instead, use calculus and differentiate Vcirc = (GM)
1/2
(r
1/2
)
with respect to getting
dVcirc = difference in velocity
dVcirc = 1/2 (GM)
1/2
r
3/2dr
where dr is the difference in distance.
Note that the text has chosen to ignore the negative sign for this question, which indicates the direction of the
change in velocity.
(b) [3] What if the particles are about 200 km apart, as in the example of Figure 317a? Compare your result
with the 60 m/s figure given there (based on a more detailed calculation including gravitational attraction of
the particles for each other).
(c) [2] How fast would a shuttle in a 200kmhigh circular orbit approach a space lab in a 201kmhigh circular
orbit? (Answer: about 0.6 m/s.)
1
3. [16 marks total] Do not solve the following question numerically unless specifically asked to do so. The flux
from the Sun’s surface is given by LJ = 4pRJ
2 sTJ
4
, where TJ is the temperature on the Sun’s surface.
(a) [3] What is the incident luminosity from the Sun seen on Earth’s surface?
(b) [4] Only a part of the luminosity incident on Earth’s surface is reflected back. The fraction of radiation
reflected from Earth’s surface, or albedo, is given by A. The other part is absorbed and converted into heat,
which is then emitted as thermal emission from the planet. Write an expression for the luminosity absorbed
by Earth and the luminosity emitted by Earth. Assume Earth has had ample time to come up to thermal
equilibrium. Use the symbol Tsurf to represent the surface temperature of Earth.
(c) [3] Find an expression for the surface temperature of Earth, when the Earth is at thermal equilibrium. Note
that thermal equilibrium means that the amount of radiation absorbed by Earth is the same as the amount of
radiation emitted by Earth.
(d) [3] Calculate the surface temperature for Earth if A = 0.36. If Earth’s observed maximum temperature is 310
K, does the calculated temperature predict an accurate surface temperature? Explain.
(e) [3] Repeat the calculation to find the surface temperature of Venus (A = 0.76). Venus has an observed
maximum temperature of 750 K. Is the calculated surface temperature for Venus accurate? Why or why not?
4. [8 marks total] Problem 16 in Chapter Three of the text (page 72):
(a) [4] In the past, when the Moon was 0.25 as far from Earth as it is now, estimate how much more massive
Earth’s average tidal bulge was than it is today.
(b) [4] How much stronger was the net accelerating effect of this bulge on the Moon?
5. [5 marks total] It has been proposed that a spaceship might be propelled in the Solar System by radiation pressure,
using a large sail made of foil. The reflectivity of foil causes the radiation force to be twice what it is normally.
Calculate the area (in square kilometers) that the sail must be for the radiation force to be equal to the Sun’s
gravitational attraction. Assume that the mass of the ship and sail is 1500 kg and Q=1.1
6. [8 marks total] Problem 17 in Chapter Three of the text (page 72):
An early measurement of the outer radius of Uranus’s rings was about 5.0 × 107 m from the center of the planet.
Compare this to the Roche limit for icy bodies near Uranus, using the expression derived here for touching
particles. What is the significance of this result? Would you expect Uranus’s ring particles to aggregate into a
satellite?
7. [17 marks total] Consider a small dust grain of mass m = 4.19 × 103 kg and radius a = 1 cm, which becomes
separated from a parent body in a circular orbit a distance 2.5 AU from the Sun. The dust is subject to a force
from the PoyntingRobertson effect given by
FP R =
a
2
4c
2
L
2
JGMJ
r
5
!1/2
where a is the size of the particle and r is the radius of its orbit. We are going to take a series of steps to determine
how much its orbital radius changes after one complete orbit around the Sun. Please do not calculate the values
explicitly unless asked to do so.
(a) [4] Use calculus to compute the acceleration that the dust undergoes, dv
dt , where v is the orbital speed of the
dust grain. dv
dt is equal to dv
dr times dr
dt. Compute
dv
dr and multiply it by unknown quantity dr
dt.
1 Adapted from Halliday, D., Resnick, R., & Walker, J. (1997). Question 46P (p. 868). In Fundamentals of Physics Extended (5th ed.). New York:
John Wiley & Sons.
2
(b) [4] Newton’s law tells us that mass times acceleration is equal to the force. Write an expression for the force
from the PoyntingRobertson effect and solve for dr
dt.
(c) [4] dt is the time the dust takes to complete one orbit—otherwise known as its period. Using Kepler’s law,
write an expression for dt or the period of the dust’s orbit, making sure you change the elements of the
standard equation to the notation given in this problem. You can simplify this expression by noting that m is
negligible compared to M.
(d) [3] Use your result for dt in the equation for dr
dt and solve for dr (the change in radius over one period).
(e) [2] Calculate how much the dust’s orbital radius changes after one complete orbit around the Sun.
8. [10 marks total] We’ve learned that due to tidal forces, the Moon is slowly spiraling outward, away from the Sun.
As this happens, the Moon’s angular diameter will get smaller in the sky. Eventually, solar eclipses will no longer
occur, since the Moon’s disk will be too small to cover the disk of the Sun.
(a) [4] Using the Moon’s current orbit, find the Moon’s angular diameter in the sky (in degrees) at its closest
approach to Earth. This is its current largest angular diameter.
(b) [2] The smallest angular size of the Sun occurs at a distance of 1.515 × 108 km at the summer solstice. What
is the Sun’s angular size at this distance in degrees?
(c) [2] How far away does the Moon have to be to match the Sun’s smallest angular size?
(d) [2] If the Moon is moving away from Earth at a rate of 4 cm/y, how many years will it take to move to the
distance calculated in part c)? What is the year of the last total solar eclipse?2
2 NASA. (n.d.) Problem 5. In The Last Total Solar EclipseEver! Retrieved October 2, 2012, from http://spacemath.gsfc.nasa.gov/
weekly/4Page28.pdf
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