Astronomy)314)–)Homework)#1)(due)Feb)2)@)9:35)AM...
Transcript of Astronomy)314)–)Homework)#1)(due)Feb)2)@)9:35)AM...
Astronomy 314 – Homework #1 (due Feb 2 @ 9:35 AM)
Show your work. On homework and exams your answers must show the steps toward the solution in order to receive the full score. Answers given with little justification will receive much lower scores. Solutions with some correct steps will receive partial credit. Part I. Carroll and Ostlie, problems 1.5, 2.6, 2.8, and 2.12. Part II. How many times stronger/weaker is the Sun’s mean gravitational force on our Moon compared to the Earth’s mean gravitational force on our Moon? (You do not need to know Newton’s gravitational constant, G, or the mass of the Moon to get the answer.) Part III. Consider a spherical asteroid of radius r. A tiny pebble orbits the asteroid just above its surface with period P. Using Kepler’s Third Law derive an expression for the mean density of the asteroid <ρ> as a function of P. A bucket of dirt and rocks from your garden has a mean density of about 2 times that of water. If we fashion an asteroid out of such material, what would be the orbital period (in hours) of a tiny pebble that orbits just above its surface? Note that the period is the same whether the asteroid is 1 meter in size or 10 km. Part IV. In the fall/winter of 2014/2015 Jupiter has been visible before sunrise in the direction of the constellation Leo. Over the course of this semester Leo will be seen earlier and earlier in the night. Kevin Krisciunas has been making some measurements of Jupiter’s position using a simple hand held cross staff:
Given the primitive nature of this instrument, the positional measurements are no more accurate than about ¼ deg. Here we plot the derived positions of Jupiter from October 18, 2015, to January 4, 2016.
Table I – positional data for Jupiter
JD-‐2457000 RA DEC λ β λSun days deg deg deg deg deg
In Table I, column 1 is the Julian Date minus 2,457,000. Columns 2 and 3 are the right ascension and declination of Jupiter. Columns 4 and 5 are the ecliptic longitude and latitude of Jupiter. The final column gives the ecliptic longitude of the Sun, as derived from chapter 18 of Astronomical Formulae for Calculators, 4th edition, by Jean Meeus, Richmond, Virginia: Willman-‐Bell, 1988. Make a plot of λ vs. JD, such as that shown below. Fit a 2nd order curve to the data: λ = A + B X + C X2, where X = the Julian Date starting with the hundreds digit, or the tens digit, or even subtracting off the JD of the 6th point. What is the root-‐mean-‐square scatter of the data points? Approximately on what date will Jupiter start exhibiting retrograde motion? In other words, when will Jupiter stop moving east against the stars and start moving west against the stars? (January 1 at 6 AM is the same as Julian Date 2,457,389.0000.)
We know the size of the Astronomical Unit (1.496 X 108 km), the number of seconds in a year, and therefore the mean orbital speed of the Earth, 29.786 km/sec. We also know that the circular velocity of an orbiting body decreases proportional to 1/sqrt(r). So for a planet with a circular orbit at rpl Astronomical Units from the Sun, its orbital velocity is 29.786/sqrt(rpl) km/sec. From the slope of the second order fit of λ vs. JD it is possible to estimate the size of the orbit of Jupiter compared to that of the Earth. What value of dλ/dt do you get at the time of the 6th data point, when the Sun’s ecliptic longitude was 237.812 deg? Assuming that both Jupiter and the Earth have perfectly circular orbits, what is the implied size of Jupiter’s orbit in Astronomical Units? The diagram shown on the next page will be helpful.