Astronomy Lab

1. Astronomy Lab Problem: How can you calculate the mass of Uranus using 3-5 of its moons?

2. General information • The planet Uranus is the seventh most distant from the sun and was discovered by William Herschel in 1781. It is a gas giant with a diameter of 50,000km. Nevertheless, its mean distance from the sun of 19.2 astronomical units meant that very little was known about the planet until the Voyager 2 fly-by in January of 1986. Up until then the planet was thought to have only 5 moons. You are given a picture of the moons of Uranus to use to calculate the mass of Uranus.

3. Mathematics Behind the calculation.

4. The formula you will be using is :R3=GM P24Π2 Where R=radius from the planet using a2 +b2=c2 a is the distance on the x-axis and b is the distance on the y-axis. P is the rotational period. You are given how many degrees the moon travels per second and need to find how many seconds in 360o. G is a constant. 6.67x 1011 M or mass is going to be in kg X1025

5. Helpful Hints and How to do one calculation. Measure the distance from the center of Uranus to a moon in the North or south direction using it orbital path. For my example, I got 8.4cm . Measure the distance from the center of Uranus to a moon in the east or west direction using its orbital path. For my example I have 5.9cm. R therefore = 10.26 cm Let say the moon moves at 2.23 degrees per second. How many seconds does it take to get around Uranus? P therefore = 161.4349776 sec

6. Plug the numbers into the formula: R3=GM P2 4Π2 7.21E14m3 = 6.67E11m3/s2 x MkgE25 2.61 E4 s2 4Π2 (2.76E10 m3/s2 )(39.48)= 6.67E11m3/s2 xMkgE25 MkgE25 =(1.090719348E12 m3/s2 )/ (6.67E11 m3/s2 ) M= 1.64 E25 (Now do this for four moon. Get an average and then check to see how close you are to the actual mass of Uranus. Do % error only if you are off by magnitude of 10.