ASTR1010L & ASTR1020L

THE MOTION OF URANUS OR NEPTUNE

LAB # 7

The planets Uranus and Neptune are two ice giants that are located in the outer Solar System (at 19 and 30 AU from the Sun, respectively). Although both planets are about 5 times the size of the Earth, they are far enough away that they are not visible to the naked eye from the Earth. Perhaps Uranus might be visible to someone with perfect eyesight who knows exactly where to look on a perfect, moonless night, but, from the roof of the Physics Building, there is no chance of seeing either one without binoculars or a telescope. Uranus is typically at a shade less than 6^th magnitude and Neptune is close to 8^th magnitude. The object of this lab is not so much to locate these worlds and observe them through the telescope (though that's pretty interesting since they are blue-green in color), but to notice that they are moving slowly with respect to the background stars. So, to do this lab, you must find the brighter and thus the easier-to-find of the two planets, Uranus, and make a hand-drawn map of its location with respect to the background stars. Then, a week or two later, you again find the planet and make another map of it and any stars you find in the field of view of the telescope (N.B.: Use the lowest power eyepiece to get the largest field of view! Try to use a 40 mm or larger focal length eyepiece if one is available); if you've made your map carefully enough, you should notice that the planet has moved with respect to the background stars. Turn in both maps indicating which objects are the background stars and which object is Uranus and that's all you have to do to finish this lab. One way of determing that you really have found Uranus is that the planet has a distinct light-blue hue and, if you swith to the highest magnification eyepiece, you should see its size increasing. In other words, at high magnification you should see or begin to see a disc rather than a point of light.

Because the planet moves with respect to the background stars, you will have to find its position at the time you make your observations. You can obtain the position in RA and Dec for any planet from:

http://ssd.jpl.nasa.gov/?horizons

just click on the “web-interface” link, change the target body (follow the directions), and then hit the “generate ephemeris” button. You will then see a table with the RA and Dec of the planet for 0:00 hours Universal Time (don't worry about that for now) for a series of dates. Write down the RA and Dec for the dates you are interested in, and then locate the position of the planet on the appropriate chart in your star atlas. This is where you will look for the planet on the night in question. You can get a graphical display of the position of the planet from websites such as:

http://www.aaa.org/aaawhatsup.htm