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 Intramural Fields, there is no chance of seeing either one. 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 one of the two planets (you need to do EITHER Uranus OR Neptune – no need to do both, unless you really want to), 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 (or all your maps, if you choose to observe the planet(s) on more than two occasions) indicating which objects are the background stars and which object is the planet in question and that’s all you have to do to finish this lab.

The principal problem will be to find Uranus or Neptune. Because these objects move, you will have to find where their position is 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