Today's lab exercise should be carried out in pairs, ideally, but it's okay to do it by yourself or in a group of three.
In order to gain credit for this exercise, you must create a PDF document which provides the answers to all the questions. Submit the PDF to the instructor via the "Assignments" tab in myCourses.
The goal of today's exercise is for you to become familiar with the "Stellarium" program, which allows you to view the sky at any date and time (and from any location). A planetarium is very, very useful for creating a plan for observing some particular object.
Start "Stellarium" on your computer. When it starts, it should show the sky for today's date, at the current time, as seen from Rochester. If it does not, please call your instructor over.
Click on the Sun; a bunch of information about the selected object should appear in the upper-left portion of the screen.
RA = 20:04:01, Dec = -20:23:03
alt = 20:03:56, az = 211:59:00
Now use the clock icon (in the pull-out menu at lower left) to change the time. Set the time to 4 PM = 16:00.
RA = 20:04:18, Dec = -20:22:15
alt = 09:10:44, az = 230:50:02
The Sun moves across the local sky, from sunrise to sunset, by about 180 degrees in a single day. That's very rapid motion in altitude and azimuth, from the point of view of a person standing outside.Due to the orbit of the Earth around the Sun, the Sun appears to drift across the entire sky, relative to the stars, by 360 degrees over the course of one year. That means its RA and Dec change by only about one degree each day.
Simply changing the time can be very useful.
About 17:04 = 5:04 PM.
Okay, let's move on to the sky at night. You should notice that the display changes from bright to dark after the Sun sets. Change the time to 8 PM = 20:00.
Jupiter
Many of the Stellarium buttons in the pull-out menus on the lower-left side and lower-left bottom of the screen are toggles: clicking on the button once activates some action, and clicking on it again de-activates the action.
Play with the constellation lines and constellation labels buttons until you figure out what they do.
Auriga
You can use the "Page Up" and "Page Down" keys on the keyboard to zoom into and out of the view of the sky. Try zooming at as far as you can; you probably won't see the entire night sky. But if you left-click-and-hold on the "S" at the southern horizon then drag down, you should be able to see the entire night sky as a circle. When you can see the entire night sky as a small circle, call the instructor over to show him.
Cygnus is to the Northwest, Ursa Major to the Northeast.
Make sure you can see the entire circle of the night sky, and then play with the "equatorial grid" and "azimuthal grid" buttons on the pull-out menu at bottom left.
The equatorial grid is centered on the North Celestial Pole, where Dec = +90, close to the North Star. The azimuthal grid, on the other hand, is centered on the zenith, the location on the sky which is directly overhead.
What happens to the positions of stars during the night? They ought to rise in the east and set in the west, right? Well, let's see. Make sure you can see the entire circle of the night sky and then modify the time from 8 PM = 20:00 to 4 AM = 04:00 the next morning.
Most stars move from East to West. The stars close to the North Celestial Pole, however, as they rotate around it, appear to move from West to East as they move "under the Pole".One could say that all the stars rotate around the North Celestial Pole in the counter-clockwise direction.
If you are interested in a particular object, left-click on it to select it, then press the spacebar to center it on the display. You can then zoom in and out to suit your needs. Let's try this with Jupiter.
Set the date to today, and the time to 8 PM = 20:00. Toggle the equatorial grid on, and the azimuthal grid off. Then select Jupiter and zoom in until you can see its large, bright moons. There are four of them: Io, Europa, Ganymede, and Callisto.
All four of them: Ganymede, Europa, Io, Callisto, from farthest to closest to Jupiter.
About three arcminutes to the northeast of Jupiter.
The view of the sky tilts back and forth between two orientations.
The RA/Dec grid is veritcal and horizontal when one chooses the equatorial view, in which the little telescope icon is lit up. The RA/Dec coordinate system is also called the "equatorial" system.
One of the tasks for which Stellarium is most useful is planning for future observing runs. It can help you to figure out when your object is visible in the sky, and, even more important, when it is "well-placed" for measurements. As we will discuss later, the Earth's atmosphere absorbs and scatters light rays, especially when the object is only a small distance above the horizon. As a general rule, it's best to observe objects only when they are at least 30 degrees above the horizon.
The very best time to observe a star, of course, is when it is highest in the sky. We call that moment "culmination," or "crossing the meridian."
Pretend that your target is the variable star V347 Aur. What is the observing window for this star? That is, when should you start and end your measurements? Of course, it's not possible to see stars at all, even with a telescope, during the day. Sometimes, your observing window is set by the times of dusk or dawn ...
The Sun sets around 17:04, but the sky doesn't grow fully dark until about 18:00. The target is more than 30 degrees above the horizon at all these times.
V347 Aur reaches 30 degrees above the northwestern horizon around 3:30 AM the next morning. So, one might observe the target for about nine or nine-and-half hours.
On the night of March 1, the sky becomes dark around 19:00 (7 PM), at which time V347 Aur is nearly overhead. The target reaches 30 degrees above the horizon at about 00:45 after midnight. On this night, the observing window is only about six hours at most.