Night Sky

Introduction

This study unit will help you explore the appearance of the Moon and bright planets. We will use Stellarium, desktop sky simulation software. While this experiment may be done on any of the computers in the lab, its is best to do it in the immersive visualization room where the night sky fills your field of view. Let one of the members of your group drive the software, while the others interact and guide the work through the following exploration.

Software and Websites

Stellarium is installed on all the lab computers, and is also available for download for Windows, Mac OSX, and Linux operating systems by clicking on this link:

http://www.stellarium.org/ Stellarium - A free planetarium for your computer

We have installed it on the lab computers. If you prefer to use your laptop, or follow up later at home, the installation is fast and simple. Follow the instructions that are provided on the site. This is beautiful "award winning" software useful throughout the semester, and worth the effort to have available if it will run on your own computer.

Other web sites that are useful for this activity are

http://www.heavens-above.com/ Heaven's Above - The sky tonight on the web

http://www.iau.org/public_press/themes/constellations/ Constellations - Maps of the constellations

http://www.google.com/sky/Google Sky - An interactive map of the sky, the Moon, and Mars

Getting Started

First, there are a few things you need to know about using Stellarium. You might look at the Stellarium Tour on their website for an overview and reference to these options:

Screen mode

Stellarium starts in full screen mode and will cover everything. You can change this by pressing the F11 key to fit it into a smaller window.

Menu bars

There are two bars of menus, one at the bottom of the screen, and one on the left. They are hidden until you run the mouse down to the bottom or over to the left.

Quitting

When you or done, you can use the bottom menu bar to exit by selecting the off button.

Setting your location

By default you will be in Paris, France. If you press F6 the location menu will pop up and you can select your city or put in longitude and latitude. Once you have done this and saved the configuration, Stellarium will come up at your chosen site. The location menu is also in the left menubar under the compass icon at the top left. When you have selected your location the menu will show a map of Earth with an arrow pointing to your site.

Setting the time

Stellarium starts with the sky over your site at this very moment. The date and time will show at the lower right, based on the computer's clock. This time is the local time at your computer. If you change your location, the time shown will still be the time at your computer, not the time at the new site. For example, if you are in Baltimore, Maryland and you set up Stellarium for that site, then the computer is in the Eastern U.S. time zone. When you run Stellarium at 3 PM it will show the afternoon daytime sky with the Sun. Should you use the location menu to change to Rome, Italy, the sky will go dark because it's nighttime there. The clock will still show 3 PM because that's the time where you are. You can change the date and time in two ways. One is with the time menu selected from the left menu bar Clock icon. The other is with the two arrow icons at the right of the bottom menu bar. These speed forward-reverse, real time rate, and now buttons let you speed up the daily motions of the sky, and then slow them down again when you have the events in view you want to see.

Looking around

Change the direction you are looking by holding down the left mouse button and dragging your direction of view, or by using the updown leftright arrow keys on the keyboard.

Identifying planets, stars, and constellations

The lower menu offers options to add labels. By default the planets will be named, and you can turn this off using the Planets labels icon that looks like Saturn. You can outline the bright stars of the constellations, add constellation names, and even overlay mythological figures to help you see the patterns by clicking on the various buttons in this menu. There are two celestial grids offered too that show the equatorial celestial coordinates of stars (right ascension and declination), and the local sky coordinates (altitude and azimuth). A menu on the left for

Sky and viewing options

allows you to change the constellations, names and associated cultural folklore. Click that option, select

Starlore and Western

to see the the typical sky labels of American and European culture, or change to one of the others offered to see the diversity of named patterns in the sky. The same menu under

Markings

lets you select whether you would also like to see the constellation boundaries as red dotted lines. Sometimes it's helpful to see these in order to identify the constellation in which a particular object is located.

Identifying an object

Move the mouse over the object and click with the left button to have its identification displayed. Click with the right button to make this go away. Click with the center button (press down on the mouse wheel) to have the display center on this object after you have selected it.

Zooming in and out

The Page Up and Page Down keys on the keyboard zoom in and out of the sky. You can move around the zoomed in sky with the arrow keys or dragging with the left mouse button. The status display at the bottom of the view tells you the field of view

FOV

in degrees among other things. For some objects there is a photo that appears when the sky view is zoomed in close. When an object is selected and centered in the view, after zooming in the view will stay centered on it even as the day progresses. Think of it as a telescope that is pointing at your target, and tracking the target as the Earth rotates.

Finding Planets

The Moon and planets will be labeled by default. You can turn these names off with the P or by clicking on the Planet icon on the lower menu bar. If you select a planet with a left mouse click, the sky view will lockto that planet and you can follow it during the night. Planets with satellites like Jupiter will show the satellites as they really would appear in a telescope, and in motion in their orbits around the planet.

You can find a planet or other objects by using the

Search window

The left menu has a Magnifying glass icon that brings up this option. So does F3.

Moon

Let's begin by watching the Moon to discover its daily, monthly, and annual cycles. Set your location in the software, and check that sunset occurs at about the right time.

Start on the first day of this month and advance the date one day at a time.

1. When does full Moon occur this month? Advance the day again, and find the next full Moon, and perhaps the one after that ...

2. How many days passed from one full Moon to another?

Notice the appearance of the Moon changes in the sky during the month , and that it rises at different times night after night.

3. From one night to the next, how many minutes later does the Moon rise?

As days pass, the Moon changes its phase, first from a thin crescent, then to first quarter, full, last quarter and new.

4. On what day and time this month does the first quarter moon set and the last quarter moon rise?

If you have time and want to look into this further, notice how the point on the horizon where the Moon rises changes during the month, and from month-to-month through the year. When the lab classes go to the planetarium will we study this under a simulated sky on the dome.

Planets

There are always several bright planets in the sky, some visible at sunset, others at sunrise. To see how this works, begin by setting the date and time to tonight at sunset and advance the time until the sky is dark enough to see bright stars and planets. Look for any of the 5 bright planets (Mercury, Venus, Mars, Jupiter, and Saturn) that may be visible.

5. Which planets are in the sky at sunset tonight?

Now advance the sky slowly and watch stars and planets set in the west, and new ones rise in the east. For each of the bright planets, find where they are, when they set if they are up at sunset, or when they rise if they are not:

6. For each planet (Mercury, Venus, Mars, Jupiter, and Saturn), what did you find?

The time that this planet rises will change as the weeks go by. Follow these into next season and find approximately on what day the outer planets, Mars, Jupiter, and Saturn, will be rising at sunset during the coming year. At our point of closest approach to a planet farther from the Sun than we are, the planet is said to be in opposition to the Sun.

In fall 2013 Saturn is on the far side of the Sun from us and is low in the western sky at sunset. Eventually Earth's orbit will carry us around until Saturn is opposite the Sun and high in our sky at midnight. See if you can find when this will happen. The behavior is similar for Jupiter, but it is moving in its orbit faster than Saturn so the oppositions of Jupiter occur less often.

7. What are the next dates of opposition for Jupiter and Saturn?

If you advance the date only one day at a time you can see how these planets change position in the sky as they appear to drift past the more distant stars. Since Earth and the planet are both moving, this apparent position depends on Earth's orbit too. If it were only the motion of the planet, it would drift past stars from west to east, in what we call "prograde" motion. At opposition, however, Earth overtakes the planet, and its motion is in the opposite or "retrograde" sense. Such retrograde motion begins before opposition and ends after it, causing the planet to appear to loop in the sky. The effect is most obvious for Mars, so find Mars in the sky now, and let the date go forward until Mars rises at sunset, in opposition to the Sun.

8. What constellation is Mars in when it is next in opposition?

Follow its motion with respect to stars from the months before opposition and notice roughly what day it starts retrograde motion. Retrograde continues for many days and then it the motion returns to the normal "prograde" pattern. Note the dates on which these changes in direction occur.

9. What were the dates on which retrograde motion started and then ended? For how many days was it moving backwards?

While the outer planets may be 180 degrees from the Sun, the inner planets (Mercury and Venus) are always close to the Sun and therefore visible only soon after sunset or before sunrise.

10. When is Venus highest in the sky at sunset this year? In this configuration it is often called the "evening" star.

After you find that date, advance the date and find about when Venus disappears into the glare of the Sun as its orbit takes it overtakes Earth with its faster orbit around the Sun. It will then reappear before dawn as a "morning" star.

11. When is Venus next highest in the sky at sunrise?

Bright Stars and Constellations

Set Stellarium for today's date and run the time to 1 hour after sunset tonight. Face south and slowly go up to look overhead and then around to the different compass directions. You can turn the constellation guide on and off, and identify individual star with the help of the cursor.

12. What is the brightest star visible in your sky tonight? Use the cursor to find its name and magnitude. Remember that magnitude is a measure of the energy we receive from the star. Magnitude 0.0 is very bright, magnitude 5.0 just barely visible from your backyard.

13. What constellation was that star in?

We'll do the same thing now, but from Sydney, Australia. Change your location in Stellarium, and change the time so that it is night in Australia, about an hour after sunset. The sky looks completely different because "up" in the sky here was under your feet when you were in the northern hemisphere.

14. What is the brightest star visible in this southern hemisphere sky tonight?

15. What constellation was that star in?