This study unit will help you explore the appearance of the Moon, planets, bright stars and constellations in the night sky from a chosen point on the Earth at any time. It uses software enabling a virtual planetarium on the desktop.
Software and Websites
Stellarium is 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
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:
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.
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.
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.
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
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
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.
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
The left menu has a Magnifying glass icon that brings up this option. So does F3.
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.
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.
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:
5. 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.
6. 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. This 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.
7. 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 when does it start retrograde motion. This will continue for a while and then it will return to its normal "prograde" motion. Note when that happens.
8. 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.
9. 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.
10. When is Venus next highest in the sky at sunrise?
Now advance the date only one day at a time and notice how the planet changes its position in the sky. It will seem to drift past the more distant stars. This retrograde miton is reversed from the direction it followed a few months before or after opposition.
Set the time to 7 P.M. on January 15, when the sky is dark and stars are clear and bright. If you are in the northern hemisphere make sure your location is set properly, turn your attention toward the southeast and look for the brightest stars. (If you are in the southern hemisphere, for the following pick a favorite spot a little north of the equator so that you can see stars you are familiar with, as well as the stars of the northern sky.
Sirius, the brightest star in the sky will be rising at this time. Left click on it and you'll see information about it displayed:
- Names: Sirius (alpha Canis Majoris) HIP 32349
Sirius is its common name. Alpha Canis Majoris means that it is the brightest star in Canis Major, the big dog. HIP 32349 means that it is the 32,349th star of the Hipparcos catalog.
- Magnitude: -1.45 (B-V: 0.00)
That's how bright it is on the astronomer's scale. Smaller numbers are brightest, larger ones fainter. Magnitude 6.0 is about as a faint a star as you can see without a telescope.
The magnitude scale is logarithmic, and a difference of 5 on this scale corresponds to a factor of 100x in energy received each second from the star. The B-V is a measure of color, and the 0.0 for Sirius means that it looks white to the eye. A positive number is red, and a negative number is blue.
- Distance: 8.60 light years
Sirius is 8.6 light years from the Sun. The light you see in the sky from Sirius tonight left it 8.6 years ago, when you were 8.6 years younger. Light leaving the star today will arrive in 8.6 years when you are 8.6 years older.
Find these other stars: Procyon, Rigel, Betelgeuse and Aldebaran. For each one, identify what constellation is it in, and how bright it is. Notice the pattern of bright stars. These are the one's you'll see first in an urban sky. Advance the date to March 15 and set the time to about 2 hours after sunset.
7. Where are Sirius, Procyon, Rigel, Betelgeuse and Aldebaran in the sky now?
Advance the date again to June 15 and you'll see Deneb, Vega, and Altair rising in the east. Arcturus will be nearly overhead if you are in a northern latitude. For each of these stars identify what constellation is it in, and how bright it is.
8. Which one of these stars is orange, that is a cool star of type K or M?
Go forward again to the fall, and set the date to September 15. Look directly overhead if you are at northern latitude. If you are elsewhere, to answer this question set your location to a northern latitude of around 40 degrees. Three of the stars that you have identified form a distinctive pattern overhead called the Summer Triangle.
Turn off the equatorial grid if you still have it on, and turn on the constellation names and outlines (use the lower menu or press C and V). Let's find some of the easily identified constellations in the northern sky for each season.
- Set the date for January 15 at 10 P.M.
- Orion with very bright stars Betelgeuse and Rigel od in the sky at this time.
- Set the date for March 15 at 10 P.M.
- Gemini, the twins , with the pair of similar stars Castor and Pollux comes in to view.
- Set the date for June 15 at 10 P.M.
- Leo, the lion, with the bright star Regulus is visible.
- Set the date for September 15 at 10 P.M.
- You will see Sagittarius, the archer , that looks like a teapot.
- Set the date for November 15 at 10 P.M.
9. Where is Pegasus, the horse, at this time?
Looking from the South
The following is a guided tour of the sky from the southern hemisphere to see how the sky would change if we moved our point of view to a southern latitude. From the north pole of the Earth you can see only the half of the sky that is north of the Earth's equator. Your horizon is the celestial equator. More of the southern sky becomes visible as you move southward. From the Earth's equator, the north pole of the sky is on the northern horizon and the south pole is on the southern horizon. Over 24 hours the entire sky will pass into and out of view.
Imagine now going south from your current location. The farther south you go, the more of the southern sky will be overhead. Let's illustrate this.
Set the date for January 15 at 10 PM by using the date and time menu "F5". Direct your view to the south and bring up the location menu with "F6". Use the map of Earth and click on a location that's south of yours, looking for a place that is at the same latitude and about 30 to 40 degrees below the equator. The sky will update each time you click a new location on Earth. Once you have a suitable location you can close the location menu.
- Look for a familiar constellation that you have already identified from the northern location nearly overhead now.
- There is a very bright star Canopus, south of Sirius, that you could not see from a northern location, nearly as bright as Sirius, in the constellation of Carina.
- The bright star Achernar is not far away, a little southwest of the star you found in the question above. Set the field of view so that you can see a field of 70 to 90 degrees with Achernar on right. (If you're lost, try using the search option and type in the name to have the software center up on the object.)
- There are two fuzzy clouds in the center of the field of view now. These are the "Magellanic Clouds", companion galaxies to our own Milky Way. If you need help in identifying them, the search term "SMC" will center the Small Magellanic Cloud. Zoom in and you'll find an object that is not a star, and is close to the "SMC". Use the search term "47 Tuc" to center on it.
- 47 Tuc is a giant globular star cluster. There's information about it in Stellarium, and you can also Google the term and find out more.
Set the date and time to June 15. If you have chosen a location about 30 to 40 degrees south of the equator from your own longitude, then the time at this site will be the same as your local time. Try 10 PM to get the sky a couple of hours after sunset.
High in this southern hemisphere sky you will now see a distinctive pattern of bright stars that outlines the "Southern Cross". It will be above the south celestial pole, while the Magellanic Clouds at this time will be below the pole.
- You can identify this pattern easily with the constellation outlines (C) and names (V) on. Look for "Crux".
See if you can find the names of two bright stars in Crux?
- Just east of Crux at this time of night is the constellation Centaurus. Look for the bright star Rigel Kentaurus. This star goes by another name, because it is brightest star in Centaurus. It's also one of the closest stars to our Sun.
10. What is the other name for this star? How far is it from the Sun?