Name: ______
Date: ______
LocatingObjects in the Night Sky: Celestial Coordinates, Time, and Angles
Introductory Astronomy Lab with Stellarium
Looking at stars is fun and romantic, but rigorous study and charting requires a set of good coordinates. (You need to install Stellariumsoftware provided by if not already done so. You should have already completed the starter exercise)
Objective: We will learn about the two celestial coordinates used to locate and mark objects in the night sky. One is theAzimuthal Coordinateand the other is the Equatorial Coordinate. We will explore how to find objects in the night sky, telling time in a couple of different ways, and measuring angles.
Estimated Time: 90 minutes
Current Location: Germantown, Maryland (or Washington, DC)
Current Time: 9:00 pm, Today – Pay attention to 24-hour setting in Stellarium
I.Coordinates: Measuring Distances and the Horizon Grid
Any local observer wishing to chart stars will naturally set up a sky coordinate grid using his or her own location. Then motions of the sky objects then can be tracked in this coordinate.
The Horizontal Coordinate is a coordinate grid, using two angles (altitude and azimuth), of the sky based on “observer’s view”. Naturally the ‘horizon’, the line between Earth (land or ocean) and the sky as seen by the observer locally, is set as 0˚Altitude, and this viewing altitude increases as the observer looks up until the zenith (directly overhead), set at 90˚ altitude. In going across in east-west direction, the northern line or direction is the default0˚Azimuth (angle), and the azimuthincreases eastward with East as 90˚, and West as 270˚ (or -90˚). In Stellarium, and as in many popular literature, the term “Azimuthal Grid” is used instead. The coordinate uses “degree-arcminute-arcsecond” as the measure of angles.
(See Wikipedia for more explanation:)
1. Launch Stellarium. It should already be in the horizontal coordinate view but without the grid lines. Position your mouse in the left part of the screen to bring up the lefticon bar. Find the “Location” option and set the location to the “Current Location” listed above.
Record the following information of the “Current Location”:
Latitude______
Longitude ______
You should already have this information from previous Stellarium starter exercise. You can double check with a quick online search.
2. In the same icon bar, find the “Date/Timewindow” and set the time to “Current Time” above.
3. Use the ‘Search Window” and find Polaris (the North Star). Position your mouse in the lower part of the screen to bring up the lower icon bar. Turn on “Azimuthal Grid”by pressing on a button at the lower icon bar. Its grid lines will appear. Determine howfar above the northern horizon Polaris is in degrees (˚) plusminutes of arc (’) and seconds of arc (’’) by manually zooming into the view to determine the position.
That is, what is the Altitude of Polaris?______.
Also, what is the Azimuth of Polaris?______
How close does the altitude of Polaris compare to the latitude of the “Current Location”? Is it not close, somewhat close, very close, exact, or completely opposite? ______.
4. You should see the famousasterism the “Big Dipper” in the northeastern sky. It is a group of 7 stars shaped like a ladle, or a large soupspoon. (Use the search function if needed). Turning on “Constellation lines” or “Constellation labels” can be helpful. Use Wikipedia or other online resources to help find the Constellation hosting the Big Dipper if needed; at least, you should know what it looks like.
Click each of the stars in the Big Dipper and record their names. ______,______, ______, ______, ______,______,______. Merak and Dubhe are known as the pointer stars because they point to the NorthStar in one direction and to the star Regulus in the constellation Leo in the opposite direction.
Make a general observation of sky objects to get a sense of their Azimuth and Latitude with respect to the grid, and use several stars as benchmark examples. You should be looking at “Horizon”, “Zenith”, any of four directional “cardinal points” in order to understand this simple grid system. Describe how Azimuth and Latitude increase or decrease with based on locations.
Describe: (One example and hint: Find a bright star below Polaris. How does its Azimuth/Latitude compare with Polaris? )
II.Coordinates: The Horizon Grid
1. Using the horizon grid, estimate the altitude and azimuth of Merak by manually zooming in.
Azimuth______; Altitude ______
a. Now you can check your estimate. Click on Merak so that it is highlighted. Look at the “Info” paneon the upper left side of the screen. Record the actual positions.
Azimuth______; Altitude ______
2.While looking north, adjust the screen so you can see Polaris a.k.a.North Star and the northern horizon. Set the time flow (the double-arrow button at the button) to simulate faster stars motion throughout the evening. You will see that for most locations in the United States, the Big Dipper will always remains above the horizon so that it is always visible from current location. Objects that are always visible from a location are referred to as “circumpolar.” Rewind and play forward several times to become familiar of the sky motions, in particular with respect to Polaris. Note that stars circle around Polaris. If you are in a very southern location such as Miami, USA, then The Big Dipper actually partly dips below the horizon – you can use “the Little Dipper” instead.
- For Polaris, how much does its Azimuth and Altitude change as the sky rotates? State some numerical values and be brief.
- Pause the time flow for a moment, and go find Merak or Dubhe again in the Big Dipper. Click on the star to show the info first then set the time flow again to the same rate as before. Follow the star and the encircled cross-mark for 4 particular positions at north, east, south and west of Polaris. See how its Azimuth/Altitude change.
What is the Azimuth of the star when it is north or south of Polaris? ______
What is the Altitude of the star when it is east or west of Polaris? ______
Look at the “grid lines” and the positions of stars carefully. Explain why the above 2 answers should be or should not be the same as that for Polaris.
- As the evening goes on, how fast are the grid lines of the “azimuth grids” changing? Give your simplest explanation why it should be this way.
- Given “Current Location”, objects out to what radial distance from Polaris will be circumpolar? Another word, within how many degrees of Polaris, or how close to Polaris in degrees, will a star always stay visible above horizon? ______(*Simply looks at the sky motions rotating night after night and see what stars always stay above horizon and never dip below. Just find one star that comes closet to the horizon without going below. All stars that stay above horizon and below Polaris will be Circumpolar, right? So what is that angular range, or angular distance, between Polaris and the horizon?)
(Bonus) Compare your answer with the latitude of “Current Location”. Why does this make sense?
- Now set your current location to Miami at 25.8˚ North latitude (or Portland, Maine at 43.7˚ N). What is the Altitude and Azimuth of Polaris from this new location?
Altitude:______Azimuth:______
Objects out to what radial distance fromPolaris will be circumpolar? ______
3. Reset the time to “Current Time” using the “Date/Time window” or simply press ‘8’ for actual current time. Also return to your “Current Location”. Click on the “Find” window in the pop up menus. Type in the star name “Arcturus” and press enter. What constellation is Arcturus in? ______. (TIP: again use some“constellation” options)
What kind of object is Arcturus? ______.
Determine its Altitude above the horizon. Alt ______.
4. From the Sky and Viewing options menu select Markings and turn on the local Meridian line. The night sky is generally divided into eastern and western hemispheres by the local meridian. From your (observer’s) view, what is the azimuth of the Meridian Line?
5. Observation: Pause the time (press the “triangle” play button on the bottom icon bar) to ensure that time-elapse simulation is turned off. Pick any bright object whose Azimuth and Latitude is clear to you at the current time.
Use the Date/Time Window, and simply jump 2 or more months ahead without changing the hour, the minute and the seconds. Again, ensure time-elapse is off. What happened to the location of the same object that you picked?
Your sky object: ______
Its Azimuth/Altitude (at Current Date &Time):______
Its Azimuth/Altitude (at Later Date & Same Time): ______
Carefully examine the difference at two different times many months apart (but at the same time of the evening). What has changed? Has the coordinate shifted, or it is the object that has shifted it position at this time of the evening at later months?
III.Coordinates: The Equatorial Grid (Right Ascension and Declination).
The Equatorial System is the default commonly accepted Celestial Coordinate used to describe the position of the celestial objects. It uses two angles,Right Ascension (RA), which is analogous to azimuth and,Declination (Dec), which is analogous to altitude. This is a fixed grid system in which the stars and other objects stay fixed on its grids. Naturally, as this grid system stays rigid against the sky then, then from our earthly view the Equatorial grid moves along with the sky motions. In addition, planets and various solar system objects will not have a fixed position in this system due to their association with Earth and the Solar System, or the center of this Celestial Sphere.
(The Celestial Coordinate System is explained in Wikipedia and other online sources
- Turn off the azimuthal grid and turn on the equatorial grid.
- Declination (Dec) is measured in degrees north or south of the celestial equator (similar to latitudes).
- Right ascension(RA)(similar to longitudes), however, is divided into “24 hours”and not in degrees (strange that we are using something similar to time, isn’t it?).
Keep the time flow and in fact speed up the time flow. How do the grid lines of the equatorial system change through out the evening?
Observe the grid lines and all the stars, constellations and various labels. Describe the relationship between the grid and (almost) all the sky objects.
(BONUS) Do the following bonus exercise if time permits:
- One (1) hour of right ascension is equal to how many (angular) degrees of arc?
**Hint: think of complete rotation in “360 degrees” or in “24 hours” of divisions.
b. One (1) minute of right ascension is equal to how many (angular) minutes of arc?
c. One (1) second of right ascension is equal to how many (angular) seconds of arc?
- Using the celestial grid for the star Altair:
Pause the time flow for a moment, find Altair and click on it to show its info. Turn ground on or off to locate Altair if the cross-mark does not appear above ground (horizon). The screen may do a funny warp, but you can manually adjust the screen again to a more comfortable view.
a. Estimate the RA and Dec of Altair from the celestial grid:
RA ______; Dec ______
(TIP: if you zoom in using the mouse scroll wheel or the + and – keys the celestial grid will increase in resolution.)
b. Click on Altair to bring up the information and record the exact coordinate:
RA______;Dec______.
c. Why is the RA and Dec of the object NOT changing (but that its Azimuth/Altitude are)? Think about what the “Equatorial Coordinate” is a coordinate of, and from whose point of view. Simply go back to the introduction in the previous page or
- From the Sky and Viewing options menu select Markings and turn on the ecliptic(should appear in red) and equator(should appear in green) if not already on. Find the location where the ecliptic crosses the celestial equator. What is the RA of the celestial equator, meaning what degree label shown in Stellarium marks the Equator line? What does this mean to you about the celestial equator?
- You may need to zoom in slightly to see the celestial equator. What are the RA and Dec of the point where the two reference lines cross?(See above for RA) It can be difficult to manuallypin-point the location so click around the point where two lines cross to get a feel of the coordinate.
RA______; Dec______
a. This is one of the “cardinal points” in Earth’s orbit. What is the significance of this point?
Do a search online for information.
b. (to be added for Spring, 2015 – ask something about seasons?)
5. The yearly motion of the Sun.
Increase the time flow and move forward until the Sun is well up into the sky. Notice that the Sun is on the ecliptic. (This part is to be improved!)
a. Stop the time flow by clicking the stop button.
b. Click on the Sun and bring it to the center of the screen.
c. Turn on the local meridian.
d. Set the time to 13:10 hours.
e. Move forward 1 day at a time by pressing the = sign. Simply observe the Sun as it changes positionrelative to the ecliptic and the meridian throughout the year. The Sun is always on the ecliptic because it is the Sun that defines the path of the ecliptic.
f.Cycle through a year by pressing and holding down the = sign key. Notice how the sun forms a figure eight about the meridian. This figure 8 is the “analemma.” (Similary the – sign key reverses the daily motion).
g. Provide a very brief explanation of the rise of the “analemma” shape. Do a quick online search.
V.Where is the Sun?
1. What is the best time to view the constellation Orion? Well, that would be when Orion in high in the nighttime sky. So when is that?
a. Find the R.A. of any star in the constellation Orion.
b. What should the R.A. of the Sun be to give you the best view of Orion?
(Should Sun be near or far away from Orion?)
- Now determine what month that will be using the explanation and the able below. (*So in approximately what month(s) will the Sun R.A be very different from the Orion RA?)
d. What is the RA of the Sun on the vernal equinox? ______
Note first that, unlike any star in Orion, the Sun R.A. will change through out the year. To determine its R.A., consider thatthere are 24 hours (divisions) of R.A. around the celestial grid. The Sun makes one trip around the celestial grid per year following the yearly Sun-Earth rotation cycle. The 0h 00m 00s R.A. point on the celestial grid is defined by the position of the Sun on the vernal equinox. Therefore since there are 24 hours (divisions) of motion in 12 months, then the R.A. of the Sun changes by 2 hours R.A. each month (on the celestial sphere).
Approximatedate / SunPosition (R.A.)March21 / 0h00m
June21 / 6h00m
Sept21 / 12h00m
Dec21 / 18h00m
V.Latitude change & Rise angle
(For FALL, 2015 Class, SKIP THIS PART! What is Rise Angle?)
1. Change your location to somewhere on the Equator. Exercise your cleverness by finding a Central American city, or something.
a. Look to the north. Where is Polaris (north star)?
b. Look to the East. Run the time forward at increased speed.Observe the stars rising in the eastanddetermine the angle the rising stars make with the horizon. Rise Angle =______.
2. Change your location to St. Louis, Missouri.
a. Look to the north. Find Polaris (north star) and determine its elevation above the horizon usingthe arc measuring tool. This will also be the latitude of St. Louis. Elevation/Lat
b. Look to the East. Run the time forward at increased speed.Observe the stars rising in the eastanddetermine the angle the rising stars make with the horizon. Rise Angle = _____ .
3. Change your location to the North Pole.
a. Where is Polaris (north star)?
b. Look at any horizon. Run the time forward at increased. What is the rising angle of stars if you are located at the North Pole. Hint: it may seem strange, but there is a number for it. Rise Angle = ______.
4. Look at how the rising angle changes with elevation/latitude, devise or guess a simple relation of the following form that describes your latitude as a function of the rising angle of stars. Use the form c – rising angle = latitude, where c is a constant. (i.e. figure out the value of c)
a. Write equation here ______
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