Stellarium - Mapping The Sky copy
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Name: Nina Mundy_____________________________
Date: 2/26/2024______________________________
Mapping the Sky
(
Stellarium
Exercise #4)
Ever tried to give someone directions to a friend's house without knowing the exact address? It’s hard! Just as buildings have addresses to help us find them, astronomers need ways to locate things in the sky. Ideally, there should be a precise way to map the sky, just as we have a map of the surface of the Earth. To locate any point on
the surface of the Earth, we specify the point’s LATITUDE
and LONGITUDE
, two angles that measure a spot's position relative to the center of the Earth. For example, Los Angeles (at least the center of it) is located at
34º North
, and 118º West
. Similarly, we need a way to tell other astronomers where to look for things in the sky. We can’t just say “Look to the left of that bright red star and then go up a little.” In this assignment, we’ll study the two ways of locating objects in the sky using angles – one called Altitude & Azimuth (Alt/Az)
, and the other called Right Ascension & Declination (RA/Dec)
- and their relative advantages & disadvantages.
PART A
Start Stellarium
. Make sure your location is Los Angeles
. If it is not, open the Location
window, enter
Los Angeles
in the search box and then click on Los Angeles, United States
, and then close the Location
Window. Turn off the Atmosphere and Fog (if they're on), by pressing the A
and F
keys. Put the horizon at the center of the screen by dragging it until it's at the center. It doesn't matter which part of the horizon – N, S, E or W, you choose. Pick a bright star that is as close to the horizon as possible. Click on it. Information on your star will appear in the upper left-hand corner
of the screen.
•
What is the common name of your star? Gru__________________________________
In the information, look for the line that starts Az/Alt
. Az/Alt
stands for Azimuth/Altitude
, and the numbers on this line list the angles that are the star's Azimuth and Altitude, as discussed in class. The first number listed is the Azimuth, and the second number, after the slash, is the Altitude. NOTE: If your version of Stellarium
lists TWO lines with Az/Alt information, use either one – the difference between the two is very small, and we don’t need to worry about what that difference is. •
What are the Azimuth and Altitude of your star? ROUND OFF ALL YOUR AZIMUTH AND ALTITUDE READINGS TO THE NEAREST DEGREE – IN OTHER WORDS, IGNORE EVERYTHING AFTER THE “°” SYMBOL!
Az 185__________ Alt 4____________
Try a few other stars along the horizon
. Click on them. Look at their altitudes. Compare the answers to what you saw for your first star. Do you notice a pattern?
•
What is the value of altitude for any star
on the horizon? 8________________°
Now find a star that is straight above one of the stars you just chose. In other words, look a little higher
in the sky. Click on the new star. Note its altitude and azimuth, and compare them to the values for the stars just
below them, on the horizon. One of the numbers will have changed, and the other will be (approximately) unchanged.
•
As you move the cursor straight up
from the horizon (in other words, as you look up from the horizon, toward the zenith), which angle changes, Altitude or Azimuth? Altitude
changes___________________________ Zoom out
and re-center the sky until you are looking at the entire sky at once. Click as close as you can
to the zenith (the point at the center of the screen). Remember, the zenith is the point directly over your head
in the sky.
•
What is the value of altitude at the zenith? 72_______________°
Altitude is an angle that tells you where a star is above the horizon. Picture two rays coming from your head – one pointing at the horizon directly below the star, and the other pointing at the star you're looking at. The angle between those rays is the altitude of a star.
Click the cursor on a star that is below
the horizon – in other words, click on the ground. Notice the value of the altitude here.
•
How can you tell by looking at a star’s altitude whether it is above or below the horizon?
You can tell that the star’s altitude will be greater than 0 if it is above the horizon. If the stars altitude is below 0, it is below the horizon.__________________________________________________________________________
Go back and click on a few more stars on the horizon
. •
As you click left or right along
the horizon
, which angle changes, Alt or Azm? Azimuth changes___________
Click around the entire horizon (you may have to move the horizon by grabbing it with the cursor and dragging it), and watch the Azimuth change. Drag the sky around so you're looking at the Northern
horizon. Click on various places on the horizon
just to the left and right of North
, and look at the values of altitude and azimuth for various points on the horizon.
•
What is the maximum value of azimuth? 357______________°
•
What is the azimuth of E? 89__________° W? 270___________° S? 180___________° NW? 314___________°
Azimuth measures the angle along
the horizon in degrees, to your chosen star, starting at North and going clockwise.
Find the star Polaris, using Stellarium's
Search
window (or by using CTRL-F
or the F3
key).
•
What are the azimuth and altitude of the star Polaris
? Az 359_____________° Alt 34_____________°
•
What is the latitude here in L.A., where you are observing from? (To find your latitude, open the Location
window – your latitude and longitude are listed there) 34_________________________°
•
Comparing the answers to the previous two questions, what might you guess is the relationship between an observer’s latitude
and the altitude
of Polaris above his or her horizon? The observers latitude may be the same as the Polaris at any given location._______________________
Stellarium
can superimpose a grid on the sky that allows you to easily measure Altitude and Azimuth. To show this grid, press the “
Z
” button on your keyboard (or go to the toolbar at the bottom of the screen and click on Azimuthal Grid)
. The lines in the grid that run parallel to the horizon
are lines of altitude, measuring from the horizon to the zenith, and the lines that are perpendicular to the horizon, and which all meet at the zenith like the spokes of a wheel, are lines of azimuth, measuring along
the horizon, from North. The grid lines are labeled, in degrees where they hit the edge of the screen.
Set the time for 8 PM
tonight by changing the time in the
Date/Time window. Label the brightest stars by opening the View
window and putting a check next to Stars
in the Labels and Markers
section of the Sky
sub-menu. Also, move the slider next to Stars
to about half way from the left. Close the View
window. It's in the end of the bowl of the Big Dipper in the constellation Ursa Major
. Look for Ursa Major
, and then click on
the end star
in the bowl
of the Big Dipper to make sure you've found Dubhe. Don't forget, if you want to see the constellation outlines and names on the screen, press C
and then V
. •
What is Dubhe’s Az and Alt at 8 PM (
Remember, if you click on a star, its data will be shown onscreen!)
? Az _21_______________° Alt _11__________________°
Change your location to New York City
(
Lat=41º N, Lon=74º W
) in the Location
window by entering New York
in the search box and then scrolling down until you find New York, United States
in the resulting list
and clicking on it.
•
Find Dubhe's azimuth and altitude in New York AT THE SAME TIME AS THE PREVIOUS QUESTION. Az 37____________° Alt 41______________°
•
Are Dubhe's Altitude and Azimuth the same in New York and L.A. at the same time? Why or why not? The Dubhe’s Alt and Az are different in LA than in NYC. This is because these are two separate locations on opposite ends of a large country.___________________________________________________________________________
Change your location back to Los Angeles
, and the date back to today's date, at 9
PM
.
•
Back in L.A., what is Dubhe’s Azimuth and Altitude at 9 PM
? Az 29__________° Alt 21___________°
•
Do a star's azimuth and altitude stay constant as the night goes on? The Alt increases as the night goes on.____________________
•
What is Polaris' altitude and azimuth at 8 PM? Az 0_________° Alt 34_________° •
What about at 9 PM? Az 359_________° Alt 34_________°
•
Why does Dubhe behave differently from Polaris? Polaris’ Az has a major increase from 8PM to 9PM, unlike Dubhe._____________________________________________________________________________
______
•
If someone told you they saw a satellite at 85
degrees altitude, where would you look (circle one)?
•
Near the horizon
•
Near the Zenith
•
Halfway up the sky
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•
None of the above
•
If you wanted to tell someone to look at a star halfway up the sky in the Southwest, what
Alt
and Az
would you give? Az 235_______________° Alt 42________________________°
PART B
As we have seen, altitude and azimuth have one large drawback - they are observer dependent
– they change, depending on both where
the observer is, and what time
he or she is observing. We need a set of coordinates that
are observer in
dependent, that never change, like Latitude and Longitude on the Earth! The coordinates we have developed that fit the bill are called Right Ascension
and Declination
, together known as the Equatorial System
.
As we have discussed in class, Right Ascension and Declination (
RA
and Dec
), are the two numbers we use to specify a star’s location, using a grid fixed on the sky
. To display that grid, first turn off
the Azimuthal
grid by pressing the Z
key, and turn on
the Equatorial grid by pressing the E
key. You can also turn these grids on and off using their buttons in the toolbar
at the bottom of the screen. Drag the sky around and zoom out until
the Southern horizon (the letter “
S
”) is at the bottom of the screen and you can see the whole sky at once.
You should see a different grid pasted over the sky – this one in blue. The lines that shoot out like curved spokes from the North Celestial Pole
are lines of Right Ascension
. The circular lines perpendicular to these Right Ascension lines are lines of Declination
. Again, the lines are labeled with their units at the edge of the screen. These lines are the grid that we use to measure fixed address locations in the sky. The Right Ascension and Declination of any star's position are listed in the information that appears when you click on that star.
Click on any star. The information about that star appears in the upper left-hand corner of the screen. Look for the line that reads RA/DE (J2000)
. The two sets of numbers separated by a slash are the star's Right Ascension and Declination. Let's look at Declination
first. •
What units
is declination measured in? degrees________________________________________
Press the “.” (the period)
key on your keyboard. This will highlight the Celestial Equator
in brighter blue. Pick a star lying right along the Celestial Equator. Click on it. Look at the star's Declination. Do this for a few more stars along the Celestial Equator.
•
What is the Declination anywhere
on the Celestial Equator? 0_______________________°
Make sure you're looking at the whole sky by zooming out
and centering the sky on the screen. Click on various stars and watch the information that appears for them. Notice that the Right Ascension (RA) and Declination (Dec) change in various places in the sky. Try to find the place where Declination reaches its maximum value.
•
What is the maximum value of declination? 77______________________________________
•
What is the name of the bright star almost exactly at the point where declination reaches its maximum value? Errai (Alarm)________________________________
•
What angle does declination measure?
38___________________________________________________ ___________________________________________________________________________________
Now let’s look at the Right Ascension
lines. Again, these are the lines that circle the celestial sphere but ALL RUN THROUGH THE NORTH AND SOUTH CELESTIAL POLES. Unlike Declination, which is measured in degrees (and fractions of a degree called arc-minutes and arc-seconds),
Right Ascension is measured in a different unit. Click on a few stars and look at their Right Ascension values.
•
What are the units RA is measured in? Hint: look at the letters that appear in the RA scale hours, minutes and seconds__________
Click on various stars along the Celestial Equator (the brighter blue line) and note their Right Ascension.
Do you see a pattern? Keep clicking on stars and try to find the maximum
value of RA. You may have to let some time pass to click all the way along the entire Celestial Equator.
•
What is the maximum value of RA (before it starts over again at zero)? 11h55m__________________________
•
What is the exact RA of Polaris? (Click on Polaris and look at its displayed information) 2h31m________
Center the whole sky on the screen, and click on a star as close as possible to the zenith to estimate the RA and Dec coordinates of the zenith itself. Don't forget to write the units for each measurement below!
•
Zenith RA 10hr23m35.2s_______________ Dec 88___________________
Now change the time to 1 hour
later. Again, click on a star as close as possible to the zenith to estimate the new RA and Dec coordinates of the zenith.
•
Zenith RA 16h07m11s_______________ Dec 87____________________
What would you guess the RA and Dec coordinates of the zenith would be six
more
hours later? •
Zenith RA 23h19m35.83s_______________ Dec 11_____________________
The answers to the previous questions should help you understand why RA is measured in hours (and minutes and seconds). Each hour that passes causes one hour of Right Ascension to pass by any point in the sky.
Right Ascension and Declination, or the Equatorial system is a much more useful way to locate stars and objects in the sky than Altitude and Azimuth. To see why in another way, turn both grids on at once
by pressing Z
and E
until you see both grids at the same time. Press
L
three
times to let time go by at 1000
times normal speed and watch the stars move across the sky.
•
Do the stars stay fixed relative to the Alt/Az grid as time passes? Yes they do_________________________
•
If we use Altitude & Azimuth to measure a star's position, would the star's Altitude and Azimuth stay the same as the night went on? No____________________________________
•
Do the stars stay fixed relative to the blue Equatorial grid as time passes? Yes they do_______________________
•
If we use Right Ascension and Declination to measure a star's position, would the star's RA and Dec stay the same as the night went on? No_________________________________
Now you can see why we use Right Ascension and Declination as our “address” in the sky. RA and Dec do not change, regardless of where the observer is on Earth, or what time of day or night it is, or what day of the year it is.
PART D
We can now give every spot in the sky a unique address that won't ever change, using Right Ascension and Declination. This applies not just to stars, but to “Deep Sky” objects, like nebulae, galaxies, and star clusters. Practice looking up some of these addresses by filling in the Deep Sky Address Table
on the next page. Find each object by either opening the Search
window from the menu, or by typing CTRL-F
or F3
. Once the object is selected and in the center of the screen (you may have to hide the ground by pressing “
G
” to see some of the objects), then zoom in until you can see each of the objects. The object's information, including its RA
and Dec
address, will be displayed in the upper left-hand corner of the screen. Just list the degrees for Declination, and the hours and minutes for Right Ascension – don't bother with the fractions of a degree or the fractions of a minute. Also add two other Deep Sky Objects of your own by choosing two other NGC objects by number from the list in the Search Box
.
Deep Sky Address Table
Deep Sky object name
RA Dec
Deep Sky Object name
RA
Dec M31 (the Andromeda Galaxy) RA: 0h42m Dec:41
Pleiades - RA: 3H46M Dec:24
Crab Nebula- RA: 5h34m Dec: 22
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Ring Nebula- RA:18h53m Dec:33
Lagoon Nebula- RA: 18h03m Dec:24
Sombrero Galaxy- RA: 12h40m Dec: -11
NGC 5139 (Omega Centauri)- RA:13h26 m Dec: -47
Triangulum Galaxy- RA: 1h33m Dec: 30
M42 (Orion Nebula)- RA: 5h35m Dec:-5
Whirlpool Galaxy- RA:13h29m Dec:47
Write a brief conclusion explaining what the advantages and disadvantages are of using Altitude & Azimuth or Right Ascension & Declination, and what you learned about them in this exercise.
Azimuth and Altitude are great for located an object in the sky precisely. I have learned that these locations are not fixed. Right Ascension and Declination are great for finding an objects coordinates in the sky independently from the observer.___________________________________________________________________________________
__________________________________________________________________________________________
__________________________________________________________________________________________
__________________________________________________________________________________________
__________________________________________________________________________________________
__________________________________________________________________________________________
_______
__________________________________________________________________________________________
__________________________________________________________________________________________
__________________________________________________________________________________________
__________________________________________________________________________________________