Lab 2 celestial coordinates
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California State University, Fullerton *
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Course
295
Subject
Astronomy
Date
Dec 6, 2023
Type
docx
Pages
6
Uploaded by CorporalTank11733
Celestial Coordinates
Student name:
Ma. Cristina Nicole Mangente
Objective
To learn about the altazimuth and equatorial coordinate systems
To describe the positions of celestial objects in the sky, astronomers commonly used two
different coordinate system. Both systems based on the convenient fiction that the Earth is the
center of the universe and that's all celestial objects are attached to an infinitely large sphere that
is centered on the Earth. This is sphere is called the
celestial sphere
, is Illustrated below.
Figure 2.1 Celestial Sphere
You are in the center of the figures. The Earth, although not infinite in size, blocks your view of
the bottom half of the celestial sphere. The shaded, circular area is the boundary between the
sphere’s top half, which you can see, and its bottom half.
We pretend that the daily rising and setting of the stars and other objects is due to the rotation of
this sphere about the axis that connects its north and south
celestial poles
, labeled N.C.P and
S.C.P. One rotation takes about 23 hours and 56 minutes, a period called one
sidereal day
(meaning star day). The circle midway between the two poles called the
celestial equator
.
To locate objects, astronomers put additional markings on this sphere. We will learn about them
in the sections below.
1
Altazimuth Coordinates
System
One way to describe an object location on the celestial sphere is by specifying its altitude and
Azimuth angles. These coordinates are based on your
horizon
, the large imaginary circle around
you where the sky meets the ground.
Azimuth
(abbreviated
az
) is the direction of a celestial object measured clockwise around the
observer horizon from north. So an object due north has an azimuth of 0º, one due east 90 º,
south 180 º, and west 270 º.
Altitude
(abbreviated
alt
) is the angle you then have to look up vertically from the horizon in
order to look straight at the object. An object exactly on the horizon has an altitude of 0 º, while
an object directly above you has an altitude of 90 º. The overhead point is called the
zenith
.
Objective below the horizon have a negative altitudes.
Figure 2.2 Altazimuth coordinates
The semicircle in the sky that runs from due north on your horizon, up through your zenith, and
back down to due south is your local
celestial meridian
. When an object on the rotating celestial
sphere crosses your local Celestial Meridian it's said to
transit
. At that time it also attains its
maximum altitude angle.
Equatorial Coordinate System
This is the preferred coordinate system to pinpoint objects on the
celestial sphere
. Unlike
the
Altazimuth coordinate system
, equatorial coordinates are independent of the observer’s
location and the time of the observation. This means that only one set of coordinates is required
for each object, and that these same coordinates can be used by observers in different locations
and at different times.
2
The
equatorial coordinate system
is basically the projection of the latitude and longitude
coordinate system we use here on Earth, onto the celestial sphere. By direct analogy, lines of
latitude become lines of
declination
(
Dec
; measured in degrees, arcminutes and
arcseconds
) and
indicate how far north or south of the celestial
equator
(defined by projecting the Earth’s equator
onto the celestial sphere) the object lies. Lines of longitude have their equivalent in lines of
right
ascension
(
RA
), but whereas longitude is measured in degrees, minutes and seconds east the
Greenwich
meridian
, RA is measured in hours, minutes and seconds east from where the
celestial equator intersects the
ecliptic
(the
vernal equinox
)
Figure 2.3
Equatorial Coordinate System
Comparing the Coordinate Systems
Go to the date and time window on the vertical menu or press F5. Set the time to 21 and
keep the date for today.
Increase the time speed by pressing “L” or pressing twice on the fast forward icon on the
horizontal menu bar.
Observe the behavior of the stars and planets for. Where do stars rise from and where
they set?
Stop the animation by pressing on the play icon on the horizontal menu bar.
Go back to the date and time window on the vertical menu or press F5. Set the time to 21.
Select one of the bright stars by clicking on it. Star data information will appear and press
on the center icon
on the horizontal menu bar.
Increase the time speed by pressing “L” or pressing twice on the fast forward icon on the
horizontal menu bar.
Observe the RA/DEC of your star while it is in animation.
Observe the Az/Alt of your star while it is in animation.
3
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The Rising, Transiting, and Setting Stars
You will now study the behavior of stars during the course of a night, and observe the influence
of Earth’s atmosphere on rising and setting times.
Go to the date and time window on the vertical menu or press F5.
Date: this year, 15
th
day of March (Spring semester) or October (Fall semester)
Time: 21 (Spring semester) or 10:30 for (Fall semester)
You should be able to see a bright star (Sirius) (in March) or Capella (in October) toward
NE.
Click on this star to get its information
Result 1:
Record the rise, transit and set time of your star in the table in result 1.
Reset the time to your star’s rise time. Click on the search icon in the vertical menu and type in
the name of your star. Record the requested information in the top line of the table 1.
Reset the time to your star’s transit time. Click on the search icon in the vertical menu and type
in the name of your star. Record the requested information in the middle row of the table 1.
Reset the time to your star’s set time. Click on the search icon in the vertical menu and type in
the name of your star. Record the requested information in the bottom row of the table 1.
Table 1
Time
RA
Dec
Alt
Az
Rise
14h41m
6h45m08.37s
-16°43’39.4
-0°01’17.5”
+109°51’55.2
Transit
19h57m
6h45m08.38s
-16°43’39.4”
+39°30’19.7”
+180°05’08.0
Set
1h12m
6h45m08.38s
-16°45’00.6”
+0°42’25.7”
+249°31’30.0
Result 2:
Use the information from table 1 to compute how long your star was visible in the sky
that night.
20h01m
Use the words “rising”, “transiting”, and “setting” to answer these questions:
Your star’s altitude is highest when the star is transitinng after it rises and before it sets
Its altitude is lowest when the stars is barely rising
and finishing to set.
Do the altitude and azimuth of your star change with time? Describe and explain the
behavior trends of each coordinate from rising to setting.
4
Yes, the altitude and azimuth change with time, when rising the alt and Az are
lower and when setting it is higher.
Do the right ascension and declination of your star and others change over the course of
the day? Describe and explain the behavior trends of each coordinate from rising to
setting.
Yes, but only barely. When rising the dec. is lower and when setting the dec. is higher.
Result 3: The effects of observer’s location
Are the altitude and azimuth of a star different for observers at different locations at the
same time? Then explain why (in a sentence or two)
Yes, a star’s altitude and azimuth are different for observers in different locations because
stars change their positioning with the horizon. The angle you are facing the stars shifts
depending on where you at.
Are the right ascension and declination of a star different for observers at different
locations at the same time? Then explain why (in a sentence or two)
No, right ascension and declination are the same for everyone viewing the star around the
Earth because these are used to measure time instead of degrees.
5
Result 4
Polaris exhibits some special behaviors. To see this.
Adjust the time to 23
Search for Polaris using the search icon in the vertical menu, CENTER Polaris.
Record the equatorial and altazimuth coordinates in the following table
Time
RA
Dec
Alt
Az
6
2h30m41.40s
+89°16’04.7”
+33°13’16.4”
+0°20’56.6”
15
2h30m40.93s
+89°16’04.6”
+34°23’53.2”
+0°13’54.2”
23
2h30m40s
+89°16’04.5”
+33°38’40.1”
+359°15’07.9
”
Do Polaris equatorial and altazimuth coordinates change significantly (much more than a
degree) during a night? Explain why or why not for both types.
Click on the location panel. Change the latitude to the latitudes listed in the table below.
Search for Polaris using the search icon in the vertical menu, CENTER Polaris and record
Polaris altitude.
Observer Latitude
Polaris Attitude
0
+33°56’20.9”
30
+33°56’20.9”
60
+33°56’20.9”
75
+33°56’20.9”
Based on your table above, what is approximately true about the altitude of Polaris and
the latitude of the observer?
Nothing changed but the observer latitude.
6
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