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Eastern Michigan University *
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105
Subject
Astronomy
Date
Dec 6, 2023
Type
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Equatorial Coordinate System (Basic Coordinates and Seasons)
Before we discuss how we locate and communicate the positions of objects in the sky, we will first look at how
we locate and communicate the positions of locations on the Earth. A solid understanding of the coordinate
system we use to identify locations on the Earth will make it easier to understand the coordinate system we use
to identify locations in the skies. The
geographic coordinate system (GCS)
coordinates of
latitude
and
longitude
are used to locate the position of an object on the Earth (neglecting elevation).
Lines of latitude are great circles that run parallel to Earth’s equator along the surface of the Earth. Latitude is
measured from the Earth’s equator, halfway between the geographic north and south pole, where it has a value
of 0°. Latitude increases in either direction with limits of 90°N at the geographic north pole and 90°S at the
geographic south pole. A capital N indicates the location is north of the equator, and a capital S indicates the
location is south of the equator.
Lines of longitude run along the Earth’s surface and connect the geographic north pole and south pole while
intersecting the equator at right angles. Longitude is measured from the prime meridian, which passes through
the Royal observatory in Greenwich, England, where it has a value of 0°. The limit of the longitudinal coordinate
is . A capital E is added to the latitude coordinate to indicate the line is east of the prime meridian, and a capital
W is added to the latitude coordinate to indicate the line is west of the prime meridian.
Using just these two coordinates, we can locate any position on Earth. The coordinate system rotates with the
Earth so that the coordinates of any location on the Earth are the same, no matter when or where the
measurement is made.
1. What is the name of the coordinate system we use to identify a specific location on the Earth?
-The geographic coordinate system (GCS)
2. What are the names of the coordinates in the coordinate system from number 1?
-Longitude and Latitude
3. Do the GCS coordinates of locations on the Earth change as the Earth rotates?
-
No
Now we will take what we learned about GCS coordinates and try to relate it to the
Equatorial Coordinate
System
, the preferred system used by astronomers to communicate the apparent positions of celestial objects.
The image above shows the
celestial sphere
surrounding the Earth. The celestial sphere is a projection of the
sky onto a perfect sphere surrounding the Earth. The yellow line on the Earth represents Earth’s equator. If we
extend the equator radially outward from the Earth onto the celestial sphere, that is the location of the
celestial equator
, shown as a yellow dotted line on the celestial sphere. If we take the straight line connecting
the Earth’s north pole and south pole (in other words, Earth’s rotational axis), and extend that line to the
celestial sphere, that is the location of the
north celestial pole (NCP)
and
south celestial pole (SCP)
. With this
information, we can now discuss the equatorial coordinate system. The coordinates of the equatorial system
are
declination
and
right ascension
.
Lines of declination on the celestial sphere are similar to lines of latitude on the Earth and are shown in the
image above in yellow. Like lines of latitude are measured from Earth’s equator ), declination is measured from
the celestial equator, where the value of declination is . Declination increases north of the celestial equator with
a maximum value of
at the NCP. Declination decreases south of the celestial equator with a minimum value of
at the SCP. A positive declination indicates the object is north of the celestial equator and negative declination
indicates the object is south of the celestial equator.
Lines of right ascension are similar to lines of longitude on the Earth and are shown in the image above in red.
Lines of right ascension run along the celestial sphere and connect the NCP and SCP while intersecting the
celestial equator at right angles. Right ascension is measured from the vernal equinox (the location of the sun
signaling the start of spring) and has a value of 0 hours (0 h). Right ascension increases eastward, toward the
summer solstice, and has a maximum value of 24 h. Right ascension is typically recorded in hours (h), minutes
(m), and seconds (s), with each hour representing
of sky from the Vernal Equinox, each minute representing
1/60 of an hour , and each second representing 1/60 of a minute .
Using just these two coordinates, we can locate any position of any object in the sky. Since objects outside of
our solar system are really far away, they don’t appear to move (much) over the course of our lifetime. The
coordinate system rotates from our perspective on Earth so that the coordinates of objects in the sky will have
the same value no matter where an observer on the Earth is observing from. Objects within our solar system
will have different coordinates depending on the moment the measurement is made.
Open
https://stellarium-web.org
in your internet browser. You can use the current time and date settings
(even if the time isn’t exactly right). In the left panel, click ‘View Settings’, then click ‘Ecliptic Line’ and close
the window. In the bottom panel, click the triangle (Constellations), the cloud and sun (Atmosphere), and
Equatorial Grid.
4. Which direction do you need to face to observe the NCP?
-north
5. Search for
Polaris
, the ‘north star’. Is Polaris located directly on the NCP?
-Polaris is not located directly on the NCP, but when it is observed from our perspective, it appears that
everything around it, rotates around Polaris.
6. Make sure you are zoomed out enough that you can see the landscape. Hit the ‘X’ to close the information
box about Polaris, then advance time for part of a day. In which direction (clockwise or counterclockwise) do
the stars move around the NCP? Does the equatorial coordinate system move along with the background stars?
-The stars do move around the NCP, and the ECS does move along with the stars except for Polaris which is from
an observing point, the NCP
7. Search for
Betelgeuse
, a red giant in Orion. Advance time by years; you can advance time by a few hundred
years if you would like. Does Betelgeuse appear to move (more than a very small amount) relative to the
equatorial coordinate system? Repeat for
Polaris
and
Vega.
Generally speaking, what does this suggest for the
coordinates of objects that do not appear to move as we observe them in the sky?
-None of these giants move at all in large amounts. This lets us know that these large objects that do not
appear to move have stand still coordiates meaning they do not change
8. Click the
ground and trees icon (Landscape)
in the bottom toolbar.
Use the search bar at the top of the
screen to find the
sun
. Advance time a few days. Does the sun move relative to the equatorial coordinate
system?
Repeat for the
moon
and
your favorite planet
. Generally speaking, what does this suggest for the
coordinates of objects that appear to move as we observe them in the sky over time?
-The sun and moon do move which tells us that their coordinates are changing over time.
9.
Turn back on the Landscape option and set the date to 02/15/2023 at 22:00:00.
This is a date from earlier in
the year that will allow you to see the constellation Orion in the night sky around wintertime. As you orient
yourself with the night sky, you can use Orion to help locate the celestial equator, which will pass through the
eastern and western points on your horizon. Search for the star Mintaka (so that we can focus on a point close
to the celestial equator) and look in the information window to see the declination is very close to zero. Click on
an empty part of the screen so the screen unlocks from Mintaka. Advance the time by hours for 24 hours. Does
the celestial equator’s height above the horizon change as you advance time?
-
The height does not change. It revolves and stay at the same height the entire time.
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10. The celestial equator is the blue gridline (labeled 0 degrees) that goes through the eastern and western
points along the horizon; you can find it by locating the gridline that emerges from either the eastern point or
the western point along the horizon. Choose any star located to the north of the celestial equator, and click on
the star so its information window pops up. Does it have a positive declination, or a negative declination? Now
choose a star located to the south of the celestial equator. Does this star have a positive declination, or a
negative declination? Test this pattern with two more stars on both sides of the celestial equator. What does
this mean, in general, about the declinations of stars on either side of the celestial equator?
-The star Alnath has a positive declination on the north side.
-the star mirzam has a negative declination on the south side
-this shows that north is positive and south is negative, it is how far they are from the equator.
There is a lot of information in this activity! The most important observations you made about the equatorial
coordinate system are those for questions 6, 7 and 8.