Equatorial Coordinate System (Basic Coordinates and Seasons)
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Feb 20, 2024
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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 180
∘
. 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?
2. What are the names of the coordinates in the coordinate system from number 1?
3. Do the GCS coordinates of locations on the Earth change as the Earth rotates?
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 0
∘
. Declination increases north of the celestial equator with a maximum value of 90
∘
at the NCP. Declination decreases south of the celestial equator with a minimum value of −
90
∘
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 15
∘
of sky from the Vernal Equinox, each minute representing 1/60 of an hour (
0.25
∘
)
, and each second representing 1/60 of a minute 0.004
∘
¿
. 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? 5. Search for Polaris
, the ‘north star’. Is Polaris located directly on the NCP?
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? 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?
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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? 9. Turn back on the Landscape option and set the date to the current date and time during our class (1/22/2024 at 8:00pm or so). You can 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, because this star sits very
close to the celestial equator, so that you can easily find it (or imagine it, if you see it outside!) in the night sky. 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, or does its height stay the same?
10. The celestial equator (from the previous question) is the blue gridline that goes through the eastern and western points along the horizon and has a declination of zero degrees. Choose any star located to the north of the celestial equator, and click on the star so its information window appears. 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?
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.