Equatorial Coordinate System (Basic Coordinates and Seasons)

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Eastern Michigan University *

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Astronomy

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Feb 20, 2024

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docx

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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

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