The Equatorial Coordinate System

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  • 0:02 Pointing Out the…
  • 0:37 The Equatorial System
  • 3:53 Lesson Summary
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Instructor: Artem Cheprasov

Artem has a doctor of veterinary medicine degree.

This lesson will teach you about the fundamentals of the equatorial system, including declination, right ascension, the vernal equinox, angular diameter, and celestial equator.

Pointing Out the Location of Something

How do you point out the location of something on Earth? Maybe you point, and just say to someone: 'Look, it's over there!' Perhaps you pull out a map and point out the location that way. Or maybe you give the coordinates of a certain location so the other person can find it themselves.

I'm sure you could think of other ways of finding or pointing to something here on Earth. Like on Earth, there is more than one way to locate or point to an object in the sky. We can use our finger, we can use the horizon system (discussed in another lesson), or we can use the equatorial system, which is this lesson's topic.

The Equatorial System

The equatorial system is a coordinate system that is used to locate a body in the sky using declination and right ascension. To help you understand what that definition entails, I must ensure you know a few other things first.

If you go outside tonight, and hopefully it's a clear night, look at a star in the sky. Then, find another star somewhere next to it. The angle formed between two lines extending from your eyes, one to each of the stars, is known as the angular distance. From your perspective, the stars and moon in the night sky seem to be tacked on to one half of a sphere. That sphere, an imaginary sphere that surrounds Earth, is known as the celestial sphere.

Like the Earth, it has an equator separating it into upper and lower halves. It is known as the celestial equator. Like the terrestrial coordinate system is based on the Earth's equator, the equatorial system is based on the celestial equator.

Declination and Right Ascension

Now you're ready to learn about the coordinates used in the equatorial system. They are not latitude and longitude, but they're sort of similar. Recall that the equatorial system is a coordinate system that is used to locate a body in the sky using declination and right ascension.

Declination is the angular distance of an object in the sky north or south of the celestial equator. This is like latitude here on Earth, a north-south coordinate. It's basically saying that the celestial object you're looking at, like a star, is so many degrees and minutes or seconds of arc up or down from the celestial equator. Remember, d in declination is measured in d degrees. Northerly declinations are considered positive, and southerly ones are negative.

Right ascension, on the other hand, is the angular distance measured starting at the vernal equinox, and from there, eastwards along the celestial equator to a point on the celestial equator underneath the celestial object's position. It is like longitude in the terrestrial coordinate system. The vernal equinox mentioned in that definition is the sun's position in the sky on the first day of spring.

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