3.3 Equatorial celestial coordinates – right ascension and declination

While the Alt-Az system of coordinates has certain benefits and uses, it also has the disadvantage that the coordinates of a given object are not fixed – they change with time as the Earth rotates and will also be different for observers in different locations. This is because the Alt-Az grid is fixed relative to the ground at a particular location.

To describe the position of an object uniquely we need a system of coordinates that is fixed relative to the objects in the sky, rather than to a point on the Earth’s surface. This can be done using a system of celestial coordinates known as the Equatorial system. This system uses coordinates called right ascension and declination (often abbreviated to RA and Dec).

To find out about this system, let’s go back to Alan in Tenerife…

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For more detailed planning, it's helpful to have a celestial coordinate system. And in astronomy, we use coordinates of right ascension and declination. These work in a similar way to latitude and longitude for defining positions on the surface of the Earth, with the added twist caused by the rotation of the Earth.

So to find a way around, we start again at the north celestial pole. And this is marked by the pole star, Polaris, which is nearby. And here in Teneriffe, we're 28 degrees north. So the north celestial pole in the sky here is 28 degrees above the horizon.

Coming down from the celestial pole 90 degrees, we have the celestial equator. And that sits out in space directly above the Earth's equator. To specify the position of an object north or south of the celestial equator, we use degrees of declination. And these work in exactly the same way as degrees of latitude. So an object that's north of the celestial equator will have positive degrees of declination. An object south of the equator of the coordinates will be negative degrees of declination. So for example, an object that's on the equator, such as the Belt of Orion, would be 0 degrees of declination. An object that's up at the pole would be 90 degrees of declination.

To specify the east and west position relative to the celestial equator is a little bit more tricky because of the rotation of the Earth. So to sort this out, let's remember that the Earth rotates once every 24 hours. So 360 degrees in 24 hours is 15 degrees every hour. So the sky will turn 15 degrees every hour. So we can mark out the celestial equator into 24 equal zones of 15 degrees each. And we call these hours of right ascension. And it makes sense to do it that way, because this helps us with the timing of our objects.

So, two objects that are the same declination and 15 degrees apart east to west, their right ascension coordinates will differ by exactly one hour. And that means that as the Earth turns, those two objects will be in the same position in the sky exactly one hour apart. So these celestial coordinates can also be used for controlling the telescope. So you can programme the telescope with the right ascension and declination coordinate for a given object. And the telescope will then go and track that object.

And in catalogues of celestial objects, such as the Messier objects, you'll always find the right ascension and declination coordinates listed alongside each particular object. So, as you become more familiar with this system, you'll be able to use these in planning your observations with Coast. And you'll also have a much better feel for how objects move across the night sky during the course of an evening and with the seasons.

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One way to visualise this system is to imagine the Earth at the centre of a great sphere on which all of the objects in the night sky and the RA – Dec grid system are drawn. This is known as the celestial sphere. In other words, rather than having a grid fixed relative to the observer, it is fixed relative to the objects in the sky and thus moves with them.

Figure 11 The celestial sphere. The RA and Dec coordinates specify the unique position of each celestial object. Importantly, these coordinates are defined relative to the sky and not the surface of the Earth, and so do not change as the Earth rotates.

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The celestial sphere. The RA and Dec coordinates specify the unique position of each celestial object. Importantly, these coordinates are defined relative to the sky and not the surface of the Earth, and so do not change as the Earth rotates.

Figure 11 The celestial sphere. The RA and Dec coordinates specify the unique position of each celestial object. Importantly, these ...

This system has the tremendous advantage of describing the location of an object in the sky to another observer in a way that will never change regardless of when or where they are looking at the time or location of observation. As before, you can explore this using Stellarium.

In completing Activity 6 you will have found that the coordinates of your chosen celestial object do not change. The coordinate grid moves with the object, meaning that each object in the night sky has its own unique position described by the RA and Dec coordinates that is independent of where on the Earth or at what time it is observed.

This equatorial grid system based on the concept of a celestial sphere is therefore the one most commonly used in astronomy and is used on star charts and in astronomical catalogues, where each object will be listed with its own unique RA and Dec coordinates. This is also the coordinate system used to control telescopes such as COAST.

One minor exception to be aware of is that while the positions of stars and other distant objects such as nebulae and galaxies are fixed in the equatorial system, the coordinates of objects within our own solar system (such as the Moon, planets, comets and asteroids) will change as they orbit around the Earth or the Sun.