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The Celestial Sphere - Right Ascension and Declination

A look at the celestial sphere and how we locate objects using Right Ascension and Declination

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The Celestial Sphere - Right Ascension and Declination
The most fundamental property that any astronomer needs to know is our place in the heavens and the position of astronomical objects that lie on the surface of the celestial sphere. We will look at the Celestial Sphere, the ecliptic and how to locate objects using Right Ascension and Declination.

The celestial sphere is a projected sphere centred on the Earth that we can imagine all the stars are painted on. The sphere appears to rotate from East to West every twenty-four hours, so celestial bodies appear to rise in the East and set in the West.

The Celestial Sphere
The Celestial Sphere

The Celestial Equator is an imaginary line around the middle of the Celestial Sphere, equidistant from the North Celestial Pole (NCP) and South Celestial Pole (SCP) and on the same plane as the Earth's equator. It intersects the Circle of the Horizon at East and due West. Think of it as an imaginary projection of the Earth's equator onto the sky.

The Ecliptic is an imaginary circle which all the other planets appear to follow during their orbits around the Sun over a year.

The Circle of the Horizon surrounds the Earth-bound observer. The horizon circle is divided into 360 degrees, with 90° between each of the four cardinal directions of North, South, East, and West.

The Zenith is the point on the Celestial Sphere directly above the observer.

The Meridian is an imaginary circle passing through the Zenith and NCP and SCP and is always perpendicular to the horizon.

Right Ascension and Declination

In order to identify the position of an object on the celestial sphere, we need a coordinate system that can cope with the position of an object on the sphere at a given time from a given location. Imagine that you are stood on the North Pole and when you look directly up you will see the Moon. Now imagine another observer stood on the Equator, where would they see the Moon? It certainly won't be overhead; instead, it will be on the horizon. In the same manner, the position of an object will appear to change if one observer was in Europe and another in America, however, over time the object will move into the position observed as the Earth rotates.

For these reasons, the celestial coordinate system is based on the latitude and longitude system used on the surface of Earth. The coordinate system consists of two figures, Right Ascension (RA) and Declination (Dec). Right Ascension is a measure of the position of an object from the First Point of Aries and can be thought of as the celestial sphere equivalent of longitude, while Declination is a measure of the position relative to the celestial equator and is similar to latitude. The celestial equator is a projection of the Earth's equator onto the celestial sphere.

In the diagram above, declination is marked on the blue lines and is analogous to latitude as it measures the angular distance in degrees from the equator, from 0° at the equator to +90° at the North Pole and -90° at the South Pole.

First Point of Aries and Right Ascension

Right Ascension is analogous longitude in that it measures the angular distance around the equator, however, this is where the comparison ends. The First Point of Aries acts as the zero point for the Right Ascension scale instead of starting from the Greenwich meridian like longitude does. Essentially the zero point of Right Ascension is the position of the Sun at the vernal equinox.

As the Earth orbits the Sun, we see that the Sun's position changes with respect to the background star over the period of a year. If you were able to the position of the Sun compared with fixed background stars, the Sun would appear to move in a large ellipse on the surface of the celestial sphere. This ellipse is imaginatively called the elliptic. As the Sun moves along the ecliptic, it crosses the celestial equator twice per year - in March we know it as the Vernal Equinox and in September the Autumnal Equinox. The First Point of Aries is the point in the sky where the Celestial Meridian, the Celestial Equator and the Ecliptic all meet. It is presently in the south-west of Pisces, moving slowly towards Aquarius.

First Point of Aries
First Point of Aries

When this point was first conceived the point was located in the constellation Aries, hence its name, however over time this position has moved westwards due to the precession of the equinoxes.

Right Ascension is measured in hours, minutes and seconds from the local sidereal time.

Circumpolar Stars

While some stars and celestial bodies appear to rise and set, some celestial bodies and constellations appear never to set. These are called Circumpolar. All circumpolar stars are within the circumpolar circle which was the original meaning of "Arctic Circle", before the current geographical meaning, meaning "Circle of the Bears", The circle consists of Ursa Major, the Great Bear; and Ursa Minor, the Little Bear.

Whether a star is circumpolar depends upon the observer's latitude, since the altitude of the NCP or SCP (whichever is visible) is the same as the absolute value of the observer's latitude, any star whose angular distance from the visible celestial pole is less than the absolute latitude will be circumpolar. For example, if the observer has latitude +50°, any star will be circumpolar if it is less than 50° from the NCP. If the observer's latitude is −35°, then all stars within 35° of the SCP will be circumpolar.

Last updated: 2017-07-22

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