Declination is measured in the same way we measure the Earth's latitude and ranges between -90 and +90 degrees measured from the celestial equator. One complete circle is 24 hours rather than 360 degrees. Right ascension is measured in hours, minutes and seconds rather than degrees (as we do with the Earth's longitude).
Positions of celestial objects are described by their Right Ascension (the angle eastward from the vernal equinox) and Declination (the angle above or below the celestial equator). In celestial navigation, this point is called the vernal equinox which is the Spring time intersection of the celestial equator (the Earth’s equator projected outwards on the celestial sphere) and the ecliptic plane (the path the Earth takes around the Sun), a k a the summer solstice (if you are in the Northern Hemisphere). To make the celestial coordinate system work, there must be a line of zero latitude and zero longitude (like the Earth’s equator and Greenwich Meridian). This is similar to how we measure locations on the Earth's surface.Ĭoordinates on Earth are measured in latitude, from 0° at the equator to 90° north and south at the poles, and longitude, running from pole to pole and measured at the Greenwich meridian to 180° east and west. So, if we imagine that all celestial objects are on the inside surface of a sphere, we only need two measurements to locate an objects position - a latitude and longitude. A sphere is a three-dimensional object, but its surface is two-dimensional. The easiest way to do this is to assume that all objects in space are at the same distance, on the inside of an imaginary celestial sphere. Although celestial objects (the sun, the moon, planets, stars, etc) are at a wide range of distances from us, to locate them in space, we only need to know their direction.
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