Of the two coordinates, declination is the easier to understand, and is equivalent to latitude on the Earth. The celestrial sphere with the declination axis indicate.Declination is similar to Latitude and objects to the North of the celestrial equater have a positive declination, those to the South are given a negative value. We call this path the ecliptic, and if we plot the motions of the Moon and planets as well, we find that they never stray too far from the ecliptic.įigure 2. We can plot the path of the Sun as it passes through the constellations on the celestial sphere. Conversely in December it resides in the constellation of Scorpius and is very low down. So in July (from the UK perspective) the Sun is located high in the sky in the constellation of Gemini. We call this imaginary globe the celestial sphere and we can divide it into two parts: the north celestial sphere above the celestial equator and the southern celestial sphere below it (see Figure 1).įrom this point of view, the Sun appears to move through the constellations of the zodiac over the course of a year, spending on average about one month in each constellation. All the objects in the sky (including the Sun and the Moon) sit on this globe, which rotates around the Earth once every 24 hours. We shall imagine that the entire sky is a vast globe surrounding the Earth. The Sun’s path across the sky, the ecliptic, is shown.In order to understand how RA and Dec actually work, we shall need to refer to a model which, unfortunately, has no physical bearing in reality, but is nonetheless a very useful tool. The entire sky visualised as a globe surrounding the Earth which astronomers call the celestrial sphere. Instead of star hopping to find that elusive galaxy, you will be able to dial up its coordinates and find it in no time at all!įigure 1. The declination circle runs from 0 degrees to 90 degrees both to the left of the 0 degree mark and to the right of it.Įverything in the sky, from the Sun to the faintest galaxy is assigned an RA and Dec coordinate, and once you understand how this system works, you will be able to use the setting circles on your telescope to track down objects. It is often further subdivided into 15 minute intervals. If you have a telescope equipped with setting circles, you will see that the RA circle is similar to a clock in that it runs from 1 hour all the way through to 24 hours (although this is usually denoted 0h on most RA circles). This celestial coordinate system of the sky is easy to understand and is rather similar to latitude and longitude used for locating landmarks here on Earth. Astronomers have devised such a system which is based on two quantities called Right Ascension (RA) and Declination (Dec). Similarly, if a new asteroid or nova is spotted, we are going to need a precise coordinate system to pinpoint its location so we can alert other astronomers who will want find it quickly – a description ‘somewhere in Leo’ isn’t really going to help! What we need is a universal coordinate system that we can all use and understand. Other objects like planets or comets for example, move rapidly through the sky, and keeping track of them is important if we wish to establish their orbits properly and observe them frequently. The bright stars of the night sky are easy to find, and from a human perspective they appear to reside permanently in constellations which remain unchanged century after century. It is alternatively defined as the ascension (point on the celestial equator which rises with a specified celestial object) as seen from any point on the Earth's equator, where the celestial equator intersects the horizon at a right angle.2016 March 22 Right ascention and declinationĪs you’ve probably realised, there are a fantastic number of objects out there beyond the atmosphere of Earth. H = LST − α, where LST is the local sidereal time.Īlong with declination, right ascension determines the direction of a point on the celestial sphere in the equatorial coordinate system. The hour-angle, H, and the right ascension, α, are related by the simple formula 1988, Peter Duffett-Smith, Practical Astronomy with Your Calculator, page 35,. in cases where great accuracy is required, as is now done in the Nautical Almanac for 24 of the brightest stars for 10 days in the year, and those numbers in the Nautical Almanac are to be preferred. The right ascensions and declinations obtained by the preceding calculations, are the mean values, to which must be applied the corrections for the Nutation and Aberration Tables XLII.
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