## Coordinates in the

Just as our globe has two poles and an equator, so does the celestial sphere. The celestial north and south poles can be located by extending an imaginary line joining Earth's north and south poles. Similarly if a plane through Earth's equator is extended to cut the celestial sphere, we get the celestial equator. Further, similar to latitudes on Earth (though imaginary), circles can be drawn parallel to the celestial equator on the celestial sphere at regular angular intervals. These celestial latitudes are called 'declination' (Dec.). Declinations north of the equator are assigned positive values (the north pole has a declination of +90°), while those south of it are given negative values (south celestial pole -90°). The celestial coordinate corresponding

Celestial coordinates.

to longitude on Earth is the 'right ascension' (R.A.) which is expressed in hours, minutes and-seconds of time.

The celestial sphere has on it another feature we do not find on a globe. It is called the 'ecliptic' and charts the positions of the Sun in the sky at different times of the year. As we know, the Earth's axis is inclined at 23 — to the plane of its orbit around the Sun. This makes the ecliptic inclined to the celestial equator at an angle of 23 \ and cut the latter at two diametrically opposite points. These two points are called 'equinoxes' because when the Sun is at any of these two positions, nights and days are of equal length everywhere on Earth. Of the two equinoxes, the Vernal or Spring equinox is also known as the 'First Point of Aries' and corresponds to 0 hour of R.A. (similar to 0° longitude of Greenwich, UK). From this point, the right ascension is measured eastward along the celestial equator.

Yet another reference line on the celestial sphere is the 'celestial meridian' which is the imaginary line joining the north and south celestial poles through the zenith (the

Motions of stars in the sky.

point in the sky directly overhead the observer). The meridian cuts the horizon at exactly the north and south points. Every star appears to cross this imaginary line once daily at the time of culmination when it reaches the highest point above the observer's horizon. The Sun crosses it every day at noon (local time).