In order to measure the distance between two points we take the help of a linear scale which could be according to the Metric, British or any other system of measurement. To spot a point on a plane surface, we use a reference point, the x-axis and the y-axis. Let us presume that from the fixed reference point, the x-axis denotes the distance in horizontal plane while the y-axis indicates the distance in the vertical plane.
The x-axis is the horizontal scale and y-axis the vertical scale. From ‘0’, the fixed reference point, location ‘A’ has a horizontal distance of 4x and vertical distance of 6y while location ‘B’ is at a greater distance horizontally (9x) and at a shorter distance vertically (3y).
Longitude and Latitude on Earth
To locate a place on the surface of the earth, we use the geographical longitudes and latitudes in lieu of the x and the y axes.
The diagram shows the earth, its equator and the north pole.
The vertical line drawn from the north pole, passing through Greenwich (a place near London), perpendicular to the equator, is referred as the prime meridian. It intersects the equator at point ‘O’. This point is the fixed reference point having a longitude and the latitude of zero degrees. Any place on the surface of the earth is located with reference to this fixed reference point.
The vertical axis of the earth, touching the surface of the earth at North pole, is passing from the centre of the earth at point C. Point ‘P’ is an imaginary place on the surface of the earth.
A vertical drawn from the north pole, passing through the place ‘P’ touches the equator at point ‘E’.
The angular distance measured along the equator, between the point ‘O’ and point ‘E’ is the longitude of the place ‘P’. The term longitude, similar to the x-axis, indicates the horizontal angular distance. The longitudes are marked up to 180°towards the east and 180° towards the west of the prime meridian.
The angular distance between the points ‘E’ and ‘P’ is the latitude of the place ‘P’. The term latitude is similar to y-axis, indicating the vertical angular distance measured from the equator. The latitude could be upto 90° to the north or the south of the equator.
Locating a Heavenly Body
To locate the position of any heavenly body in the sky, there are three distinctive methods of measurement.
1. Right Ascension and Declination
Just as the geographical longitudes and latitudes are used to locate a place on the surface of the earth, the Right Ascension and Declination are used on the celestial sphere to spot a heavenly body on the surface of the celestial sphere. Right Ascension, identical to the geographical longitude, is the angular distance measured from the vernal equinox along the celestial equator upto the point where the meridian drawn from the north celestial pole through the centre of the heavenly body cuts the celestial equator. Declination, similar to the geographical latitude, is the angular distance measured between the celestial equator and the heavenly body. Right Ascension is marked from the vernal equinox upto 360°. Declination could be upto 90° towards the north or the south of the celestial equator.
2. Celestial Longitude and Latitude
Though similar in nomenclature, this system is different from the geographical longitude and latitude. Celestial Longitude is the angular distance measured from the vernal equinox along the ecliptic upto the point where the meridian drawn from the north ecliptic pole through the centre of the heavenly body cuts the ecliptic. Celestial Latitude is the angular distance measured between the ecliptic and the heavenly body and can be upto 90° toward the north or south of the ecliptic. The Sun is always on the ecliptic so it’s latitude is always zero.
It needs to be appreciated that whereas the Right Ascension and the Declination are to be considered in relation to the plane of the celestial equator, the Celestial Longitude and the Latitude are to be considered with reference to the plane of the ecliptic.
3. Azimuth and Altitude:
Horizon of a place is the circle where the earth appears to meet the sky. It varies according to location of the person on the surface of the earth. Equally so the map of the heavens and the location of the celestial bodies as apparent to a person varies according to his location on the earth. The zenith for a place is the point located directly overhead for an observer. Any great circle that joins the celestial north and south poles is called a meridian; for any given place, it passes through the zenith.
Altitude is the angular distance between the celestial body and the horizon of the observer.
Azimuth is the angular distance along the horizon between the meridian of a place and the vertical drawn from the zenith passing through the celestial body upto the horizon. In northern latitudes, the azimuth is generally measured east and west from the south point and in southern latitudes from the north point. This concept is important to understand the exact location of a heavenly body in the sky as apparent from a particular location on the surface of the earth at any given moment.
© Rajeev Jhanji, Vedic Astrology