Astronomical Terms & Definitions

Earth and Sky

Hindu sages perceived perfect harmony and order in the movements of the heavenly bodies, through their keen observation of the sky above and by their yogic powers. Our Rishis had no telescopes but they were well aware of astronomical phenomena that can be observed with the naked eye. Ancient Astronomy tells us:

v  The Earth is a sphere and rotates on its axis once every 24 hours, which causes the rising of Sun Stars and Planets in the East and setting in the West daily.

v  The Earth revolves around the Sun and it completes one revolution around Sun in nearly 365 days.

v  The Earth spins on its own axis and this axis tilted at 23.5 degree to the ecliptic, causes change in the seasons.

v  The Moon revolves around the Earth and completes its one revolution in nearly 28 days causing the Phases of Moon and their correlation with the time of Moonrise and Moonset.

v  The planes of orbit lie almost - but not quite - in the equatorial planes of the major body. As viewed from Earth, therefore, the Sun, Moon and planets follow paths across the sky along the ecliptic.

An astronomer can only see half the sky at a time, that is, only half the sky is above the horizon at any time. However, the sky keeps moving as the earth rotates. Just as the sun rises and sets every day, so does every star in the sky each night. To understand the movements of the Moon, the planets, the Sun, the stars, and other objects through the sky, we need to define a coordinate system and become comfortable with a few terms describing various positions.

The occurrence of eclipses, both solar and lunar - Earth, Moon and Sun are all lined up perfectly, causes an eclipse. 

The retrograde motion of the planets and the variation in the speed a planet moves around the sky. The usual motion of planets as they "wandered" on the celestial sphere was eastward against the background stars. This is called "Direct" Motion". However, it was observed that at times the planets moved westward for some period on the celestial sphere; this was termed "Retrograde Motion".

 There are differences in motion of the inferior and superior planets.

The Earth: The shape of Earth is spheroid like an orange, a bit flat on the poles. The sphere of Earth is called The Terrestrial Sphere.     

 An understanding of the relationship that exists between the earth and the celestial bodies is essential for establishing the position of the celestial bodies in relation to the observer's position on earth, and for understanding the phenomenon of time. 

To determine the observer's position on the earth's surface, we assume that the earth is a sphere and that a co-ordinate system (grid) based on the earth's axis of rotation, which is perpendicular to the equatorial plane. The ends of the axis are designated as the north and south poles of the grid.

The great circles that pass through both the poles and intersect the equator at right angles are meridians. The meridian, which passes through the Greenwich Observatory, England, is the reference meridian or zero meridians.

The two co-ordinates, LONGITUDE and LATITUDE, determine the observer’s position on the earth’s surface.

Longitude is the angle measured from the reference meridian, east or west, along the equator, to the meridian that passes through the site of interest. It varies from 0° to 180° east, or west. 

Latitude is the angle measured from the equator, north or south along a meridian, to the latitude circle that passes through the site. It varies from 0° to 90° north, or south. Thus, the position of the boat in fig.1 is 45° E, 20° N.

THE SKY: To any observer on the ground looking at the sky with the naked eye, the sky appears to be a vast spherical bowl, This spherical bowl we will refer as Celestial sphere (An imaginary sphere with Earth at its center). The stars, planets, Sun, Moon, and other celestial bodies  appear  to be  located  on  this  sphere, and the sphere  appears to rotate around  the  Earth's  extended  axis  once  every 24 hours, giving them their diurnal motions.

About 6,000 stars are visible to the naked eye in the darkest conditions. These stars over the course of the night, stars move across the sky from east to west because of Earth's rotation on its axis. They appear to move in a circular path.

The celestial sphere is an imaginary sphere of gigantic radius with the earth located at its center. The poles of the celestial sphere are aligned with the poles of the Earth. The celestial equator lies along the celestial sphere in the same plane that includes the Earth's equator.

Thus if you were standing at the North Pole and looked straight up, you would be looking at the North Celestial Pole in the sky. If you were at the South Pole, you would see the South Celestial Pole directly overhead.

The rising and setting points of celestial bodies

The rising and setting of celestial bodies (sun, moon, stars, and planets) are determined by their positions on the celestial sphere. During the night, we can see most of stars also rise in the East and set in the West like Sun and Moon. Indeed, the entire celestial sphere seems to rotate slowly--one turn in 24 hours--and since half of it is always hidden below the horizon. An astronomer can only see half the sky at a time, that is, only half the sky is above the horizon at any time this rotation constantly brings out new stars on the eastern horizon, while others to disappear beneath the western one.