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.