New Delhi: Sky gazers in the capital are looking forward to watch the spring equinox on Saturday - when the sun shines directly overhead as viewed from the earth, making the night and day equally long.

According to scientists, equinox marks the beginning of spring in the northern hemisphere and the word 'equinox' is derived from the Latin aequus (equal) and nox (night). Around the equinox, the night and day are approximately equally long.

In Delhi, Science Popularisation Association of Communicators and Educators (SPACE) will be organising a public outreach programme at Jantar Mantar on Saturday to explain the concepts of equinox, using the ancient instruments located there.

The sky watch will take place between 10 a.m. and 12.30 p.m.

"Astronomers are able to measure shadows created by the sun during equinox day to determine the earth's location. Finding your location by just observing the movement of the sun is something unimaginable for a layman. But astronomers can do that easily using simple tools to create shadows, and measuring the angle of the sun," said SPACE president C.B. Devgun.

An equinox happens twice each year. During this period the centre of the sun can be observed to be vertically overhead. It occurs approximately around March 20/21 (Vernal) and September 22/23 (Autumnal) each year.

Explaining the process, Devgun said, "We know that the earth rotates about itself and it is tilted at an angle of approximately 23.44 degrees. It also revolves around the sun in an elliptical orbit. Because of the tilt of the earth, for the whole year, the sun shines at different altitudes above the horizon as viewed from earth.

"But there are two days in the year when the sun shines directly overhead as viewed from the earth's equator. On these two days, day and night are of equal lengths anywhere on the surface of the earth, called Spring Equinox and Autumnal Equinox," he added. IANS