EQUINOXES and TWILIGHT

EQUINOXES Two days in a year when day and night are equal throughout the world are equinoxes. Falling midway between the dates of the solstices, on these dates the earth's axis lies at 900 to the line joining the centres of the earth and the sun and neither the northern nor the southern hemisphere is inclined towards the sun. The vernal equinox occurs on March 20 or 21, also called the spring equinox in the northern hemisphere, while the autumnal equinox occurs on September 22 or 23. On these two days, every place on the globe experiences 12 hours daylight and
12 hours darkness. The sun rises due east and sets due west, and is seen directly overhead on the equator.

TWIUGHT Following sunset and preceding sunrise, the diffuse illumination is called the twilight. It is caused by the refraction of the sun's rays to the earth's surface, even after the sun disappears below the horizon of the ground level. This refraction occurs due to the scattering action of air molecules and the presence ofJX\inute dust particles and moisture in the earth's atmosphere. Duration of twilight depends mainly on latitude and date, which determine the angle that the sun's path makes with the horizon. As this angle is always about 90° in the tropics, twilight is of short dUration. But in high latitudes, due to the acute angle or the low slant of the sun's path as it goes below the horizon, twilight is greatly lengthened.