By Ellalyn de Vera Ruiz
Daylight and darkness will be almost of equal length on Sunday (September 23), which is one of only two occasions in every year.
Sun
KAZUHIRO NOGI / AFP (MANILA BULLETIN)
Thereafter, the Philippine Atmospheric, Geophysical and Astronomical Services Administration (PAGASA) said nights will begin to last longer and days get shorter in the country.
PAGASA explained that autumnal equinox occurs when the sun’s declination is zero degrees and there are 12 hours of day and 12 hours of night at all points on the earth’s surface, hence day and night will have equal length on Earth.
It usually falls on September 22 or 23 of every year, but this year, autumnal equinox will occur on September 23.
Philippine nights will be longer thereafter as the sun moves below the celestial equator towards the southern hemisphere, PAGASA said.
Every year, equal lengths of day and night happen twice a year during the vernal and autumnal equinox, on March and September, respectively.
PAGASA said equinoxes are the only times when the solar terminator, or the "edge" between night and day, is perpendicular to the equator.
"On an equinox, day and night are of approximately equal duration all over the planet. They are not exactly equal, however, due to the angular size of the sun and atmospheric refraction," it pointed out.