Daylight and nighttime will be about the same length on Sept. 23, in an astronomical event called "autumnal equinox."

Thereafter, Philippine nights will be longer as the sun moves below the celestial equator towards the southern hemisphere, the Philippine Atmospheric, Geophysical and Astronomical Services Administration (PAGASA) said.

PAGASA explained that the autumnal equinox is when "the sun will be exactly above Earth’s equator, moving from north to south" which usually falls on September 22, 23, or 24 every year.

This year, the autumnal equinox will occur on Sept. 23.

Every year, equal lengths of day and night happen twice a year during the vernal and autumnal equinox, in 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," PAGASA further explained.

After the autumnal equinox, nights will gradually get longer until the winter solstice on Dec. 21, 2021, which is the longest night of the year.