Daylight and nighttime will be almost of equal length this Tuesday, Sept. 22, in an astronomical event called "autumnal equinox."

It is caused by the Earth’s tilt on its axis and continuous orbit around the sun

The Philippine Atmospheric, Geophysical, and Astronomical Services Administration (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 22.

Thereafter, PAGASA said nights will begin to last longer and days get shorter in the country.

Philippine nights will be longer thereafter as the sun moves below the celestial equator towards the southern hemisphere, it explained.

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," it said.

"They are not exactly equal, however, due to the angular size of the sun and atmospheric refraction," it said.