Nighttime will begin to slightly last longer than daytime in the country after the autumnal equinox on September 22, the day that daylight and darkness are almost of equal length.

The Philippine Atmospheric, Geophysical and Astronomical Services Administration (PAGASA) explained that 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, PAGASA said. This happens twice a year during the vernal and autumnal equinox, on March and September, respectively.

They are not exactly equal, however, due to the angular size of the sun and atmospheric refraction, PAGASA pointed out.

Autumnal equinox usually falls on September 22 or 23 of every year, but this year, autumnal equinox will occur on September 22.

After the autumnal equinox, Philippine nights will be slightly longer as the sun moves below the celestial equator towards the southern hemisphere, PAGASA said.

In countries with four seasons, the autumnal equinox signals the start of fall season.

This month, PAGASA also noted that stargazing will give a fine display of celestial bodies such as stars and constellations after sunset and before sunrise.

It said the famous "summer triangle" of the stars Vega, Deneb, and Altair of the constellations Lyra, Aquila, and Cygnus respectively, is well placed above the eastern horizon.

Likewise, this month, PAGASA said the rich band of constellations and stars along the Milky Way from the constellations Cygnus in the north to Sagittarius and Scorpius in the south, begin to give way to fainter constellations, many of them with watery associations such as the constellations of Capricornus, Aquarius, and Pisces.