Longer nights start today

MANILA, Philippines - People around the world will experience today the “autumnal equinox” which marks the beginning of shorter days and longer nights in the northern hemisphere, which includes the Philippines, according to the weather bureau.

The Philippine Atmospheric, Geophysical and Astronomical Services Administration (Pagasa) said that at the autumnal equinox, day and night would have equal length on Earth.

“The autumnal equinox occurs on Sept. 23 at 11:19 p.m. (Philippine Standard Time) 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,” Pagasa said in its monthly astronomical diary.

“The noon altitude of the Sun is 90 degrees (the zenith) at the equator and diminishes to zero degrees at the poles. Beginning Sept. 24, 2009 until March 20, 2010, the sun will lie south of the celestial equator, signifying the approach of winter for the northern hemisphere and summer for the southern hemisphere,” the weather bureau added.

In the northern hemisphere, this astronomical event marks the seasonal shift from autumn to winter.

Pagasa explained that because the North Pole tilts away from the sun, countries within the northern hemisphere could observe that the sun stays low in the sky, which makes the days shorter, and the nights are longer and cooler.

The weather bureau said an equinox occurs twice a year. The vernal or spring equinox occurred last March.

The term “equinox” is derived from the Latin word “aequus” which means equal, and “nox,” which means night.