Equinox: When day and night are at same length

All things being equal, a celestial phenomenon that occurs twice a year will happen tomorrow, Sept. 23: The "equinox," that time of year when day and night are of equal length, with each being 12 hours in duration.

The word "equinox" derives from the Latin term "equinoctium," which in turn comes from "equus"(equal) and "nox" (night).

According to astronomers, equinoxes occur when the Earth’s axis of rotation is not aligned with the plane of its orbit around the Sun. With the Earth tilting over by about 23.45 degrees, the Sun appears to cross the celestial equator either from north to south or from south to north.

Astronomer Elmor Escosia of the Philippine Atmospheric, Geophysical and Astronomical Services Administration (PAGASA), told The STAR that after the equinox day (Sept. 23), the Sun would begin moving again from south of the equator.

"The Sun will be exactly at the southernmost part of the equator in December when the winter solstice happens on Dec. 22," Escosia said.

Astronomers explain that the point at which the Sun crosses the celestial equator on its way from south to north is called a vernal equinox, occurring about March 21 and marking the beginning of spring in the Northern Hemisphere.

On the other hand, the point at which the Sun appears to cross the celestial equator from north to south, occurring about Sept. 22 or 23, is called the autumnal equinox, marking the beginning of autumn in the Northern Hemisphere, he said.

On Friday (Sept. 23), Escosia said, the Sun directly rises in the East and will set directly in the West, traveling through the sky for almost exactly 12 hours.

After Friday, nights begin to grow longer while the days become shorter, and will continue to do so until the winter solstice (Dec. 22) in the Northern Hemisphere, Escosia also said.

"Avid night sky watchers will then have longer periods to stargaze and, possibly, they can observe on Friday night the occultation involving the star clusters of Pleiades in the constellation Taurus," he noted.