Amman, Sept. 21 (Petra) -- Astronomical fall will begin at 3:05 a.m. Wednesday, Sept. 23, as the autumn equinox ushers in nearly 90 days of progressively shorter daylight hours in Jordan, the Jordanian Astronomical Society said Monday.

Society President Ammar Sakaji said the equinox occurs as the sun, in its apparent annual movement, crosses the celestial equator from north to south, marking the start of fall in the Northern Hemisphere and spring in the Southern Hemisphere.

While the equinox is commonly associated with equal periods of day and night, Sakaji said the two are not precisely equal in observational terms.

Daylight on the equinox remains slightly longer than 12 hours because of atmospheric refraction, which makes the sun visible before it geometrically rises above the horizon and after it has set. Sunrise and sunset are also calculated using the upper edge of the sun's disk.

In Amman and surrounding areas, the closest balance between day and night will occur several days later. Daylight will last around 12 hours and one minute on Sept. 26, falling to about 11 hours and 59 minutes on Sept. 27.

On the equinox, the sun rises almost exactly in the geographic east and sets almost exactly in the geographic west.

In the days that follow, its rising and setting points in Jordan will gradually shift southward. The sun will appear lower in the sky at midday, daylight hours will continue to decrease and nights will grow longer until the winter solstice in December.

Sakaji explained that the seasons are caused by the approximately 23.4-degree tilt of Earth's rotational axis rather than changes in the planet's distance from the sun. As Earth travels around the sun, the tilt changes the angle at which sunlight reaches different parts of the planet and alters the duration of daylight throughout the year.

Astronomical fall this year will last 89 days, 20 hours and 45 minutes, beginning with the equinox at 3:05 a.m. Wednesday and ending with the winter solstice at 11:50 p.m. on Dec. 21.

The exact length of the astronomical seasons varies slightly from year to year because Earth's orbit is elliptical and its orbital speed changes during its journey around the sun, Sakaji said.

He added that the weeks around the equinox offer a clear opportunity to observe seasonal changes in the sky, particularly the shifting positions of sunrise and sunset and the steady reduction in daylight.

The Jordanian Astronomical Society plans to document the changes through astronomical observations.

//Petra// RZ