Summer has officially come to an end for the northern half of the world. Today, Sep. 22, marks an important celestial event called "autumnal equinox," during which the sun crosses the equator to leave the northern half of the sky and reach the southern half.

That is, for the northern half of the world, the nights will now start to become a little longer and the days would be shorten than before. One half of the world would start to get colder, as the sun drifts away to provide warmth to the southern world.

The autumn equinox was preceded by "harvest moon" on Sep.16, Friday. Harvest moon is actually a full moon day, which marks a rare celestial event just before the autumn equinox. It makes the moon look bigger and darker than usual.

According to The Sun, the autumn equinox for the lower half of the sun would fall on March 20, 2017. That is when the sun would cross the equator again to reach back to the northern half of the sky.

The autumn equinox is celebrated across the world in different cultures because of several reasons.

According to the International Business Times, the fall or autumn birthstones include sapphire, opal and tourmaline.

The following are some of the facts to know about the autumnal equinox:

The word "equinox" has a Latin origin meaning "equal nights." It refers to the fact that on this day, the night and the day are roughly of the same duration (12 hours).

Equinox does not last for the entire 24 hour. It's just refers to the exact time when the sun crosses the invisible point on the equator to reach the other half.

On this day, Pagans used to celebrate the festival of Mabon, thanking the summer that went by and welcoming the darkness that will arrive soon.

People with fall allergies are more susceptible to be affected dust mites, mold and ragweed during the autumn months.

The testosterone levels are the highest during fall.

Alaska remains the coldest during the fall and Florida remains the warmest state.

Some believe that equinoxes trigger ancient structures to reveal secrets that have been long hidden from the world.

Other planets in the solar system also experience similar equinoxes.