People raise their arms toward the sun while standing near the Great Pyramid of Cholula in Puebla, Mexico, on March 20. Visitors come every year to the world's largest pyramid near the Popocatepetl volcano to welcome the spring equinox. Imelda Medina/Reuters
Hot air balloons float above the Pyramid of the Sun of Teotihuacan outside Mexico City during a festival as part of the spring equinox on March 20. Hundreds of Mexicans and tourists gather at Teotihuacan every year to welcome the spring equinox. Henry Romero/Reuters
People raise their arms towards the sun while standing in front of the Kukulcan pyramid in Chichen Itza, Mexico, on March 20. Thousands of visitors came to welcome the spring equinox. Gerardo Garcia/Reuters
Artists stand in a circle during a performance to celebrate Nowruz in the Kyrgyz capital of Bishkek on March 21. Nowruz, an ancient holiday marking the spring equinox, is widely celebrated across Central Asia. Vladimir Pirogov/Reuters
Artists take part in a theatrical performance to celebrate Nowruz, the spring equinox, in Almaty, Kazakhstan, on March 21. Shamil Zhumatov/Reuters
Actors in traditional costumes perform with axes and shields during a ceremony adapted from an ancient Qing Dynasty tradition where emperors offered prayers to the sun, at the Temple of the Sun in Ritan Park, central Beijing, on March 20. The ceremony is held on Chunfen, and also known as the vernal equinox. Petar Kujundzic/Reuters
Folk dancers in national costumes perform during the festivities marking the holiday of Nowruz in Tashkent, Uzbekistan, on March 21. Nawruz, Central Asia's spring 'New Year' holiday, falls on March 21. Anvar Ilyasov/AP
An Iraqi family celebrates Nowruz at a park in central Baghdad, Iraq, on March 21. Nowruz, the Farsi-language word for 'new year,' is an ancient Persian festival, celebrated on the first day of spring in countries including Iraq, Turkey, Afghanistan, and Iran. Karim Kadim/AP
Residents tour on boats at marshes on Nowruz Day, a festival marking spring, in Basra, Iraq, on March 21. Atef Hassan/Reuters
A vendor prepares to grill fish for customers on Nowruz Day, a festival marking spring, in Basra, Iraq, on March 21. Atef Hassan/Reuters
As Sunday ushered in the first official week of autumn, with the autumnal equinox setting Earth's Northern Hemisphere on a path to shorter days and colder temperatures. But what does the equinox really signify, and what's actually happening in the night sky? We break it down for you.
Sunday's equinox marked the transition from summer to autumn in the Northern Hemisphere and from winter to spring in the Southern Hemisphere. Here, we answer your most pressing equinox-related questions.