The vernal equinox, the halfway point between winter and summer, arrived shortly after noon to the pyramids of Teotihuacan, a sprawling pre-Hispanic ghost town 30 miles (50 kilometers) north of Mexico City.

People from across the country make an annual pilgrimage to the ruins, climbing the tallest structure, the Pyramid of the Sun, and throwing their arms skyward. Others stream to the Moon Pyramid, which is not as high but is older.

The pilgrims believe the pyramids hold a special energy on the equinox that can be transmitted to people, especially those who dress in white.

Even larger crowds were expected at Teotihuacán on Tuesday, the first full day of spring, which this year falls on a national holiday commemorating the 200th anniversary of the birth of reformist president Benito Juárez.

Authorities are expecting so many revelers that they will charge 45 pesos (US$4) for entry into the ruins, which are normally free on holidays.

The equinox also is an important event at Chichen Itzá, the Maya ruins on the Yucatan Peninsula. There, thousands turn out to watch a serpent-shaped shadow slither down the Temple of Kukulkán at daybreak.

Teotihuacán emerged about two centuries before the birth of Christ and was home to as many as 200,000 people at its peak.