File:Summer Solstice Marker.jpg

English: A solstice is an astronomical event that happens twice annually as the Sun reaches its highest or lowest points in the sky. Here in the Northern Hemisphere, June 20th is usually the longest day of the year and is referred to as the summer solstice.

These photographs illustrate how sunlight from the rising summer solstice Sun flows down the cleft in the boulder in front of you. The play of light and shadow on the spiral petroglyph changes as the Sun rises and moves across the sky. From between June 14th to the 28th a shaft of light forms, moving down the side of the adjacent boulder, until it touches the center of the spiral within a few minutes of 9:00 am. The full interaction of the solar marker takes about an hour, shown in each photograph below.

Why would the inhabitants of Puerco Pueblo want to mark the date of the solstice? Prehistoric peoples used solar calendars to plan their lives around the changing seasons. For agricultural people knowing when to plant crops or expect summer rains is vital for survival. Solstice days marked important points in the seasonal calendar and formed the cornerstones of annual ceremonial cycles. This importance persists in the ceremonial calendars or contemporary indigenous communities where the year is divided in two by the summer and winter solstices.

In Petrified Forest National Park researched have identified over a dozen calendric petroglyph sites and many more exist throughout the Southwest. These features demonstrate the importance of marking a passage of the changing season to prehistoric peoples and their descendants.

Village of Puerco Pueblo @ Petrified Forest National Park, Arizona