Welcome to the September edition of Searching the Sky, Discovery Place’s monthly sky watching and astronomy blog.

At the end of the month, we will officially transition into astronomical fall. Our seasons are marked by two different kinds of celestial events: equinoxes and solstices. The autumnal equinox will occur on Wednesday, September 22 and marks the official start of fall. We will remain in astronomical fall until the winter solstice on Friday, December 21.

During the autumnal and vernal (or spring) equinoxes, there are roughly 12 hours of daylight and 12 hours of night. The winter solstice is the shortest day of the year, meaning that people in the Northern Hemisphere will see very little daylight. The farther north one is, the shorter the length of the day on the winter solstice. In fact, there are 24 hours of darkness at the North Pole during the winter solstice. Conversely, the summer solstice is the longest day of the year in the Northern Hemisphere and, on this day, there are 24 hours of sunlight at the North Pole.

The solstices and equinoxes, and the seasons themselves, are caused by the tilt of the Earth and its position in space as it orbits around the Sun. The tilt of the Earth on its axis is about 23 degrees, as illustrated below. During the summer, the Northern Hemisphere is tilted toward the Sun, and we experience longer days. Because the Northern Hemisphere receives more sunlight during these longer days, the atmosphere is able to trap more heat and we experience warmer weather. The opposite is true during winter in the Northern Hemisphere; the Northern Hemisphere is tilted away from the Sun and receives less direct sunlight, resulting in colder weather.

Although ancient peoples had a limited understanding of astronomy, equinoxes and solstices are easily observable phenomena that occur on a regular basis and thus hold an important place in many different cultures around the world. The fall equinox is especially important since it often indicated a time to plant certain kinds of crops. In addition to using the equinoxes and solstices to mark time, some cultures based religious holidays and rites around them. In many cases, these religious rites are reflected in ancient architecture.

My favorite instance of architecture aligning with the equinoxes occurs at Chichen Itza, a Mayan city located in Mexico. With their observations of the sky, the Mayans were able to build a temple, known as El Castillo or the Temple of Kukulkan, which creates a curious seasonal spectacle. On the autumnal and vernal equinoxes, the feathered serpent god Kukulkan appears to descend down the steps of the temple. As the Earth rotates on its axis and the Sun appears to change position in the sky, light bouncing off the temple creates triangular shadows that gradually combine into the shape of a serpent. Throughout the day, the shadow serpent appears to grow larger in size and move down the steps of the temple. It’s a truly impressive sight that demonstrates how much science and engineering knowledge the Mayans possessed, and a reminder of how important the sky and the changing of the seasons have been to humans throughout history.

Check out the media gallery below for photos of El Castillo.