It’s official. Winter is here. Well, it will be at 4:19 this afternoon.

The first day of winter is more than a simple day on the calendar, it is a celestial event that people have been studying for thousands of years. The winter solstice has been marked by civilizations for eons.

Everyone knows that the earth orbits the earth once a year. Because the earth is tilted on its axis 23.5 degrees we have seasons. When the top of the axis tilts away from earth we get winter in the northern hemisphere, and we get less daylight. Today the tilt, in relation to the sun, is on the side that faces away from the sun, resulting in today having the shortest amount of sunlight and the sun setting the furthest south that it will do all year. Thus begins the first day of winter for the 90% of the world’s population that live north of the equator. For those living south of it, like the people of Australia, today is the first day of summer.

From today, until June 20 at 10:41 p.m. the amount of daylight each day will grow as the earth travels roughly 470 million miles to reach the other side of the sun. The tilt of the earth will then point toward the sun and bless us all with more hours of daylight.

Globally, several ancient civilizations built large stone structures such as Stonehenge that aligned with the sunrise and sunsets on the special day.

In Marietta, a group of native Americans 1,000 years ago constructed earth mounds that also aligned with the setting sun on the day of the winter solstice. When the first group of European settlers arrived here in 1788 they found mounds all over the area that would become Marietta.

Some were preserved; others were flattened as the town grew.

The area that is now Sacra Via Park had large mounds that ran down either side of the park from Third to Front streets. The mounds that were in Marietta may have influenced those plotting the streets. The streets running toward the Muskingum River for instance were all parallel to the Sacra Via mounds. A block away Quadranaou is parallel to nearby Fourth Street.

The clay that the Sacra Via mounds were built from was sold off in the 1800s to make bricks. The open space remained though, and it is in that space that every year residents gather to watch the sun dip behind Harmar Hill and observe the solstice from the same spot where people likely stood 1,000 years ago.

The annual Winter Solstice Sunset Watch hosted by The Castle Historic House Museum has become a tradition for many. The program starts today at 4:15 p.m. in Sacra Via Park and will be led by Castle archaeologist Wes Clarke who will talk about how the earthworks played a role in marking the passage of seasons in ancient times.

Art Smith in the online manager of The Times, he can be reached at asmith@marietatimes.com