It’s leap year, and as civilizations have done for centuries, we will add an extra day to the calendar this month, bringing the annual total to 366 instead of the usual 365.

But why?

You may think the modern system was designed to keep the calendar aligned with the seasons (not exactly). You might believe leap years occur every four years (uh-uh). You may even be fuzzy on the definition of a year.

If so, you’re not alone. The whole scheme is convoluted, and we have the Roman Catholic Church to thank for the modern arrangement.

“People think the length of the year is to regulate the seasons,” said Duncan Steel, author of “Marking Time: The Epic Quest to Invent the Perfect Calendar.” “That’s totally untrue. It’s based upon religious considerations.”

There are different ways to measure a year, but for the purposes of the civil calendar, it is the amount of time that elapses between successive vernal equinoxes — the date in spring when day and night are of equal length.

The date is important to Catholics, and some other Christians, because they celebrate Easter on the first Sunday after the first full moon after the vernal equinox.

The current pattern of leap years was designed to keep March 21, the ecclesiastical date of the vernal equinox, aligned with the astronomical event. (It’s a close approximation; this year, the vernal equinox will occur on March 20.)

The reason the two get out of whack is because the span of time between vernal equinoxes, also known as the solar year, is actually 365.242374 days — a bit longer than 365 days. Dropping an extra day into the calendar at regular intervals makes up for the difference.

The Gregorian calendar, launched in 1582, is the basis for today’s international civil standard. It includes a leap day almost every four years, with exceptions in century years that aren’t evenly divisible by 400. So, 2000 was a leap year, but 2100, 2200 and 2300 won’t be.

We take the measurement of time for granted, yet our lives are punctuated by dates: Christmas, New Year’s, Halloween, Mother’s Day, tax day, birthday, payday. Some events are worldwide holidays, while others are celebrated by only a few. The calendar is an indispensable tool for ordering and recording events.

A long-time custom of the Elizabethton STAR is to pick 12 lovely girls to represent the 12 months on the calendar, thus they are known as the STAR Calendar Girls. The girls are presented each January, and not only do they represent the 12 months of the year, but they are chosen from local high schools.

The 2020 version of STAR Calendar Girls include:

• JANUARY: Jessalin Foster, a senior at Happy Valley High School. She is the daughter of Jessica Foster and is HOSA President. Jessalin enjoys hiking and spending time with her family.

• FEBRUARY: Olivia Vaughn is a junior at Elizabethton High School and is the daughter of Misty Buck and Jack Vaughn. She is a member of Southside Christian Church and enjoys dancing and cheering for her favorite teams.

• MARCH: Zoie Grindstaff is a senior at Unaka High School. She is the daughter of Todd and Christie Grindstaff. In her spare time she enjoys HOSA activities and playing golf and basketball.

• APRIL: Destini Milhorn is a junior at Hampton High School. She is the daughter of Kristi and Craig Milhorn. She enjoys playing basketball and softball.

• MAY: Kaylen Shell is a senior at Elizabethton High School and is the daughter of Jason and Sandy Shell. Among the activities she enjoys are soccer, softball, basketball, snowboarding, hiking, and spending time with her family and friends.

• JUNE: Jayln Hill is a senior at Hampton High School. She is the daughter of Holly Fraley and J.B. Hill. She is a member of the school’s pep club and enjoys golf.

• JULY: Sara Grubb is a senior at Unaka High School and is the daughter of Mark and Amy Grubb. She is a member of the Key Club, Beta Club, National Honor Society, FCA, FCCLA and attends Riverview Baptist Church.

AUGUST: Kenzie Birchfield is a senior at Cloudland High School and the daughter of Matt and Kerri Birchfield. She enjoys playing basketball, softball and cheerleading

SEPTEMBER: Mandy Benfield is a junior at Cloudland High School. She is the daughter of Lee and Lisa Benfield and her favorite thing to do is play basketball.

OCTOBER: Sadie Stout is a junior at Johnson County High School. She is the daughter of Chasity Hart and Joey Stout. Sadie enjoys hanging out with friends, playing basketball, photography and attending concerts.

NOVEMBER: Casey Campbell is a junior at Happy Valley High School. She is the daughter of Johnny and Elizabeth Campbell. Casey enjoys playing basketball.

DECEMBER: Rhiannon Icenhour is a junior at Johnson County High School. She is the daughter of Billy and Jennifer Icenhour. Rhiannon enjoys playing basketball, volleyball, tennis, going to concerts, and hanging out with friends.