Thursday — the extra day tacked onto the month of February this year — isn’t a holiday, but its historical significance lives on due to its quirky ties between space science and religion.

While most everyone notices the additional 29th day in February that comes around almost every four years, not everyone knows why there is a Leap Year, said Professor Jeffrey Scholes, director of the Center for Religious Diversity and Public Life and chair of the department of philosophy at the University of Colorado campus in Colorado Springs.

“There are so many things we celebrate now that have a religious origin but have become so secularized that we don’t ask questions,” Scholes said. “A long time ago religion was the cause of all change in society, so it’s not surprising that Leap Year has religious origins.”

Thousands of years ago, before the roundness of the Earth was a gleam in anyone’s eye, the prevailing belief was that the Earth revolved around the sun in precisely 365 days.

That clearly was not the case.

“It was actually a little bit more — a fourth of a day more every year,” Scholes said.

To be exact, it takes 365 days, five hours, 48 minutes and 45 seconds for the Earth to complete a full trip around the sun, said Sean M. Wright, an unofficial historian and writer in Santa Clarita, Calif., who has studied Leap Year.

When combined, those slices of time add an entire day to the calendar every four years.

Civilizations handled the discrepancy differently, Wright noted.

Mediterranean cultures kept a 355-day calendar and threw in an additional month every few years to realign the vernal and autumnal equinoxes, he said.

And the 360-day Egyptian calendar ended each year with five or six added days.

“The Hebrews knew they were wrong because the seasons started changing, but they didn’t know there were days missing,” Wright said.

When Julius Caesar took over as ruler of the Roman Republic in 46 B.C.E., he ordered his astronomer to reform the timetable.

That produced the Julian calendar, with 365 days in a year and a Leap Day added every four years at the end of February, which at the time was the last month in the year.

But that system was flawed, Wright said, because the Earth’s yearly solar rotation is 11 minutes and 15 seconds short of six hours — which adds another day every 128 years.

“By the 1600s, you had 10 extra days,” Wright said.

That not only thrust the Julian calendar out of alignment with the seasons, but more importantly, Scholes said, sent Christianity’s most holy day of Easter, marking the resurrection of Jesus, spinning out of whack.

An initial calendar correction arose at the earliest Christian Council of Nicea, held 325 years after Jesus’ death. The meeting of religious leaders unified the celebration of Easter on the first Sunday following the first full moon of spring.

There’s an exception, Wright said. If Easter lands on the first day of the first full moon of spring, it’s observed a week later. Go figure. “It’s confusing,” he said.

Also, Scholes said, though Easter falls on different dates it is always on a Sunday because of biblical accounts of Jesus’ crucifixion happening on a Friday and his resurrection in three days.

“They wanted it in late March or early April,” Scholes said. “If they let it go without correction of the calendar, Easter would be in summer.”

And that would not have worked.

“Easter represents hope because of the resurrection, and spring always represents hope,” Scholes said. “It wouldn’t make sense theologically to have Easter in the summer.”

The second adjustment came in 1578 when Catholic countries adopted the Gregorian calendar, orchestrated by Pope Gregory XIII, as both a church and a civil calendar.

Widespread approval was slow to develop, though.

Protestant countries protested the Gregorian calendar, Wright said, some calling it the “work of the anti-Christ.” It took another 200 to 300 years for Protestants to agree to the new calendar.

The Orthodox Church continues to use the Julian calendar for its liturgical year, which Wright said is now 13 days behind.

The astronomers and mathematicians that Pope Gregory had convened created a new methodology to right the marching of time that continues today.

Under the Gregorian system, Leap Year is skipped in turn-of-the-century years not divisible by four, starting in the year 1600.

So, while the millennial year of 2000 had a Leap Day, or a Feb. 29, the years 1700, 1800 and 1900 did not.

Neither will 2100, 2200 and 2300, Wright said. But 2400 will.

“And 35 centuries will pass before this error of five hours, 48 minutes and 45 seconds results in a single extra day, since the length of the Gregorian year exceeds the true astronomical year by 26 seconds,” he said.

Wright believes the history of Leap Year solidifies that science and religion go hand-in-hand and should not be viewed as contradictory.

“Science is never at odds with faith; you use reason to understand some aspects of faith,” he said. “Creation is something like a jigsaw puzzle — we may not know how it fits together, but it exactly fits together.”