The science of leap day

Feb 29, 2024

3 min

John Gizis

The arrival of a leap year brings with it myths, legends and superstitions about its origin. John Gizis, professor of physics and astronomy at the University of Delaware, takes part in a Q&A to share the truth and science behind this “extra” day added to the Gregorian calendar every four years.

Why do we have leap day?

Unfortunately, the amount of time the earth takes to go around the sun is not exactly 365 days. It’s off by about ¼ of a day each year.

It would be hard to have a calendar for 365 ¼ days. When was the extra day added to the calendar?

Julius Caesar introduced the Julian calendar in 46 B.C., which added one extra day every four years. Of course, the year is not exactly 365 ¼ days either, so after a while, that extra time built up. Pope Gregory XIII established the Gregorian calendar [the current calendar] in 1582 to correct the fact that the calendar had gotten off by about 12 days, enough that it was noticeable.

How was it noticeable?

The seasons were gradually shifting, so that what we think of as summertime in the northern hemisphere had gradually become more like autumn. The shortest day of the year is Dec. 21. Basically, the shortest day of the year drifted and eventually was in early December.

How did they institute the reforms?

First, they skipped a couple of weeks, so they returned to the original calendar lining up with the sun and stars as it’s supposed to. Then they instituted a couple of rules to keep this from happening again. Leap year happens in years divisible by four, but every 100 years, there is not a leap year. However, every 400 years, you do have a leap year. This happened in the year 2000. And the next time it will happen most of us won’t be around — 2100 will not be a leap year.

What would happen if the extra day had not been introduced?

If we didn’t have it at all, we would be off by ¼ of a day every year. The seasons would completely shift through the calendar and anything that ties to the seasons would be affected, like farming. It would create havoc with the time to plant and to harvest crops, for example. You also would lose the meaning behind sayings such as “April showers bring May flowers.”

Multiply the 2,000 years since it was introduced by ¼ day per year, and that would be 500 days we would have shifted over history. In the northern hemisphere January would have become summer, then gone back to being winter, then shifted off again.

Does adding the extra day make up the difference exactly and keep the astral year in sync with the calendar year?

Yes, but this relates to a bigger issue. Astronomers want time to match up so that the positions of the stars match up year after year. Because the length of day changes slightly over time, astronomers sometimes would like to add an extra “leap second” to keep the stars in sync with our time system. But adding a second is an annoyance for computer and tech systems.

Did you know that people born on leap day are sometimes called “leaplings?” According to Google, in 2020, there were about 5 million people with Feb. 29 birthdays. Do you think they have any advantages or disadvantages to being born on this day?

No, I didn’t know that, and I know someone who refused to be induced on that day because she didn’t want confusion for her child, although I think it might be cool. After all, in this day and age, everyone always knows how old they are.

To set up an interview with Gizis, visit his profile and click on the contact button.

Connect with:
John Gizis

John Gizis

Professor, Physics and Astronomy

Prof. Gizis focuses his research on improving the understanding of stars and brown dwarfs (failed stars).

Brown Dwarfs and BinariesCool StarsStarsSolar SystemCosmology

You might also like...

Check out some other posts from University of Delaware

2 min

Gold medal-worthy experts for Olympic Summer Games coverage

The University of Delaware boasts several experts who can comment on health-related topics such as injuries and training and business-focused areas like marketing and team behavior as they relate to the 2024 Olympic Games in Paris, which begin next week. Matt Robinson Professor, sport management Relevant expertise: Will be in Paris and can discuss the Olympics from an onsite perspective; can give the backstory on The International Coaching Enrichment Certificate Program (ICECP) and what’s new in the Paris Olympics. Link to profile and contact Tom Kaminski Professor, kinesiology and applied physiology Relevant expertise: Has studied heading in soccer (women's football) and the risks of concussions for nearly three decades. Link to profile and contact Karin Silbernagel Professor, physical therapy Relevant expertise: Research aims to advance the understanding of tendon and ligament injuries and repair. Can also discuss sailing. Link to profile and contact Tim DeSchriver Associate professor, sport management Expertise: Sport finance, economics and marketing Link to profile and contact Other experts: INJURIES: Tom Buckley Associate professor, kinesiology and applied physiology Expertise: Head impacts from boxing. Stephanie Cone Assistant professor, biomedical engineering Expertise: Studies the structure-function relationship that exists in tendons and ligaments with a special interest in changes in this relationship during growth and following injury. Mike Eckrich Clinical instructor, physical therapy Expertise: Weightlifting; can talk about the difference between men’s and women’s injuries and form in the sport. Donald Ford Physical therapy Expertise: Shoulder injuries/rehab expert Jeffrey Schneider Senior instructor, kinesiology and applied physiology Expertise: Athletic training and injury prevention, with a particular interest in ice skating injuries. Worked with athletes competing in Winter Olympics (2002, 2006) as a strength and conditioning coach and athletic trainer. EVENTS: Jocelyn Hafer Assistant professor, kinesiology and applied physiology Expertise: Race Walk events and how biomarkers are used in walking studies. Airelle Giordano Associate professor, physical therapy Expertise: Gymnastics; she was a collegiate gymnast Kiersten McCartney Doctoral student Expertise: Can chat about Paralympic Triathlon (running, hand cycling, swimming). Steve Goodwin Associate professor, health behavior and nutrition sciences Expertise: He is also in Paris leading a study abroad cohort. He has been to multiple Olympics, and can also speak to on-site experience, differences in games, etc. George Edelman Adjunct professor, physical therapy Expertise: Works with USA Swimming. BUSINESS: John Allgood II Instructor, sport management Expertise: Sport business management, event management SCIENCE: Joshua Cashaback Assistant professor, biomedical engineering Expertise: Specializes in neuromechanics and control of human movement. His research falls under two major themes: The neuroplasticity and adaptation research line tests how reinforcement feedback can subserve our ability to acquire new motor skills.

1 min

Researchers find ‘narrow’ depictions of fatherhood in children’s literature

When children read picture books, they are often greeted with depictions of family and life lessons their young minds soak up. What happens then, when those depictions don’t offer a thoughtful image of gender or family as they have changed over the years? That is one of the questions University of Delaware Professor Bill Lewis and Social Science Research Analyst and UD alumna Laura Cutler explored in their recent paper, published in the quarterly journal Children's Literature in Education. In “Portraits of Fatherhood: Depictions of Fathers and Father–Child Relationships in Award-Winning Children’s Literature,” Lewis and Cutler looked at more than 80 children’s books to analyze how authors and publishers depicted fathers and fatherhood. What they found was that over a span of nearly 20 years, from 2001 to 2020, these books presented "a narrow view of fatherhood," both in what roles fathers have in familial units and which types of fathers are presented. They also noted that these portrayals have remained relatively static over the last two decades. Lewis, a professor in the College of Education and Human Development’s School of Education, broke down what spurred the research and what he and Cutler hope changes as a result of the research in a new Q&A. Contact to set up an interview with Lewis.

1 min

Professors address students' climate anxiety

Professors at the University of Delaware preparing students for careers working on climate change are making sure to consider mental health issues as they send them out into the world. UD's Climate Change Science and Policy Hub, led by director A.R. Siders, is starting a series of initiatives – on campus and in the region – to tackle the challenge of what is known as climate anxiety. This involves traditional trainings but also innovations with creativity, art, video games and play. "Learning about and working on climate change causes climate anxiety, ecogrief, solastalgia – there’s a whole new set of terms being created just to describe the problem," said Siders, also an associate professor in UD's Disaster Research Center and Biden School of Public Policy and Administration. "This is a real mental health concern." This new way of approaching climate education has become even more critical as universities expand climate education – such as new climate schools, degrees, courses and even embedding it in general education courses, Siders said. The U.S. government is supporting a growing climate workforce, and it is expected that more people will work in climate-related careers. To reach Siders and set up an interview, visit her profile and click on the "contact" link. This will automatically send an email directly to her.

View all posts