Featured Post from University of Delaware

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
Powered by

You might also like...

Check out some other posts from University of Delaware

3 min

Rethinking AI in the classroom: A literacy-first approach to generative technology

As schools nationwide navigate the rapid rise of generative artificial intelligence, educators are searching for guidance that goes beyond fear, hype or quick fixes. Rachel Karchmer-Klein, associate professor of literacy education at the University of Delaware, is helping lead that conversation. Her latest book, Putting AI to Work in Disciplinary Literacy: Shifting Mindsets and Guiding Classroom Instruction, offers research-based strategies for integrating AI into secondary classrooms without sacrificing critical thinking or deep learning. Here is how she is approaching the complex topic.  Q: Your new book focuses on AI in disciplinary literacy. What is the central message? Karchmer-Klein: Rather than positioning AI as a shortcut or replacement for student thinking, the book emphasizes a literacy-first approach that helps students critically evaluate, interrogate, and apply AI-generated information. This is important because schools and universities are grappling with rapid AI adoption, often without clear guidance grounded in learning theory, literacy research, or classroom practice. Q: What inspired this research? Karchmer-Klein: The book grew directly out of my work with preservice teachers, practicing educators, and school leaders who were asking practical but complex questions about AI: How do we use it responsibly? How do we prevent over-reliance? How do we teach students to question what AI produces? I also saw a gap between public conversations about AI which often focused on fear or efficiency and what teachers actually need: research-informed strategies that support deep learning. My long-standing research in digital literacies provided a natural foundation for addressing these questions. Q: What are some of the key findings from your work? Karchmer-Klein: AI is most effective when it is embedded within strong instructional design and disciplinary literacy practices, not treated as a stand-alone tool. The research and classroom examples illustrate that AI can support student learning when it is used to prompt reasoning, reveal misconceptions, provide feedback for revision, and encourage multiple perspectives. Another important development is the emphasis on teaching students to evaluate AI outputs critically by recognizing bias, inaccuracies, and limitations, rather than assuming correctness. Q: How could this work impact schools, teacher education programs and the broader public? Karchmer-Klein: For educators, this work provides concrete, evidence-based literacy strategies coupled with AI in ways that strengthen, not dilute, student thinking. For teacher education programs and school districts, it offers a research-based framework for professional development and policy conversations around AI use. More broadly, the work speaks to a public concern about how emerging technologies are shaping learning, helping to reframe AI as something that requires human judgment, ethical consideration, and strong literacy skills to use well. ABOUT RACHEL KARCHMER-KLEIN Rachel Karchmer-Klein is an associate professor in the School of Education at the University of Delaware where she teaches courses in literacy and educational technology at the undergraduate, graduate, and doctoral levels. She is a former elementary classroom teacher and reading specialist. Her research investigates relationships among literacy skills, digital tools, and teacher preparation, with particular emphasis on technology-infused instructional design. To speak with Karchmer-Klein further about AI in literacy education, critical evaluation of AI-generated content and teacher preparation in the era of generative AI, reach out to MediaRelations@udel.edu.

2 min

How AI can improve poor leadership writing and boost productivity

Poor written communication from leaders can create the kind of confusion it intended to avoid. University of Delaware career expert Jill Gugino Panté suggests using AI to sharpen emails, clarify expectations and reduce unnecessary calls. Getting through to employees with strong messaging can boost productivity by saving time and reducing unwanted meetings, she says. Panté, director of UD's Lerner Career Services Center, says that good leadership writing should be direct and outcome-driven, with no fluff, and offered the following advice for improvement. ✅ Don’t bury the lead. Start with what decision needs to be made, what action is required, and the deadline. If your writing doesn’t reduce ambiguity, it’s going to add to it. Vague communication can create interpretation gaps which, in turn, can create more meetings. When ownership isn’t defined, decisions aren’t documented, or outcomes aren’t clear, teams default to “Let’s hop on a call.” Meetings then become the fallback for unclear thinking. ✅ Generative AI can be a powerful clarity tool if it’s used intentionally. When used well, it can sharpen your ask and structure communication for action. The key is prompting it to refine your message, not just polish it. Leaders can use prompts like: • “Rewrite this message so the action, owner, deadline, and success metrics are explicitly stated" • “What assumptions or ambiguities exist in this message?” ✅ Good writing can replace unnecessary meetings. If communication is not direct, outcome-driven, and structured for action, it will cost you time somewhere else. Here are some practical actions that leaders can make in their writing: • Start with the Ask - Be explicit about what decision or action is needed. Don’t make people search for it. • Define Outcomes - Clarify deliverables, timelines, budgets and state what success looks like. • Clarify Ownership - Identify who is responsible for the request. • Document Decisions - Write down what has been decided and reiterate next steps, owners, and deadlines. To connect with Panté directly and arrange an interview, visit her profile and click on the "contact" button. Interested media can also send an email to MediaRelations@udel.edu.

1 min

Epidemiologist: Winter Olympics fortunate to dodge norovirus outbreak

Finland's Olympic women's hockey team overcame a norovirus scare last week, but they couldn't get past Team USA, who shut them out 5-0 Saturday in Milan. The University of Delaware's Jennifer Horney can discuss the difficult-to-contain virus, which also hit the Winter Games in 2018. - Horney, a professor of epidemiology at UD, said that the outbreak –  which forced Finland to cancel its first game after 13 players had either been infected or quarantined – is not surprising. Norovirus spreads rapidly in crowded environments through direct contact with surfaces or airborne droplets. - It is difficult to limit the spread of norovirus, as witnessed by the major outbreak that spread at the 2018 Winter Olympics in South Korea. - Consideration is often given for the potential of these types of outbreaks being intentional, which requires public health to work closely with law enforcement. To reach Horney directly and arrange an interview, visit her profile and click on the "contact" button. Interested journalists can also send an email to MediaRelations@udel.edu.

View all posts
Powered by