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

1 min

March Madness: Experts comment on picking underdogs, prop bets and economic benefits

Why do people pick underdogs when filling out their brackets for the NCAA men's and women's basketball tournaments? How do people consume March Madness? How does the tournament benefit host cities and teams economically? University of Delaware experts have the answers. The following UD faculty members can provide their expertise for journalists working on stories about the tournament, which begin this week. • Jackie Silverman, assistant professor of marketing: Why people can't resist choosing underdogs when filling out their brackets. • John Allgood, instructor of sport management: How people consume March Madness (streaming vs. cable TV) and how major collegiate sports events can help brand individual schools. Contact UD media relations to reach him. • Tim DeSchriver, associate professor of sport management: Sports gambling (including prop bets) and advertising targets. • Matthew Robinson, professor of sport management: Economic benefits of the host city. • Matt McGranaghan, assistant professor of marketing: Consumer attention span during commercial breaks. To reach these experts directly and set up interviews, visit the expert profiles below and click on the contact button.

2 min

Understanding waterfowl movement and bird flu

Scores of waterfowl are currently on the move, and experts are worried they could further spread highly pathogenic avian influenza, commonly known as bird flu. But these flocks, which can number in the thousands and include Canada geese and snow geese, are of most concern around poultry houses, said University of Delaware’s Christopher Williams. The risk to humans and dogs getting bird flu from geese who inhabit their neighborhoods remains low, Williams said. Williams, a professor wildlife ecology in UD’s College of Agriculture and Natural Resources, explained why this is happening: • Smaller ducks such as blue-winged teal, larger ducks such as mallards, and larger geese like Canada geese and snow geese are heading north to their breeding grounds. • Waterfowl are a primary reservoir for avian influenza viruses, meaning they can harbor bird flu strains viruses, spread them, and in some cases show no signs of disease. They can shed the virus through their saliva, naval secretions, and feces. • Waterfowl serve as the primary vector of avian influenza to the U.S. poultry industry which has affected more than 8 million commercial and backyard chickens and turkeys that have been culled to keep the virus from spreading. “While we are worried about these flocks congregating near poultry houses, the risk of humans getting bird flu from waterfowl remains low,” Williams said. “Further, walking dogs around areas with geese flocks should pose little threat to dogs’ health as avian influenza is very rare in dogs.” Nevertheless, Williams cautions people and dogs to avoid touching sick birds or dead birds. A large segment of the Canada geese population are not migratory, Williams said. They will stick around the Mid-Atlantic year-round and tend to congregate and breed near grocery store parking lots, golf courses and neighborhoods. “Those birds are being drawn to needing two things. No. 1 is they need water including any natural pond or water containment area,” Williams said. “And all the geese species are herbivores, so No. 2 they eat grass. There are places that have both water and grass that will provide ideal nesting habitat for the geese and their chicks.” Reporters can connect with Williams by visiting his profile and clicking on the “contact” button.

1 min

Spring break could compound West Texas measles outbreak

Most Texas schools go on spring break this week, which could be a major issue compounding the ongoing measles outbreak in West Texas, said Jennifer Horney, an epidemiologist at the University of Delaware. The outbreak, which has grown to more than 200 cases and claimed the life of a child last week, is likely to continue to grow as new cases are identified in both surrounding communities as well as other states, Horney said. Horney, founding director of UD's epidemiology program, can also comment on vaccination of measles, which is one of the most highly contagious infectious diseases. • Through maintenance of vaccination coverage of greater than 90%, the U.S. was able to gain eradication status in 2000. • Although there have been sporadic cases, and even outbreaks, since then, worsening vaccine hesitancy has led to vaccination rates in some communities of 80% or lower. • According to data, of 20 outbreaks involving three or more cases reported in the last five years, 40% were in 2024. With outbreaks currently in at least nine states in the U.S., 2025 looks to be a very bad year for measles infections. Visit Horney's profile below and click on "contact" to arrange an interview.

View all posts