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The Leap Year and Orbital Dynamics on Earth

Author: Abhipsha Sahu


Introduction

Happy Leap Year! A quirk of our current calendar system is the fact that every four years, an additional day gets added to the month of February, giving us the 29th of February: the leap day.

The leap day owes its existence to the fact that the Earth takes about 365.25 days to orbit the sun. That quarter of a day adds up to a full missing day every four years.

The concept of leap years often leads us to feel like the passage of time is rather arbitrary- and to some degree, it is! The leap day is why, even across timescales of hundreds of years, January is always a cold month in the northern hemisphere, and why the southern hemisphere always has warm christmases. It keeps things consistent between the solar year and our calendar year.

Leap years are just one consequence of the earth’s orbital characteristics, but what else does it mean for us?


Goldilocks and the Origin of Life

Perhaps the most unique thing about the Earth’s orbit around the sun is its distance from the sun. The Earth is often described to lie in what is known as the “Goldilocks Zone” which is a region where water can exist in a primarily liquid form. Liquid water on earth has long been thought to be the reason life exists, as biological models and existing archaeological evidence indicate that early life originated in the Earth’s oceans.

By far the most important quirk of the Earth’s orbit is that it exists within the region that allowed life to exist. Given that Earth is the only planet currently known to be inhabited by living organisms, our unique orbit is crucial to our very lives. An orbit too far or too close to the sun would’ve made the evolution of life on earth completely impossible.


Seasons in the Sun-Earth Orbital System

The most obvious effect that the Earth’s orbit around the sun has is that it creates seasons. It is a popular misconception that seasons arise due to the physical distance from the earth to the sun changing throughout the course of a year. While it is true that due to the Earth’s orbit being elliptical, it is sometimes further away from the sun than at other times of the year, this difference is not significant enough to cause significant seasonal variation.

In reality, seasonal variation is almost entirely a result of the tilt of the earth’s rotational axis, also known as its “obliquity”, as the planet moves around the sun. This results in the distribution of sunlight across the earth being uneven across the two hemispheres. During summer months, the incoming solar radiation is simply more “direct”, resulting in hotter weather.

This axial tilt also explains why days are longer in the summers and shorter in the winters. The sun’s rays are more direct throughout summer months, and therefore cover the surface for a longer time.

The axial tilt is also why seasons in the northern and southern hemispheres are always opposed. While one hemisphere receives higher intensity solar radiation and experiences summer, the other is tilted away, thus giving rise to winter.


What do Ice Ages and the Pole Stars Have in Common?

The earth’s orbit is also responsible for long-term variation in the Earth’s climate, through a series of cyclic changes known as Milankovitch Cycles. These cycles are governed by changes related to three main characteristics: the Earth’s axial tilt or Obliquity, the shape of Earth’s orbit or Eccentricity, and the direction in which the Earth’s rotational axis points, or Precession.

The angle at which the Earth’s axis is tilted varies between two extremes, about 21.4 degrees and 24.5 degrees. At greater angles, the differences between seasons is sharper. Therefore, over millions of years, seasonal variation becomes more extreme before gradually becoming more uniform.

The eccentricity of the Earth’s orbit is a measure of how elliptical it is. While planetary orbits generally have quite low eccentricities, the fact that the Earth is sometimes closer to the sun and sometimes further away does have small impacts on its climate by impacting the length of seasons. The eccentricity of the earth oscillates in an approximately hundred-thousand year long cycle. While this variation doesn’t significantly impact the Earth’s climate across short-term time scales, it will over long time scales. At higher eccentricities, certain seasons will be significantly longer than others. When the orbital eccentricity is lowest, this variation decreases to almost none.

Precession is a phenomenon by which the direction in which the Earth’s axis periodically “wobbles” and changes direction. Although this change is slow, it eventually leads to variation in the Earth’s climate by controlling how extreme seasons are in each hemisphere by controlling which one experiences summer at perihelion. Aside from climatic variation, precession is also why the pole stars change every few tens of thousands of years.


The Milankovitch cycles combined are responsible for long-term climatic patterns like ice ages. The effect of Earth’s orbit on seasonal variation is still an active field of research, as mapping out these long-term patterns can get quite complicated and many questions regarding the subject still remain unanswered.


Conclusion

Seasonal weather patterns and climatic cycles similar to the Milankovitch Cycles are not unique to the earth, and are an almost universal consequence of planetary orbital dynamics. However, at this point in human history, it is only on Earth that they directly affect us. Perhaps one day, a multiplanetary human race will investigate the various ways in which planets’ orbits affect everyday life. For now, plenty remains to be understood about our own planet’s movement around our sun. In 2024, we can celebrate the 29th of February as one such lovely consequence. Once again, Happy Leap Year!


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