This orbital revolution is the basis for all human timekeeping systems, but precisely measuring this journey introduces complexities. The duration of this solar trip is not a neat, whole number of days, which necessitates the adjustments found in our modern calendar. Understanding the mechanics of Earth’s path and the various ways astronomers measure it reveals why “one year” is a layered concept.
Defining the Orbital Journey
Earth’s path around the Sun is an ellipse, not a perfect circle. This elliptical shape means the distance between the Earth and the Sun is constantly changing throughout the year. The time required for Earth to complete one full revolution, returning to the same position relative to the distant background stars, is called the sidereal year.
This complete circuit takes approximately 365.256 days, or about 365 days, 6 hours, 9 minutes, and 9.8 seconds. During this time, the planet travels roughly 940 million kilometers (584 million miles). Earth’s orbital speed averages about 29.78 kilometers per second (18.5 miles per second).
The slight deviation from a circular orbit is measured by its eccentricity, which is low at about 0.0167. This low value means the path is very close to circular. The elliptical path results in a point of closest approach to the Sun, called perihelion, and a farthest point, known as aphelion.
Why Our Calendar Needs a Fix
The primary challenge in creating a calendar is that the length of the planet’s physical orbit does not align with a whole number of days. The tropical year, which governs the timing of the seasons, is slightly shorter than the sidereal year, clocking in at approximately 365.2422 days. This fractional quarter-day is the source of the discrepancy between the solar cycle and our 365-day calendar.
If the calendar year were simply 365 days, the accumulated difference of almost six hours annually would cause the dates of the seasons to drift backward. Over a century, the calendar would be off by about 24 days, meaning summer would begin in late spring. The system of adding a 366th day every four years, known as a leap day, was introduced to re-synchronize the calendar with the seasonal cycle.
The Gregorian calendar system refines this correction by skipping the leap day in years divisible by 100, unless they are also divisible by 400. This mechanism averages the length of the calendar year to 365.2425 days over a 400-year cycle. This calculation is close to the actual length of the tropical year, maintaining the alignment of our dates with the seasons over long periods of time.
How Astronomers Measure Different Years
Astronomers recognize that the definition of a year depends on the reference point used for the measurement. The two most common astronomical definitions are the tropical year and the sidereal year, which differ because of axial precession.
The tropical year is measured by the time it takes for the Sun to return to the same point in the seasonal cycle. Its duration is about 365 days, 5 hours, 48 minutes, and 45 seconds.
The sidereal year is the time required for Earth to complete one full orbit relative to the fixed, distant stars. Earth’s axis experiences a slow, conical wobble, much like a spinning top, which causes the equinox point to shift slightly each year. Because of this wobble, known as precession, the seasons begin roughly 20 minutes before Earth completes its orbit relative to the distant stars.