Calendar Year vs. Earth Orbit

The Earth does not orbit the sun in exactly 365 days.
••• Thinkstock Images/Stockbyte/Getty Images

The calendar year is typically 365 days. However, Earth's orbit around the sun takes slightly longer than this. Because of this difference, every fourth year of our calendar is called a leap year, and has 366 days. The differences arise because it actually takes the Earth around 365.25 days to make a full orbit. This value is rounded down for the sake of our timekeeping.

Sidereal Day vs. Solar Day

Astronomers can refer to two different types of days when tracking the motion of the Earth and heavens. A sidereal day is the time it takes for a star to rotate 360 degrees, completely around the sky. This length of time is roughly 23 hours, 56 minutes and 4 seconds. A solar day is the time it takes for the sun to travel completely across the sky, crossing the meridian twice. Because the Earth moves around the sun as it rotates, the sun's position changes relative to the stars. Therefore, a solar day is slightly longer than a sidereal day. A mean solar day is exactly 24 hours long.

Sidereal Year vs. Solar Year

The discrepancy between a sidereal day and solar day results in slightly different lengths of the entire year. A sidereal year is 365 days, 6 hours, 9 minutes and 9 seconds. A solar year is 365 days, 5 hours, 48 minutes and 46 seconds. The resulting 20-minute, 23-second discrepancy does not have too many immediate repercussions. However, the positions of the equinoxes change gradually relative to the stars, and astronomers must make note of this in their observations.

Integer Timekeeping and Leap Years

Ultimately, both sidereal years and solar years are slightly longer than our 365-day calendar year. However, in order to maintain the day as a significant marker of time, we round our calendar to the nearest day. Therefore, even though the Earth itself takes longer than 365 days to orbit the sun, we round this to the nearest integer. In order to account for this difference, we add a day to every fourth year. These years are called "leap years."

The Julian and Gregorian Calendars

The Julian calendar was the first 365-day calendar. It was created in 46 B.C. by Julius Caesar. Because the actual length of the year was roughly 365.25 days, the Julian calendar added one day every four years. However, the true length of the solar year is 365.242199 days. This difference causes a discrepancy of three days every 400 years, even accounting for leap years. In 1852, Pope Gregory XIII altered the calendar so that any century year not divisible by 400 would not be a leap year.

Related Articles

Why Do the Positions of the Stars Change Each Month?
The Difference Between Solar & Lunar Years
How Much Time Is One Day on Mars?
Why Are Days Longer and Shorter?
Similarities of the Sun & Moon
How to Calculate the Sun's Declination
What Is Solar Altitude?
What Causes the Day/Night Cycle on Earth?
What Are the Two Motions of the Earth?
How to Calculate the Julian Date
How to Convert the Julian Date to a Calender Date
How to Calculate a Planet's Revolution Around the Sun
How to Calculate 180 Days From a Date
What Types of Measurements Are Used for Measuring in...
What Are the Causes of the 4 Seasons on Earth?
Which Planet Moves the Slowest Along Its Orbital Path?
How Are a Solar & Lunar Eclipse Alike?
What Is the Distance From the Sun to Mercury?
The Moon's Effect on the Seasons

Dont Go!

We Have More Great Sciencing Articles!