How Long Is A Year On Earth?
QUICK ANSWER
A year on Earth is about 365.25 days, the time it takes our planet to complete one full orbit around the Sun. The extra quarter-day is why we add an extra day to the calendar every four years as a leap year. The more precise figure is 365.2422 days.
A year is the time Earth takes to orbit the Sun once. The basic answer is 365 days, but if you watch carefully, the real number is slightly longer. That little discrepancy is the entire reason calendars get complicated.
What is Earth's exact orbital period?
According to NASA, Earth completes one orbit around the Sun in 365.256 days when measured against the distant stars (called a sidereal year). The tropical year, which tracks the position of the seasons, is slightly shorter at 365.2422 days. The difference is small but real, and it accumulates over time. Earth travels about 584 million miles in a single orbit, at an average speed of roughly 67,000 mph.
Why do we have leap years?
Because Earth's orbit doesn't fit a whole number of days. The actual year is 365.2422 days, but our calendars only count whole days. If we just used 365 days every year, we'd drift about 1 day off the seasons every 4 years. To correct for this, we add a leap day (February 29) every 4 years. That overcorrects slightly, so we skip the leap year in years divisible by 100, unless they're also divisible by 400. That's why 2000 was a leap year but 1900 wasn't.
How fast is Earth moving in its orbit?
About 67,000 miles per hour (108,000 km/h) on average. That's roughly 18.5 miles per second. Earth's orbital speed isn't quite constant; the planet moves slightly faster when it's closer to the Sun (perihelion in January) and slightly slower when it's farther (aphelion in July), by about 3 percent in either direction. The variation is a consequence of Kepler's laws of planetary motion: orbiting bodies sweep equal areas in equal times.
Has a year on Earth always been the same length?
Almost. Earth's orbit is very stable, so the length of a year (in absolute time) has remained roughly the same for billions of years. What has changed is the length of a day. Earth's rotation is slowly decelerating due to tidal interactions with the Moon, making days slightly longer over time. So while a year is still about 365 of our current days, in the distant past, a year would have contained more days because each day was shorter. Around 1 billion years ago, a year was probably closer to 400 to 420 days long.
A year on Earth is 365.2422 days, and the fractional remainder is the whole reason we have leap years. Earth travels at 67,000 mph through space to make that orbit happen, faster than any human-made object has ever traveled in any direction. Everyone gets carried along for the ride.
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