Photo: Getty Images
Photo: Getty Images

How Many Days Are In A Year

Days in a year calculation

Gregorian calendar year

One calendar common year has 365 days:

1 common year = 365 days

One calendar leap year has 366 days

1 leap year = 366 days

Leap year occures every 4 years, except for years that are divisable by 100 and not divisable by 400.

So the mean length of the Gregorian calendar year is:

1 mean year = (365+1/4-1/100+1/400) days = 365.2425 days

Photo: Twinkl
Photo: Twinkl

Julian year

First, there's the Julian year, which is exactly 365.25 days long. It's not very precise, since it's just a number someone decided on, rather than an exact measurement of an astrophysical phenomenon. But when it was introduced by the Julius Caesar in 46 B.C., it was revolutionary.

Before then, the Romans knew that it took about 365.25 days for the Earth to orbit the sun, but they decided to stick to a 355 day calendar anyway. Every so often the high priest of Rome would call for an “intercalary month” to put the calendar back on track. Theoretically these intercalations were supposed to be systematic, but they often were abused — priests would call for an extra month when their friends were in power, or omit a needed month if an enemy was consul. Things got pretty wonky pretty quickly, and the last years before Caesar swept in with his new system were known as “the years of confusion.”

We don't use the Julian year for calendars any more, but it is used to define the light-year as a measurement of distance.

Sidereal year

A sidereal year is the time taken by the Earth to orbit the Sun once with respect to the fixed stars. Hence, it is also the time taken for the Sun to return to the same position with respect to the fixed stars after apparently travelling once around the ecliptic. It equals 365.256 363 004 Ephemeris days for the J2000.0 epoch.

One sidereal year is the time it takes the Earth to do a single rotation around the sun.

One sidereal year has 365.25636 days:

1 year = 365.25636 days

Tropical year

One tropical year is the time it takes the Earth to complete a single cycle of the 4 seasons.

One tropiical year has 365.242189 days:

1 year = 365.242189 days

Modern calendars are set according to the tropical year, which tracks the amount of time it takes to get from spring equinox to spring equinox — about 365 days, 5 hours, 48 minutes, and 46 seconds, or 365.2422 days. It's probably the most well-known measure of a year because it's the most useful for people here on Earth. If you want to know when the seasons will change, the tropical year will tell you.

But there's a not-so-tiny problem with the tropical year: the Earth wobbles. Instead of spinning like a globe on an axis, we turn like a top. This means that the orientation of Earth's equator is constantly shifting ever so slightly; thus the moment of the equinox (when the equator passes through the center of the sun) also changes. The consequence of all this wobbling is that a tropical year ends about 20 minutes before Earth actually completes an orbit of the sun.

Photo: Getty Images
Photo: Getty Images

Tracking the Moon's Orbit

The months originated as a way to mark time and break up the year into shorter periods based on the Moon’s orbit around Earth. The word month is even derived from the word Moon.

As far as we know, months were first used in Mesopotamia sometime between the years 500 BCE and 400 BCE to measure the natural period related to the lunar month, or synodic month, which is the time it takes for the Moon to go through all the Moon phases.

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