How Do We Define the Length of a Year on Earth?

Time the number of days it takes for the Earth to travel completely around the sun to define a year. Every 365.25 days, the Earth returns to the same place in its orbit. This period of time has become known as a year. The extra quarter of a day is made up every four years with a leap day that is tacked on to the end of February 29. This is the most basic and common definition for the length of time known as a year. Calendars are designed to help people with this task.

Track the constellations as they move through the sky. Certain constellations are only visible at certain times of the year because of where the Earth is in its orbit. When a constellation returns to a certain point in the sky, a year has passed. Without precise equipment, however, being exact is difficult using this method. A common person can track the movement of constellations by using landmarks as a reference point. Stand in a specific place relative to the landmark and track the location of the stars. The technical term for this type of year is the sidereal year.

Measure the year from one spring equinox to the next. The spring equinox is the period during spring when the day is the same length as the night. Keep a record of the times of dawn and dusk. When the time between the two is identical (after winter has ended), you have found the spring equinox. The spring equinox falls on or around March 20 every year. This type of year is called the tropical year.