The length of a year is not a straightforward matter

By MUNGAI KIHANYA

The Sunday Nation

Nairobi,

20 December 2020

How long is a year? The answer is simple and straightforward: it is either 365 or 366 days. Perhaps, when this question is asked, the answer being sought is the average length of a year. If we ignore leap centuries, the answer is 365.25 days. We get this from the understanding that, in every four-year period, there are three with 365 days and the fourth has 366.

Thus, a 4-year period has (365 x 3) + 366 = 1,461days. Dividing this by four gives an average of 365.25. This calculation brings out one of the dangers of working with averages: even though this is the average number of days per year, there has never ever been a calendar with 365.25 days!

If we include the concept of leap-centuries, that is only a century year divisible by 400 is a leap year, then, in every 400 years, there are 97 leap-years of 366 days while all the other years have 365. This makes a total of 146,097 days every 400 years. Thus, in this calculation, the average length of a year comes to 365.2425 days.

It is important to recognise that these two values (365.25 and 365.2425) are the average lengths of calendar years. The first one is for the (old) Julian version while the second is for the (modern) Gregorian style.

Calendars are an attempted to track the movement of the earth around the sun so that the change of seasons as well as positions of stars in the sky repeat their cycles on the same date each year.

The duration that the Earth takes to go round the sun is not a straightforward matter. It depends on what is being used as a refence. The “tropical year” is the duration that elapses from the moment the sun crosses the equator heading to the next crossing in the same direction. This has been measured accurately as 365.24219 days.

Then there is the “sidereal year” which the duration that elapses for a distant star to return to the same position in the sky. This one comes to 365.25636 days. This is almost 6 hours longer than the tropical year.

Finally, there is the “anomalistic year” which the duration from moment the Earth is at its closest to the Sun to the next. This is 365.25964 days – about 5 minutes longer than the sidereal year.

So, when you ask “how long is a year?” it is important specify which year you are referring to.