# How long does it take the center of the Earth to complete a full orbit?

If we take midnight of 1/1/2017 in any country and mark in space where the center of the Earth is located, at what date, in the same country, has the center completed a full orbit.

I am almost certain that has to be 1/1/2018, but I am not so sure about the hour.

• What do you mean by a full translation? We'd first need to define a reference frame (for example, heliocentric or barycentric), and, even then, the Earth never returns to exactly the "same" position again: the Earth's orbit isn't quite that perfect.
– user21
Jan 1 '18 at 13:04
• There was a study in Nature magazine that said the inner core rotates in the same direction as the Earth and slightly faster, completing its once-a-day rotation about two-thirds of a second faster than the entire Earth. Every year it does 365 rotations about 4 seconds faster than a mark on a country... You said orbit, which implies heliocentric rather than a rotation, so the country marker shifts and arrives at a full sun orbit after the core does. I don't think you mean orbit though! Jan 1 '18 at 21:18
• While not precisely accurate, because the entire orbit moves, but quite slowly, you can look up the precise Perihelion and aphelion dates and times or the precise solstice dates and times to give you an approximation of 1 complete orbit. None of those occur on January 1st, and they're not exact, they'll be pretty close to one orbit start to finish. Jan 2 '18 at 1:51

It's not exactly one year because of the 28/29 February thing.

The exact period is 365 days 6 hours 9 minutes 9 seconds [Random House Encyclopedia. New York: Random House: 174.]

• A year in which we assign an extra day is called a leap year. Every now and then we also add leap seconds. Jan 1 '18 at 16:55

A sidereal year is 365.256363004 days (365 d 6 h 9 min 9.76 s).

One of the well known Web calendars and date calculators is at: https://www.timeanddate.com/.

Here is the result for your specific question 1/1/2017+1year = 1/1/2018. Adding exactly one year (on a calendar) gives you one year later, everything is the same except the 'year number'.