That’s annus, the Latin for “year”.
Today, July 2, is the middle day of 2007. 182 days of 2007 have already passed, and we have 182 more to go after today. That means that at noon it will be exactly the middle of the year. On leap years, the middle occurs at midnight before July 2, because the extra day was added earlier in the year, on February 29.
Well, not exactly.
For one thing, it depends on where you were at the beginning of the year, and where you’ll be on its last second. If you were in California on January 1, and you’ll be in Sidney next New Years Eve, then you’ve shortened the year for yourself by the majority of a day. On what days of the year you traveled would affect where on your personal calendar the middle moment of the year falls for you. That would also apply to anyone who was in a different time zone on July 2 than they were at other times of the year.
The same thing occurs wherever Daylight Saving Time is observed. Since July 2 falls during the observation of DST but the end of the year does not, that means that the middle of the year for a time zone that observes DST actually occurs at 1PM (or 1AM on leap years).
Occasionally, UTC (the official time kept by atomic clocks) is adjusted by 1 “leap second” to keep it in sync with irregularities in the Earth’s rotation. If the US Naval Observatory continues to add that extra second onto the end of December 31, then the mid-point of those years would fall a half-second later than usual.
But who’s counting?