Earth Travel On Time, Again

burgburgburg writes "The NY Times has an interesting article about a rather puzzling phenomena: for the fifth year in a row, the Earth's travel through space is right on time. The rate that the Earth travels through space has slowed ever so slightly for millenia. To compensate for this, since 1972, scientists have added a "leap second" at the end of each year. The problem: Since 1999, the Earth has been on time. The recognition of a need for a leap second was an unintended consequence of the invention of the atomic clock. Suggested reasons for the unexpected punctuality: the tides, weather and changes in the Earth's core."

Comments

Intercalation, Calendar Calibration, Leap Seconds

[HopeOS]

I recently had to implement code to convert terrestrial time (TT) to martian solar day (MSD). Some interesting tidbits in that research follow.

As you might guess, the extra days in leap years help keep our calendars synchronized with our actual position about the sun (heliocentric longitude). This is called intercalation, and the general rules governing the gregorian calendar cover 400 year periods. Other methods exist which are in a sense more "accurate," but less useful for predicting future dates. Fortunately, the earth is pretty regular in its movement around the sun.

The 0 degree mark for heliocentric longitude occurs at the vernal equinox, an event that can be easily determined from earth, and has been for centuries. In the Iranian calendar, the new year begins on the day of the vernal equinox. Since this event occurs later in the day each year, eventually an extra day must be added. Such calendars are based on observation rather than rule-based model and consequently are implicitly self-calibrating.

Leap seconds, as pointed out, are an entirely different beast, and are meant to shore up the discrepency between our actual rotation and the atomic clocks we use. The current offset is 22 seconds slow officially. Oddly enough, a NASA document from 1997 uses a value of 63 seconds as the offset between TT (terrestial time) and UTC (Greenwich Mean Time). Another from 2000 shows a 32.184 second offset from TT to TIA (atomic). It doesn't exactly correlate or add up, and I'm not precisely sure why that is. Perhaps someone could enlighten me on the matter.

Curiously, our leap years follow the mathematical model while our leap seconds follow the observation method of calibration. Consequently, you can determine the correct date in the future, but not the correct second.

-Hope