Question

Earth's perihelion passed about nine hours ago. How accurately do we know the moment of closest approach of the Earth to the center of the sun? How do we make this measurement?

Answer 1

Two things that we are really good at measuring in astronomy: time and angles. With respect to the sun and other planets, we can measure our relative orientation in the solar system to better than a milliarcsec ( of a circle). A year is sec, so we know passage of pericenter to about 0.02sec. This precesion can be built up even more by observations over many years (the greeks knew the average length of a month to much better than a second by comparing eclipse times separated by hundreds of years). Of course, all our atomic clocks keep absolute time much better than this, and in practice we know (because of these accurate astronomical measurements) that the earth rotation is not nearly so constant, so we often have to add or subtract leap-seconds.

EDIT: from radar ranging, we can actually measure our instantaneous location in the solar system to within 3m. With an orbital velocity of about 4.7km/s, that gives pericenter passage to better than a millisecond!