Today on June 21 we mark the summer solstice in the northern hemisphere.  This is the day when the Sun reaches its maximum northern declination (Tropic of Cancer).  Given the 0.1' precision displayed for celestial navigation purposes, this declination appears constant for an extended period of time around the exact moment of the solstice.  In order to pinpoint that special moment down to a minute (or even a second), we would have to display more decimal places of the Sun's declination in order to spot the instant when it reaches its maximum.



Alternatively, however, we can look further down the sun.xls spreadsheet and look at the intermediate result of the Sun's right ascension ("Alpha").  The input UT in row 2 has been adjusted so that Alpha = 90º (or 6 hours, in astronomers' lingo).  This means that the Sun has completed exactly one quarter of its annual roundtrip starting at Alpha = 0 (the vernal equinox).  This places the moment of the 2011 northern summer solstice on June 21 at 17h 16min Universal Time (10h 16min U.S. Pacific Daylight Saving Time).