Published: 2020-06-26 12:04:00
Updated: 2020-06-27 11:28:29
By Tony Rice, NASA Ambassador
Last Saturday, June 20, the Sun reached its highest point in the sky. This was also the "longest day of the year" as that high point kept the Sun above the horizon for the greatest amount of time. You might expect the earliest sunrise and latest sunset would also happen on this day, but they don't.
The earliest sunrise always happens before the solstice and latest sunset after. How far apart they occur depends on latitude. Each is separated by a little more than a week here in the Carolinas. Similarly, the latest sunrise happens a little more than a week before the December solstice and the earliest sunset a bit more than a week after.
The latest sunset of 2020 will be Sunday, June 28, just before 8:35 p.m. The Sun has been rising just a little later each day since June 12.
This happens because the Sun's path through the sky is driven not just by Earth's daily rotation, but also its yearly trip around the sun.
While 12 noon occurs every 86400 seconds (24 hours), the sun reaches that highest point in the sky for the day every 86413 seconds.
This is because Earth must rotate just a little more as it continues to move around the Sun each day to put the Sun in that highest point, also known as solar noon. This takes about 13 seconds, give or take a few microseconds this time of year. Because sunrise and sunset are spaced equally around solar noon, they also get shifted.
Henry Reich of Minute Physics created some animations that help make this a lot clearer:
All the dates, times and numbers above are based on calculations which are really just predictions. They take into account all of Earth's motion described above, plus a standard 34 arcminutes (about half a degree) for atmospheric refraction, the bending of sunlight, published by the United States Naval Observatory (USNO).
But temperature, air pressure and other atmospheric conditions at the time affect that refraction, and when actually we first and last see the Sun.