The Longest Day

The summer solstice marks the official start of summer. It brings the longest day and
shortest night of the year for the 88% of Earth’s people who live in the Northern
Hemisphere.

Astronomers can calculate an exact moment for the solstice, when Earth reaches the
point in its orbit where the North Pole is angled closest to the sun. That moment will
be at 15:54 UTC on June 21. Six months from now, the sun will reach its southern
extreme and northerners will experience their shortest day of the year, at the winter
solstice.

The angle of the sun around the time of the solstice changes so gradually in relation to
the equator that the everyday observer almost can’t tell it is changing. Without
instruments, the sun appears to be in the same place for about 10 days. This is the
origin of the word solstice, which means “solar standstill.”

This slow shift means that June 21 is only about 1 second longer than June 20 at midnorthern
latitudes. It will be about a week before there’s more than a minute change to
the calculated amount of daylight. Even that’s an approximation – Earth’s atmosphere bends light over the horizon by different amounts depending on weather, which can introduce
changes of more than a minute to sunrise and sunset times.

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