Science of the Summer Solstice

SCIENCE OF THE SUMMER SOLSTICE

The word Solstice comes from the Latin solstitium meaning “Sun, standing-still.” This year the Summer Solstice occurs on June 21 at 05:04 UTC, or Coordinated Universal Time, or Zulu Time, or roughly Greenwich Mean Time.

This is also known as the Northern Solstice as the Sun is positioned directly above the Tropic of Cancer in the Northern Hemisphere. This time of year is known as Midsummer, though the official Midsummer Day is actually celebrated on June 24, thanks to differences between the Julian and Gregorian calendars. Christian festivals during this time of year are related to the Birth of St. John the Baptist. In Bolivia and Peru it’s called the Festival of San Juan.

Earth enjoys different seasons because the planet is tilted 23 degrees and 27 minutes off the perpendicular to the plane of orbit. This means that the earth revolves like a tilted spinning top. The Summer Solstice is the longest day of sunlight as the Sun is at its highest arc in the sky, at least in the Northern Hemisphere. The farther north one is from the Equator, the more pronounced this is in Summer. However, as the Earth continues its orbit, the hemisphere that is angled closest to the sun changes, and the seasons are reversed.

In the Northern Hemisphere the sun appears at its highest point in the sky, and its noontime elevation appears to be the same for several days before and after the solstice, so that it looks like the Sun is “standing still” until following the Summer Solstice, the days begin to grow shorter and the nights longer.

Bill Petro, your friendly neighborhood historian
www.billpetro.com

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