Summer Solstice 2013: Northern Hemisphere Welcomes Longest Day Of The Year

on June 20 2013 3:01 PM
The sun rising over Stonehenge on the summer solstice, 2005 Wikimedia Commons/Andrew Dunn

Summer is fully upon the Northern Hemisphere this week, no matter what the thermometer says.

This year’s summer solstice will occur at exactly 1:04 a.m. Eastern time Friday morning. At that moment, the sun will seem to pass over the Tropic of Cancer, the imaginary line that runs through Mexico, the Sahara Desert and India. For all points north of the Tropic of Cancer, Friday will be when the sun is at its highest point in the sky. And for most places in the Northern Hemisphere, Friday will be the longest day of the year: 16 hours, 10 minutes, 49 seconds in Paris; 15 hours, 5 minutes, 39 seconds in New York City; 13 hours, 16 minutes, 38 seconds in Mumbai.

After the summer solstice passes, the sun’s apparent path through the sky will move south again, and the days will become shorter. As Earth keeps spinning toward the winter solstice, the sun’s apparent path will continue to move south until it crosses the equator on September 22, the autumnal equinox.

Whether the summer solstice represents the first day of summer or midsummer is more a matter of opinion or culture than science. While many Americans refer to the solstice as the first day of summer, like dubbing the vernal equinox the first day of spring, European celebrations of the solstice (often involving bonfires) are meant to mark Midsummer’s Day.

Though the summer solstice is the longest day of the year, it's not always the warmest day of the year. And there’s more to that discrepancy than the whims of weather.

For instance, in Albuquerque, N.M., “the maximum daily temperature occurs nearly 3 weeks [after the summer solstice], in mid-July,” the Albuquerque outpost of the National Oceanic and Atmospheric Administration writes. “This lag in temperature occurs because even though the minutes of daylight are decreasing, the earth's surface and atmosphere continues to receive more energy than just what it receives from the sun.”

Another upcoming celestial quirk involves Earth’s distance from the sun, which varies thanks to our planet’s elliptical orbit. At the time of the summer solstice, the Earth is actually nearing aphelion, the point of its orbit where we are furthest from the sun. At aphelion, which usually falls sometime near July 4, the Earth will be 1.6 million miles further from the sun than its normal average of 93 million miles. That might not seem to square with the warmth of summer, but our seasons are not primarily determined by Earth’s distance from the sun. The reason for the season is the Earth’s 23.5-degree tilt on its axis, which is always pointing in the same direction. Summer for the Northern Hemisphere comes when the planet’s northern half is inclined toward the sun, as shown in the NASA graphic below:

nasaseasons An illustrated guide explaining how axial tilt is the reason for the season.  NASA