WRAL WeatherCenter Blog

June solstice marks the longest day of the year...usually

Posted June 21

There are many different definitions for when summer starts. The WRAL meteorologists look to June 1 as the start of meteorological summer.

Wake County Public School System students have five different dates for the start of their summer break. Astronomers look to the June solstice for the beginning of summer in the northern hemisphere.

The June solstice occurs at 12:24 a.m. on Wednesday, June 21, marking the beginning of astronomical summer. The Aug. 21 total eclipse is also just two months away.

The solstice occurs the moment the sun is directly over the Tropic of Cancer (23 degrees north latitude). Anyone standing on this line at solar noon, when the sun is at its highest, will cast no shadow.

This is also when the Earth's tilt toward from the sun is at its relative maximum. It’s this 23.5-degree tilt that creates the seasons.

EUMETSAT

Our planet has an elliptical orbit that varies the distance from the sun. Earth’s distance from the sun has very little effect on the seasons. Now that astronomical summer is here and temperatures continue to rise, you might think Earth is near its closest point to the sun. But we are actually two weeks away from reaching the furthest point.

Earth reaches aphelion on July 3, 2017, or the furthest point from the sun in its orbit. By the Jan. 4, 2018 Perihelion, Earth will be 3 million miles closer to the sun, and then the cycle starts all over again.

Aphelion/Perihelion

The solstice is also generally the longest day of the year, but not in 2017.

To accurately calculate the time the sun is visible above the horizon (day length), you must to take into account the atmospheric distortion, or the bending of sunlight through the atmosphere. This distortion is significant, adding over six minutes to the time the sun is above the horizon this week. But this happens at every sunrise and every sunset.

The key to which day will be the longest is when the solstice occurs. The longest day is the one which the rising or setting of the sun occurs closest to the solstice.

This year, the solstice occurred at 12:24 a.m. on June 21. June 20 was about a second longer. Next year, the solstice will occur just after sunrise at 6:07 on June 21 so that day will get the extra second, at least according to the mathematical formulae that predict these things.

Ultimately, when you see the sunrise, and the amount of daylight that day is a function of the atmospheric pressure at the time.


Tony Rice is a volunteer in the NASA/JPL Solar System Ambassador program and software engineer at Cisco Systems. You can follow him on Twitter @rtphokie.

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