Northern Hemisphere 2017 Summer Solstice Happens Wednesday, June 21

Summary: The Northern Hemisphere 2017 summer solstice, which opens astronomical summer, happens Wednesday, June 21, according to Coordinated Universal Time.

axial tilt, seasons and solstices; orthographic projection rendered from NASA Blue Marble Next Generation: cmglee, NASA, Public Domain, via Wikimedia Commons

The Northern Hemisphere 2017 summer solstice, which opens astronomical summer for northern latitudes, takes place Wednesday, June 21, at 4:24 a.m., according to Coordinated Universal Time (UTC), the world’s official time standard.

The mid-year event that opens astronomical summer in the Northern Hemisphere is also known as the June solstice or as the northern solstice. The Northern Hemisphere experiences the June solstice as a summer solstice. The Southern Hemisphere, where seasons contrast with those experienced in northern latitudes, experiences the June solstice as a winter solstice. The June solstice opens astronomical winter in the Southern Hemisphere.

Earth’s local time zones either place the astronomical event on the previous day, Tuesday, June 20, or agree with the UTC’s Wednesday date. The highest time zone, which is set 14 hours ahead of Coordinated Universal Time, experiences the 2017 June solstice on Wednesday.

The June solstice’s designation as the northern solstice references the sun’s appearance directly over the Northern Hemisphere’s Tropic of Cancer. Known as the Northern Tropic, the Tropic of Cancer is located at 23.43709 degrees north latitude. The Northern Tropic marks the most northerly circle of latitude at which the sun reaches its zenith. At the Tropic of Cancer, sun’s rays touch the Earth’s surface perpendicularly rather than obliquely.

Earth’s rotational axis is an imaginary line, running through the North Pole and the South Pole, around which the Blue Planet rotates. Earth’s rotational axis tilts with respect to the ecliptic, plane of Earth’s orbit around the sun. The tilt of the rotational axis is termed obliquity (Latin: obliquus, “oblique”) because Earth’s rotational axis forms an oblique, or slanted, angle with the plane of the ecliptic.

Earth’s current obliquity angle measures about 23.4 degrees. Earth’s obliquity angle changes cyclically. The axial tilt varies between 22.1 and 24.5 degrees during a cycle averaged at about 40,000 years.

During the northern solstice, Earth displays the maximum tilt of the north polar axis toward the sun. The tilt favors Earth’s Northern Hemisphere with astronomical summer. During the northern solstice, Earth’s south polar axis displays its maximum tilt away from the sun. This tilting away ushers in astronomical winter for the Southern Hemisphere.

The tilt of Earth’s rotational axis interacts with light coming from the sun to affect the angle formed by sunrays with Earth’s equatorial plane. The angle, known as the sun’s declination, displays variation through the year. The sun’s declination ranges from plus 23.4 degrees to minus 23.4 degrees.

Solar declination of plus 23.4 degrees occurs during the June solstice. This declination is responsible for over 12 hours of daylight in Northern Hemisphere locations and less than 12 daylight hours in the Southern Hemisphere.

Solar declination of minus 23.4 degrees happens during the December solstice, the last of the year’s two solstices. This declination is responsible for over 12 hours of daylight in Southern Hemisphere locations and less than 12 daylight hours in Northern Hemisphere locations. As the opposite of the June solstice, the December solstice opens astronomical winter in the Northern Hemisphere and astronomical summer in the Southern Hemisphere.

The takeaway for the Northern Hemisphere 2017 summer solstice is that Coordinated Universal Time’s Wednesday, June 21, astronomical event concerns Earth’s seasons by welcoming astronomical summer in the Northern Hemisphere and astronomical winter in the Southern Hemisphere.

tilted reason for Earth’s seasons: NASA Science, Public Domain, via NASA Space Place

Acknowledgment

My special thanks to talented artists and photographers/concerned organizations who make their fine images available on the internet.

Image credits:

axial tilt, seasons and solstices; orthographic projection rendered from NASA Blue Marble Next Generation: cmglee, NASA, Public Domain, via Wikimedia Commons @ https://commons.wikimedia.org/wiki/File:Axial_tilt_vs_tropical_and_polar_circles.svg

tilted reason for Earth’s seasons: NASA Science, Public Domain, via NASA Space Place @ http://spaceplace.nasa.gov/seasons/en/

For further information:

“11 Things About the June Solstice.” Time And Date > Sun & Moon.
Available @ https://www.timeanddate.com/astronomy/facts-about-june-solstice.html

Graham, Steve. “Milutin Milankovitch (1879-1958): Orbital Variations.” NASA Earth Observatory > Features. March 24, 2000.
Available @ http://earthobservatory.nasa.gov/Features/Milankovitch/milankovitch_2.php

“June Solstice: Longest and Shortest Day of the Year.” Time And Date > Sun & Moon.
Available @ https://www.timeanddate.com/calendar/june-solstice.html

Marriner, Derdriu. “2016 June Solstice Signals Summer Start But Also Begins Summer End.” Earth and Space News. Wednesday, June 15, 2016.
Available @ https://earth-and-space-news.blogspot.com/2016/06/2016-june-solstice-signals-summer-start.html

Marriner, Derdriu. “Full Strawberry Blue Moon Seasonally Welcomes 2016 June Solstice.” Earth and Space News. Wednesday, June 8, 2016.
Available @ https://earth-and-space-news.blogspot.com/2016/06/full-strawberry-blue-moon-seasonally.html

Marriner, Derdriu. “Martian New Year and Summer Solstice 2015: Happy New Year From Mars.” Earth and Space News. Saturday, June 20, 2015.
Available @ https://earth-and-space-news.blogspot.com/2015/06/martian-new-year-and-summer-solstice.html

Marriner, Derdriu. “Two Weeks After 2016 June Solstice Earth Reaches 2016 Aphelion.” Earth and Space News. Wednesday, June 29, 2016.
Available @ https://earth-and-space-news.blogspot.com/2016/06/two-weeks-after-2016-june-solstice.html

“What Causes Seasons on Earth?” Time And Date > Sun & Moon.
Available @ https://www.timeanddate.com/astronomy/seasons-causes.html

“What Is Earth’s Axial Tilt or Obliquity?” Time And Date > Sun & Moon.
Available @ https://www.timeanddate.com/astronomy/axial-tilt-obliquity.html

“Why Is It Hot in Summer and Cold in Winter?” The Library of Congress > Researchers > Science Reference Services. Jan. 21, 2011.
Available @ https://www.loc.gov/rr/scitech/mysteries/seasons.html