2018 June Solstice Happens Thursday, June 21, or Friday, June 22

Summary: The 2018 June solstice happens Thursday, June 21, at 10:07 Greenwich Mean Time/Coordinated Universal Time (6:07 a.m. Eastern Daylight Time).

June solstice places the noon sun directly overhead at the Tropic of Cancer: Alexander Novati, Public Domain, via NASA SciJinks

The 2018 June solstice happens Thursday, June 21, at 10:07 Greenwich Mean Time/Coordinated Universal Time (6:07 a.m. Eastern Daylight Time) as three-month successor to March's equinox and as three-month predecessor to September's equinox in Earth's orbit around the sun.

The astronomical event of the June solstice happens everywhere on Earth at the same instant. Time zones, however, convert the date and hour of the event from the world’s time standard, Coordinated Universal Time (UTC), to local time.

The 2018 June solstice occurs on Thursday, June 21, for all time zones except one. UTC+14 time zones, with local time 14 hours ahead of Coordinated Universal Time, have a 2018 solstice date and time of Friday, June 22, at 12:07 a.m.

Kiritimati, a raised coral atoll in the central Pacific Ocean’s Republic of Kiribati, observes Line Islands Time (LINT) year-round. LINT’s standard time is 14 hours ahead of Coordinated Universal Time. Kiritimati places the 2018 June solstice at 12:07 a.m. LINT, Friday, June 22.

The June solstice is an astronomical event occurring annually between June 20 and June 22. The Northern Hemisphere recognizes the June solstice as a summer solstice that marks the beginning of astronomical summer. The Southern Hemisphere observes the June solstice as a winter solstice that opens astronomical winter.

The June solstice pairs with the December solstice as the year’s two astronomical events celebrating a directly overhead sun at noon. The June solstice bestows the Northern Hemisphere’s noon, overhead sun upon the Tropic of Cancer. Also known as the Northern Tropic, the Tropic of Cancer is currently located at 23 degrees 26 minutes 14 seconds north latitude.

The Tropic of Cancer numbers among five imaginary circles of latitude on Earth. The other four circles of latitude consist of the equator, the Tropic of Capricorn and the polar circles of the Arctic and the Antarctic.

The Tropic of Cancer represents the most northerly latitudinal circle at which the sun can appear directly overhead. Its Southern Hemisphere counterpart, the Tropic of Capricorn, signifies the most southerly latitudinal circle for the sun’s overhead appearance. Also known as the Southern Tropic, the Tropic of Capricorn experiences the noon, directly overhead sun during the December solstice.

The tilt of Earth’s rotational axis is responsible for the astronomical event of the June solstice. Earth’s rotational axis tilts at an angle of 23.43711 degrees to its perpendicular orbital axis.

In Earth’s orbit around the sun, Earth’s rotational axis tilts toward the sun in June and away from the sun in December. June’s toward-tilt allows the sun’s rays to make perpendicular contact with Earth’s surface at the Tropic of Cancer. December’s away-tilt allows for sunlight’s perpendicular contact with Earth’s surface at the Tropic of Capricorn.

The June solstice effectuates the Northern Hemisphere’s longest day of the year and the Southern Hemisphere’s shortest day of the year. The Northern Hemisphere receives and reflects the most sunlight.

Six months later, the December solstice switches the effects to Northern Hemisphere’s shortest day and Southern Hemisphere’s longest day. Now the Southern Hemisphere receives and reflects the most sunlight.

Earth’s somewhat elliptical orbit around the sun yields two extreme points, known singularly as apsis (Ancient Greek: ἁψίς, hapsís, “arch, vault”) and plurally as apsides. The extreme point of aphelion (Ancient Greek: ἀπό, apó, “from” + ἥλιος, hḗlios, “sun”) registers the farthest center-to-center distance between Earth and sun. The extreme point of perihelion (Ancient Greek: περί, perí, “near” + ἥλιος, hḗlios, “sun”) references the closest center-to-center distance between Earth and sun.

Reaching the extreme points usually occurs within about two weeks of the solstices. Aphelion is attained in July, after the June solstice. Perihelion is achieved in January, after the December solstice.

Earth reaches 2018 aphelion Friday, July 6, at 16:47 GMT/UTC (12:47 p.m. EDT). Aphelion takes place 15 days 2 hours 40 minutes after the 2018 June solstice’s occurrence Thursday, June 21, at 10:07 GMT/UTC (6:07 a.m. EDT).

Earth’s 2018 perihelion happened Wednesday, Jan. 3, at 5:35 GMT/UTC (12:35 a.m. Eastern Standard Time). Perihelion took place 12 days 13 hours 6 minutes after 2017 December solstice’s occurrence Thursday, Dec. 21, at 16:29 GMT/UTC (11:29 a.m. EST).

The 2018 June solstice takes place during the moon’s first quarter phase of 50 percent surface illumination. Earth’s moon transitioned from its waxing crescent of less than 50 percent surface illumination to enter its first quarter phase Wednesday, June 20, at 10:51 GMT/UTC (6:51 a.m. EDT).

June's 2018 solstitial counterpart occurs Dec. 21, at 22:22 GMT/UTC (5:22 p.m. EST). The year's two equinoxes join the June and December solstices as seasonal markers. The March equinox announces the Northern Hemisphere's spring and the Southern Hemisphere's autumn. The September equinox ushers in the Northern Hemisphere's autumn and the Southern Hemisphere's spring. The 2018 September equinox happens Sunday, Sept. 23, at 01:54 GMT/UTC (Saturday, Sept. 22, at 9:54 p.m. EDT).

The takeaways for the 2018 June solstice that happens Thursday, June 21, or Friday, June 22, are that the first of two annual solstices opens the Northern Hemisphere’s astronomical summer and the Southern Hemisphere’s astronomical winter and that the Tropic of Cancer experiences the June event as the day of directly overhead sun at noon. Also, the Northern Hemisphere receives and reflects more sunlight than the Southern Hemisphere.

amount of sunlight reflected from Earth during the June 20, 2005, solstice: Takmeng Wong and CERES Science Team/NASA Langley Research Center, Public Domain, via NASA Earth Observatory

Acknowledgment

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

Image credits:

June solstice places the noon sun directly overhead at the Tropic of Cancer: Alexander Novati, Public Domain, via NASA SciJinks @ https://scijinks.gov/solstice/

The CERES (Clouds and Earth’s Radiant Energy System) instrument on NASA’s Terra satellite measures the amount of sunlight reflected from Earth during the June 20, 2005, solstice: Takmeng Wong and CERES Science Team/NASA Langley Research Center, Public Domain, via NASA Earth Observatory @ https://earthobservatory.nasa.gov/IOTD/view.php?id=6125&src=ve

For further information:

Byrd, Deborah. “All You Need to Know: June Solstice 2017.” EarthSky > Astronomy Essentials > Earth. June 20, 2018.
Available @ http://earthsky.org/?p=2319

Espenak, Fred. “2018 Calendar of Astronomical Events.” Astro Pixels > Planetary Ephemeris Data -- Earth > SKYCAL -- Sky Events Calendar Generator > Astronomical Events Calendars for 2011 to 2022.
Available @ http://astropixels.com/ephemeris/astrocal/astrocal2018gmt.html

Espenak, Fred. “SkyCal -- Sky Events Calendar.” NASA Eclipse Web Site.
Available @ https://eclipse.gsfc.nasa.gov/SKYCAL/SKYCAL.html

Espenak, Fred. “Solstices and Equinoxes: 2001 to 2100.” Astro Pixels > Ephemeris.
Available @ http://www.astropixels.com/ephemeris/soleq2001.html

“LINT -- Line Islands Time (Standard Time).” TimeAndDate > Time Zones > Abbreviations.
Available @ https://www.timeanddate.com/time/zones/lint

Marriner, Derdriu. “2017 June Solstice Happens Tuesday, June 20, or Wednesday, June 21.” Earth and Space News. Wednesday, June 21, 2017.
Available @ https://earth-and-space-news.blogspot.com/2017/06/2017-june-solstice-happens-tuesday-june.html

Marriner, Derdriu. "2018 Northern Hemisphere Spring Equinox Happens Tuesday, March 20." Earth and Space News. Wednesday, March 14, 2018.
Available @ https://earth-and-space-news.blogspot.com/2018/03/2018-northern-hemisphere-spring-equinox.html

Stoller-Conrad, Jessica. “What’s a Solstice?” NASA SciJinks.
Available @ https://scijinks.gov/solstice/

“Winter and Summer Solstice.” NASA Visible Earth. Dec. 21, 2005.
Available @ https://visibleearth.nasa.gov/view.php?id=6125