The 1999 summer solstice occurs today at 19:49 UT
June 21, 1999
: Later today, at 19:49 UT (3:49 p.m. EDT), Earth's north pole points more directly at the Sun than at any other time during the year. For polar bears and other denizens of the Arctic it will be noontime, the middle of a 6-month long day, as the Sun climbs to 23 1/2 degrees above the horizon.
June 21st marks the beginning of summer in the Northern Hemisphere and the beginning of winter in the Southern Hemisphere. In the North it's the longest day of the year. At mid-latitudes there is sunlight for over 16 hours. Above the Arctic Circle the sun doesn't set at all!Above:
The Earth's axis (the black line) is tilted by 23.5 degrees relative to the plane in which our planet orbits around the Sun. That's why we have seasons. For three months of the year, centered on June 21, the north pole is tilted toward the Sun and the south pole is tilted away. Six months later the situation is reversed. The tilt of the Earth's spin axis is exaggerated in this figure.
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Although the June solstice marks the beginning of Northern summer, it is often called midsummer. In traditional Gaelic culture the summer solstice represented the mid-point between the commencement of the Celtic summer on May 1 and autumn on August 1. The setting for William Shakespeare's play A Midsummer Night's Dream
takes place at this time of year.
Normally, there is nothing special about the Sun at the time of the solstice. It is high in the Northern sky and the day is generally warm simply because of the tilt of Earth's axis, not because of any special solar activity. However, next year's summer solstice will coincide with the peak of the Sun's 11 year sunspot cycle.
Left: Click on the image to see an animated gif simulating the Earth's rotation on a single day - June 21 - as seen from the Sun. The North Polar Cap is clearly visible throughout this 24 hour period - the sun doesn't set. And Antarctica cannot be seen - there, the sun doesn't rise. The images were generated by JPL's Solar System Simulator; the animated gif is 349KB.
During the "sunspot maximum" there are solar flares, coronal mass ejections, and -- of course -- lots of sunspots. The action isn't limited to the Sun. We feel the effects of the solar maximum here on Earth, too. Power grids, radio communications, and the intensity of aurorae are all affected.
Unlike the summer solstice, which is technically just an instant in time when the North Pole points directly sunward, the solar maximum lasts for many months. Even now, nearly a year before the projected peak of solar activity, there are frequent solar flares and the Sun's disk is peppered with sunspots. The image, left, shows what the sun looked like on June 20, 1999.Right
: The Midsummer Sun. This white light image from the Big Bear Solar Observatory was recorded at 14:53:08 (UT) on June 20, 1999 with a 8-bit Kodak MegaPlus 1.4i CCD camera. [
While the sun beats down on the North Pole from its highest point today, it is pitch dark at the other end of the Earth. It is midnight at the South Pole where the long night is just reaching its midpoint. The temperature there is a frigid -57 deg. C and the sun won't rise again for 3 months. Although conditions at the South Pole are undeniably harsh, astronomers are increasingly interested in making observations from Antarctica. The nights are long and the skies are dark. Southern stars are visible above the horizon for 24 hours a day during the winter, and the South Pole is very, very dry. This means that stars don't twinkle much and the atmosphere is exceptionally transparent. In the parlance of astronomy: "Seeing is good."
The images below show today's solstice as viewed from web cams both ends of the Earth. Pictured right is a vista from Tromso, Norway above the Arctic circle. Pictured left is a view from the Automated Astrophysical Site-Testing Observatory near the US South Pole Station.
: This is an image of the Automated Astrophysical Site-Testing Observatory near the US South Pole Station captured by a web cam just 3 hours before the June 1999 solstice. The image is dark because it is near the midpoint of the South Pole's 6 month long night. Click for images updated every 10 minutes. Above right
of Tromso Norway (69 42' N, 19 00' E) 3 hours before the 1999 summer solstice. Tromso is just above the Arctic Circle, so the sun does not set today.
for a live view of Tromso.
This week's sky at a glance -- from Sky &Telescope
The Sun - from the SEDS Nine Planets web site
Earth's Seasons -- A table of solstices and equinoxes from the US Naval Observatory
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|For more information, please contact:
Dr. John M. Horack , Director of Science Communications
|Author: Dr. Tony Phillips
Curator: Bryan Walls
NASA Official: John M. Horack