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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.
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. [larger
image]
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.
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Above Left: 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: A web cam
view 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. Click
for a live view of Tromso. |
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.
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