<p><strong>A Russian pagan puts on a fiery show Saturday during Kupalo, a summer solstice festival held in a forest about 37 miles (60 kilometers) from <a id="h2-l" title="Moscow" href="http://travel.nationalgeographic.com/travel/city-guides/moscow-russia/">Moscow</a>.</strong></p><p>(See <a id="qx3-" title="&quot;Summer Solstice 2010: Why It&amp;squot;s the First Day of Summer.&quot;" href="http://news.nationalgeographic.com/news/2010/06/100621-summer-solstice-2010-first-day-of-summer-longest-year-science/">"Summer Solstice 2010: Why It's the First Day of Summer."</a>)</p><p class="Body">The first day of summer officially kicked off today at 7:28 a.m. ET, the beginning of the summer solstice and the longest day of the year in the Northern Hemisphere.</p><p>The summer solstice is a result of <a href="http://science.nationalgeographic.com/science/space/solar-system/earth.html?nav=A-Z">Earth</a>'s north-south axis being tilted 23.5 degrees relative to the <a href="http://science.nationalgeographic.com/science/space/solar-system/sun-article.html">sun</a>, experts say. This tilt causes different amounts of sunlight to reach different regions of the planet over the course of the year. Today the <a href="http://news.nationalgeographic.com/news/2004/11/1108_041108_north_pole.html">North Pole</a> is tipped closer to the sun than on any other day of 2010. The opposite holds true for the Southern Hemisphere, for which today is the <a href="http://news.nationalgeographic.com/news/2009/12/091221-winter-solstice-2009-first-day-winter-shortest-day-year.html">winter solstice</a>, the shortest day of the year.</p>

Pagan Fire Show

A Russian pagan puts on a fiery show Saturday during Kupalo, a summer solstice festival held in a forest about 37 miles (60 kilometers) from Moscow.

(See "Summer Solstice 2010: Why It's the First Day of Summer.")

The first day of summer officially kicked off today at 7:28 a.m. ET, the beginning of the summer solstice and the longest day of the year in the Northern Hemisphere.

The summer solstice is a result of Earth's north-south axis being tilted 23.5 degrees relative to the sun, experts say. This tilt causes different amounts of sunlight to reach different regions of the planet over the course of the year. Today the North Pole is tipped closer to the sun than on any other day of 2010. The opposite holds true for the Southern Hemisphere, for which today is the winter solstice, the shortest day of the year.

Photograph by Konstantin Zavrazhin, Getty Images

Summer Solstice 2010 Pictures: Fire Rites, Druids, More

Pagans play with fire, druids flock to Stonehenge, and revelers build a "friendship nest" to mark the first day of summer 2010.

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