<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>