An astronomical explanation of the summer solstice was first put forward in the 1500s and 1600s by the astronomers Nicolaus Copernicus, Johannes Kepler and Galileo Galilei who disagreed that the earth was at the center of everything. They insisted that the earth was actually a planet moving around the sun. This was a controversial and dangerous theory in their time and they put their lives and professional careers in jeopardy whilst trying to maintain the certainty of their beliefs. Many religious leaders refused to accept that these theories had any truth in them, threatening Galileo and abolishing the books written by Copernicus.
Later day astronomical calculus has reinforced the accuracy of these great astronomers initial calculations and have given the scientific explanation of the validity and accuracy of the many different rituals around the world that have celebrated the beginning of the various seasons of the year since early times.
Ancient people were able to map the stars and predict the coming of the seasons by observing the path of the sun in relation to the constellations which over time became known as the Zodiac. Each year the pattern of movement in the celestial heavens although changing in appearance throughout the year, always returned to their original locations when the year had come to an end.
The Summer solstice marks the one such time of the year when the sun reaches its highest point in the sky at noon and is recognized as being the longest day in the year.
Looking from the Earth, the Sun appears to make a path through the sky in a great circle which is inclined 2326'45" from the equator. The polar axis of the earth is tilted by this amount away from the vertical of the Earth's orbit around the sun. Because of its consistency in the amount of tilt and the direction of inclination, the Sun looks like it is traveling about 23 above the equator in the northern summer and 23 below the equator in the northern winter.
The points, at which the elliptic crosses the equator in the celestial sphere, are called the equinoxes, and the points at which the sun is either furthest above or below the equator is known as the solstices. The summer solstice occurs around about the 21st of June, and is officially known as the first day of the Summer season.
A cause for great celebration, not only for past ancient cultures where the coming of the different seasons marked important events in regards to crop management and religious activity, but nowadayds it means that people can shake off the winter blues and a wet spring, and look foward to planning their vacation time.
101 Questions answered about astronomy J. S. Pickering. Lutterworth press