The Earth warms up in the summertime for two reasons: because the Sun is higher in the sky, and also because days are longer. Astronomically, both of these reasons are because the Sun has been hanging out near the summer solstice.
The summer solstice is one of the two moments in the year when the Sun's apparent motion around the Earth takes it as far from the equator as it ever goes, when it turns back and begins to approach the equator. In the northern hemisphere the summer solstice is the moment in late June when the Sun is farthest north of the equator; in the southern hemisphere it is the moment in late December when the Sun is farthest south of the equator. With apologies to our friends south of the equator, we're going to talk about the situation in the northern hemisphere (almost all of the Adler collection was made by and for people in the northern hemisphere).
The Sun's passage through the summer solstice technically marks the beginning of summer. It is the longest day of the year. The hottest days of the year, when we say it is summer, follow the summer solstice. That's because the Earth, its atmosphere, and its oceans retain their heat for a while after the actual solstice.
On some of them it is easy to identify the representation of the summer solstice. Earth-centered armillary spheres, as shown in figure 1, are perhaps the most intuitive depictions of early ideas about the Universe. On such an armillary sphere, a broad band decorated with the signs of the zodiac indicates the apparent path of the Sun and planets around the Earth. The summer solstice is the place where this zodiacal band reaches farthest north of the equatorial ring, and momentarily runs parallel to it.
Representation of the summer solstice is not always so obvious. Celestial charts employ a great variety of map “projections” to flatten the celestial sphere onto a sheet of paper, so finding the summer solstice can take a bit of thought. On the celestial chart shown in figure 2, again from the Adler collections, the sky and its constellations are flattened onto a pair of large circles, representing the northern and southern hemispheres of the sky. (To save space we show only the circle for the northern hemisphere.)
So where is the summer solstice in figure 2? The solstice is a point on the ecliptic, so first we must locate the ecliptic. We observe that the familiar figures representing the “signs of the zodiac” are shown (and named) around the circumference of the circle, so on this chart the circumference must represent the ecliptic.
In order to find the summer solstice, we need to find the celestial north pole, because the solstice is the point where the ecliptic most closely approaches the north pole.
You do not have to know all the stars in the sky to identify the celestial north pole if you simply remember that the pole star is the star at the tip of the Little Bear's tail. It's easy to spot the Big Bear on the chart; if you look in that neighborhood you'll have no trouble picking out the Little Bear. (It is facing straight down, with its unnaturally long tail stretching up and to the left.) At the tip of the Little Bear's tail is a star that is helpfully labeled “Stella Polaris.” It is almost directly left of center in the big circle of northern constellations. The pole itself, marked “Polus Arct” is directly on the center line of the chart.
That tells us just where to look for the summer solstice on the circumference of the big circle. The summer solstice is the point on the left side of the big ecliptic circle, right where the horizontal diameter meets the ecliptic. On the constellation figures, the solstice falls on the back of the left calf of Castor, one of the Twins in the constellation Gemini. Alas, there is no Solstice Star bright enough to be marked on the chart.
Written by Bruce Stephenson, former curator, the Webster Institute for the History of Astronomy at the Adler Planetarium.