The Trip around the Sun
The apparent path of the Sun in the sky is known as the ecliptic. The ecliptic is actually the intersection between the Earth's orbit around the Sun and the celestial sphere. The Earth is tilted at a 23.5 degree angle from the plane of its orbit, and so the figure above shows the ecliptic inclined at that angle from the celestial equator. The ecliptic and the celestial equator intersect at two points. The first point is where the Sun crosses the celestial equator moving from south to north. This occurs at the vernal equinox on March 21, when day and night are of equal length. The vernal equinox marks the beginning of spring in the northern hemisphere. The second point of intersection, when day and night are also of equal length is the autumnal equinox on September 21. This is where the Sun crosses the celestial equator moving north to south. The autumnal equinox marks the beginning of autumn in the northern hemisphere.
Now, here is the key concept. Remember that the celestial sphere rotates around the celestial pole, a vertical axis in the figure above. As the year marches on the Sun gradually moves counterclockwise along the ecliptic, but each day the celestial sphere makes one complete rotation. So, imagine that each day as the celestial sphere rotates around the Earth, the Sun travels along a circle parallel to the celestial equator. This is not the same as the circle described by the ecliptic, as that circle describes the Sun's motion for the entire year. So, on the vernal equinox, the circle the Sun will travel along is the celestial equator itself. The next day, the circle will be slightly above (north), but parallel to the celestial equator, and each day the circle will be further above the equator. Finally on the summer solstice, the circle will be 23.5 degrees north of the celestial equator, and then in the days that follow, the circles will begin to travel back down toward the celestial equator until the autumnal equinox. The day after the autumnal equinox the circle will be below the celestial equator, and will continue to travel south until the winter solstice when it will be 23.5 degrees south of the celestial equator.
So, as you can see, as the Sun moves from the vernal equinox to the summer solstice, its maximum altitude in the sky increases day after day until it reaches a maximum on the summer solstice, which occurs on June 21 for the northern hemisphere. Then, as the Sun travels toward the autumnal equinox, its maximum altitude begins to decrease day after day until it reaches a minimum on the winter solstice, which is on December 21 in the northern hemisphere. For the southern hemisphere the solstices are reversed, with their summer solstice occurring on December 21 and their winter solstice on June 21.
Near to the poles on Earth, changes in the maximum altitude of the Sun have some interesting effects. For example on the Arctic Circle, at 66 degrees and 34 minutes north latitude, the sun never sets on the day of the summer solstice. On the day of the winter solstice, it never rises. Fairbanks, Alaska is only 125 miles south of the Arctic Circle, so its residents see the fringe of this effect having nearly 22 hours of sunlight on June 21 and only 4 hours of sunlight on December 21. The further north you travel, the more pronounced this effect gets. For example, Barrow, Alaska is in the extreme northern part of the state, at 71 degrees and 18 minutes north latitude, and here the Sun is continuously in the sky for 85 days from May 10 to August 2. The Sun never rises for 67 days from November 18 to January 23. Places on the Earth south of the Antarctic Circle at 66 degrees and 34 minutes south latitude experience the same effects, though at the opposite times of the year.
At the summer and winter solstices the Sun is directly overhead at noon at the Tropics of Cancer and Capricorn, respectively, named after the zodiacal constellations associated with those parts of the ecliptic where the Sun is at these times.
The Four Seasons
A common misconception is that the seasons are cause by changes in the distance of the Earth from the Sun. This is not so. The Earth is tilted at 23.5 degrees to the plane of its orbit around the Sun. As the Earth revolves around the Sun, this tilt causes the Sun to strike the surface of the Earth at different angles during different times of the year, pictured above. Remember, at the summer solstice the maximum altitude of the Sun in the sky is the highest it will be for the entire year. So, at this time of the year the sunlight is striking the surface of the northern hemisphere at a more direct angle, which allows more thermal energy to pass through the atmosphere warming the earth below. The amount of solar energy transferred to an area is called insolation. Also, the day of the summer solstice is the longest of the year, meaning that there are more hours of sunlight heating the surface of the Earth. These two factor are what cause the seasons. They are at their maximum during summer solstice and at their minimum at winter solstice.
The Earth-Sun Distance
The Earth's orbit is slightly elliptical, so that the Earth is closer to the Sun at some times, and farther away at others. The point in the Earth's orbit when it is as the greatest distance from the Sun is called the aphelion. The point when the Earth is closest to the Sun is called the perihelion.
You now know that the Sun's position in the sky changes throughout the year due to the 23.5 degree tilt of the Earth's axis. The picture above is a spectacular illustration of this fact, as it shows the Sun's position in the sky over the course of one year taken at the same time each day. The shape traced out by the sun is called an analemma. The highest point in the analemma occurs during the summer, the lowest during the winter. Because the Earth's orbit around the Sun is elliptical, the figure-eight shape of the analemma is distorted. If the orbit was circular the two loops of the analemma would be equal in size. Analemma photographs taken from different latitudes or at different times of day would appear slightly different.
Here is an image of the Martian Analemma.
Images are shown here for noncommercial educational purposes.
The image of the ecliptic intersecting with the celestial sphere is from an online astronomy course Birth and Death of Stars presented by Dr. James Schombert at University of Oregon.
The image of the four seasons if from an Astrophysical and Planetary Sciences slide at the University of Colorado.
The image of the analemma was taken by Vasilij Rumyantsev of the Crimean Astrophysical Observatory and was found on NASA's Astronomy Picture of the Day site.