There are two solstices in a year, the June solstice (between June 20-22) and the December solstice (between December 20-23). On the June solstice, the Sun reaches its highest point in the sky for an observer at the North Pole. On the December solstice, the Sun reaches its highest point in the sky for an observer at the South Pole. Solstices also mark the change from fall to winter or spring to summer.
The solstices happen because of the tilt of Earth’s axis of rotation with respect to its orbital path around the Sun. The axis of rotation is an imaginary line drawn straight through Earth connecting the North Pole to the South Pole. This line is tilted 23.5° from Earth’s orbital path around the Sun. This means that the Sun’s light does not hit the Earth evenly at a particular latitude above and below the equator. This tilt is the cause of Earth’s seasons.
The equator is an imaginary line drawn right around Earth’s middle, like a belt. It divides Earth into the Northern and the Southern Hemispheres. The imaginary lines around Earth that are parallel to the Equator are called lines of latitude. They are numbered from 0° to 90°. The one at 0° is the equator itself. The higher the number, the farther north (if it’s a + number) or south (if it’s a − number).
Because of Earth’s tilt is 23.5° the latitudes of +23.5° and −23.5° are special and have special names. The one in the Northern Hemisphere, +23.5° latitude, is called the Tropic of Cancer. The one in the Southern Hemisphere, − 23.5° latitude, is called the Tropic of Capricorn.
These are the latitudes where the Sun is directly overhead at noon once a year. In the Northern hemisphere, on the Tropic of Cancer, that is the June Solstice. In the Southern Hemisphere, on the Tropic of Capricorn, that is the December Solstice. These solstice days are the days with the most (for Summer) or fewest (for Winter) hours of sunlight during the whole year.
The Sun gives us heat and light, which changes over the seasons. The same is true over the solar activity cycle. Instead of solar seasons changing over a year, they change over about 11 years. We call this the solar activity cycle. During this 11-year period, the Sun goes from very low activity with little or no sunspots to high activity with a lot of sunspots, solar flares, and explosions of material back to low activity at the end.
Over the course of each cycle, the star transitions from relatively calm to active and stormy, and then quiet again; at its peak, the Sun’s magnetic poles flip. Now that the star has passed solar minimum, scientists expect the Sun will grow increasingly active in the months and years to come.
Understanding the Sun’s behavior is an important part of life in our solar system. The Sun’s outbursts—including the releases of huge amounts of light and material known as solar flares and coronal mass ejections—can disturb the satellites and communications signals traveling around Earth, or one day, Artemis astronauts exploring distant worlds. Scientists study the solar cycle so we can better predict solar activity.
Not only is this time of year exciting because of the June solstice, but it also corresponds to the release of a new set of forever stamps by the United States Postal Service, the Sun stamp series on June 18, 2020.
Credit: NASA Goddard Space Flight Center, NASA Scientific Visualization Studio, and the Royal Observatory of Belgium – Solar Influences Data analysis Center