Planet Earth a Multitasker for Seasonal Weather and Daylight Hours

Ken Walcheck  |   Saturday Jun. 1st, 2024

The month of June is named for the Roman goddess Juno, patroness of marriage and the well-being of women.

Thursday, June 20, 2024 is when the summer solstice will occur at the same instant for all of us, everywhere on Earth, only our clocks will register different times. In the eastern United States, the solstice starts at 4:51 PM (EDT). For Montana, that time is 2:51 PM (advance one hour for states using DST). The word “solstice” comes from the Latin words sol “sun” and sister “stand still.” It’s where the Sun’s path stops advancing northward each day and appears to stand still in the sky before going back the other way.

According to the astronomical definition of seasons, the summer solstice marks the beginning of summer, which lasts until the autumnal equinox, occurring on September 22 or 23 in the Northern Hemisphere. For most of us, the June solstice simply means the start of summer, a time associated with numerous outdoor activities to look forward to, from swimming, getting a tan, or enjoying a backyard barbecue with family or friends.

Technically, the summer solstice is not the longest day of the year because all days have the same number of hours, but the summer solstice is the longest day of the year in providing us with more than 15 ½ hours of daylight. On that day, the North Pole is tilted toward the Sun, while the South Pole aims away from it. At the Arctic Circle, it’s the only day of the year in which the Sun stays above the horizon for twenty-four hours. Meanwhile, at the Antarctic Circle, it’s the only day of the year in which the Sun doesn’t rise above the horizon for twenty-four hours. For those living in the Southern Hemisphere, it’s the first day of winter, and the shortest day of the year in reference to daylight hours.

Even though we get the most hours of our sunshine on the summer solstice date, it’s not the hottest day of the year, as some might believe. Those hotter days are still typically weeks away. At the start of this event, the land and oceans are still relatively cool, due to spring’s temperatures, so the maximum heating effect on air temperature is not felt just yet. Eventually, the land, especially the oceans, will release stored energy back into the atmosphere. This usually results in the year’s hottest temperatures in late July, August, or later, depending on latitude and other factors.

If you have taken a course in earth science, astronomy, or meteorology, you learned that the Earth is a master multitasker; it orbits the Sun and rotates on its axis at the same time. As the Earth spins on its axis, producing night and day, it also moves about the Sun in an elliptical (elongated circle) orbit that requires about 365 ¼ days to complete. The Earth’s axis is tilted at 23 ½ degrees with respect to its orbital hemisphere, and it’s this tilting that causes the changes of our seasons across the globe.

Why should the tilt of the Earth’s axis matter to our weather? To better understand this, the National Weather Service suggests taking a piece of paper and a flashlight, and shining the light from the flashlight straight onto the paper, so you see an illuminated circle. All the light from the flashlight is in that circle. Now slowly tilt the paper, so the circle elongates into an ellipse. All the light is still in that ellipse, but the ellipse is spread out over more paper. The density of light drops. In other words, the amount of light per square centimeter drops, while the number of square centimeters increases, even though the total amount of light stays the same.

The same is true on the Earth’s surface. A solstice isn’t a whole day. Instead, it’s a moment —when the Sun is farthest in our sky. It’s the Earth’s tilt, not the distance from the Sun, that causes winter and summer. In fact, our planet is closest to the Sun in January, and farthest from the Sun in June, during summer in the northern hemisphere.

The days that mark the beginning of each of the four seasons are either solstices or equinoxes. These astronomical terms concern the direction the Earth is tilting in relation to the Sun at those four moments during the year. In the northern hemisphere, June and December have a solstice each, and spring vernal and autumnal equinoxes occur in March and September, respectively. During the summer solstice, the Sun is overhead at the Tropic of Cancer, and during the winter solstice it’s overhead at the Tropic of Capricorn.

What, exactly, is an equinox? It’s the time when the Sun crosses the equator at 0 o solar declination, making night and day of equal length (12 hours) in all parts of the Earth. In the northern hemisphere, the vernal equinox occurs about March 21st, the autumnal equinox about September 22nd. On these dates, the Sun is directly overhead at the equator.

The above scientific facts about solstices, equinoxes, and solar declinations may seem confusing (understandably so) to some folks. Most people, admittedly, only care about one single June solstice fact: It’s the start of summer, and time for all the outdoor fun and relaxing things we can do!  

About the Author(s)

Ken Walcheck

Ken Walcheck is a Bozeman resident, and a retired Montana Department of Fish, Wildlife and Parks Information Wildlife Biologist. He continues to write Montana natural history wildlife articles.

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