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Earth from Orbit: Summer Solstice

June 24, 2022
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June 21 marked the official start of astronomical summer in the Northern Hemisphere. The summer solstice — the longest day and shortest night of the year — occurred at 5:14 a.m. EDT. 

The seasons change at various times of the year due to the planet’s axial tilt of 23.5 degrees as it orbits the sun. This tilt means the two hemispheres are exposed to different sun angles and variable lengths of daylight throughout the year. The solstices mark when the sun reaches its most northerly or southerly position relative to the Earth’s equator. 

Thus, the summer solstice in the Northern Hemisphere is the exact moment each year when this region of Earth reaches its greatest possible tilt toward the sun. The sun's direct rays reach their northernmost position with respect to Earth's equator, along the Tropic of Cancer, at 23.5 degrees north latitude. As the Earth rotates on its axis, the North Pole experiences 24 hours of daylight, while the South Pole is obscured in darkness. The opposite occurs at each pole in December, when the Northern Hemisphere sees its shortest day and longest night of the year.

Of course, "astronomical" summer is not to be confused with "meteorological" summer, a term used by meteorologists and climatologists, who break the seasons into three-month periods based on the annual temperature cycle and the calendar. We generally think of winter as the coldest time of the year and summer as the warmest time of the year, with spring and fall being the transition seasons, and that is what the meteorological seasons are based on. 

Meteorological spring includes March, April, and May; meteorological summer includes June, July, and August; meteorological fall includes September, October, and November; and meteorological winter includes December, January, and February.

On the contrary, the natural rotation of Earth around the sun forms the basis for the astronomical calendar, in which we define seasons with two solstices and two equinoxes. Earth’s tilt and the sun’s alignment over the equator determine both the solstices and equinoxes. Therefore, astronomical summer runs from June 21, 2022 to September 23, 2022.

From their position 22,236 miles above the equator, NOAA’s GOES-16 and GOES-17 satellites orbit at the same rate Earth rotates, so they can keep constant watch over the same region. This allows them to view the terminator—the edge between the shadows of nightfall and the sunlight of dusk and dawn—as it moves across the Western Hemisphere. The slope of the terminator curve changes with the seasons.