With the final days of spring upon us and summer approaching, people look forward to the summer solstice, which marks the official astronomical start of the new season.
In an astronomical calendar, the seasons are defined by two solstices and two equinoxes, according to the NOAA.
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The planet’s tilt and the sun’s alignment over the equator determine both the solstices and equinoxes.
People have used this observable method for thousands of years.
"The equinoxes mark the times when the sun passes directly above the equator," experts at NOAA said.
"In the Northern Hemisphere, the summer solstice falls on or around June 21, the winter solstice on or around December 22, the vernal or spring equinox on or around March 21, and the autumnal equinox on or around September 22," they continued.
The elliptical shape of Earth’s orbit around the sun causes the lengths of the astronomical seasons to vary.
The two solstices are exact moments when the sun is in direct alignment with three specific bands of latitude.
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"The summer solstice occurs when the sun is directly over the Tropic of Cancer, which is located at 23.5° latitude North, and runs through Mexico, the Bahamas, Egypt, Saudi Arabia, India and southern China," the National Weather Service explained.
The Tropic of Cancer marks the most northerly latitude at which the sun can appear directly overhead at noon.
It is worth noting that meteorologists and climatologists consider June 1 the first day of summer, as opposed to people who prefer the astronomical calendar.
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Meteorologists break down the seasons by grouping them into three months based on the annual temperature cycle as well as the calendar.
Dates and season lengths vary with astronomical seasons.
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The summer solstice is known as the longest day of the year, as many cities such as Seattle will experience as much as 16.5 hours of daylight, while New York City and Boston will see 15.5 hours of sunlight.
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The summer solstice will begin on Sunday, June 21, at 4:24 a.m. EDT.
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