Just in case you missed it, take note that the summer solstice occurred last night at 7:04 p.m. as the sun reached its northernmost point of 23.26 degrees latitude in its annual journey through the sky. As seen from our vantage point on Earth, the sun appears to move along the horizon at sunrise and sunset from day to day. Over the course of a year, sunrise will oscillate between northeast and southeast like a flat pendulum, while sunset does the same between northwest and southwest.
As seen from space, the sun’s apparent movement would trace out the ecliptic on Earth’s surface due to the 23.26-degree tilt of Earth’s rotational axis to its orbital plane.
The movement is accompanied by changes in the sun’s arc through the sky, which is most northerly in June and most southerly in December. This corresponds with the longest and shortest days due to the arc’s greatest length in June and shortest in December.
The solstices are reversed in the Southern Hemisphere with June being the winter and December being the summer.
Like a pendulum, a time-lapse movie of the sun’s position along the horizon throughout the year would be fastest in March and September at the equinoxes as the sun’s overhead position crosses the equator.
The word “solstice” is French, derived from the Latin “solstitium.” “Sol” is Latin for sun, as in solar.
“Stice” is a diachronic variation of the Latin “sistere,” “stand still,” from which we also get “static,” “stasis” and “assist.”
The sun literally stands still as it sits at nearly the same point on the horizon for two weeks, like a slow pendulum at the top of its swing.
From June 14 to 29 it varies only one-tenth of a degree, or about one-fifth of the sun’s diameter. This compares with the daily movement at the fastest part of the swing, the equinoxes, of two-tenths of a degree per day or nearly half of the sun’s diameter every day.
The imperceptible daily movement at the solstice is not visible with the naked eye, which is the main reason why our ancient ancestors had a hard time figuring the exact number of days in a year. The fact that there are actually slightly fewer than 365 and one-fourth days in a year made it even more difficult.
The Gregorian calendar solved that problem by modifying the ancient Julian calendar. It took away one leap year each century and ignored one each millennium.
Without leap year the dates of the solstices and of the corresponding equinoxes would shift forward six hours each year, gaining a day every four years. With leap year the date jumps back one day, bringing the calendar in sync with the sun.
In Hawaii we note Lahaina noon, which occurs May 26 in Honolulu as the noon sun passes overhead on its northward journey. With less notoriety, it passes overhead again July 18 as it moves southward to cross the equator at the September equinox. The technology of timepieces and calendars masks the complexity of the sun’s annual motion such that we barely take notice of its height in the sky or the location of sunrise and sunset.
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Richard Brill is a professor of science at Honolulu Community College. His column runs on the first and third Friday of the month. Email questions and comments to brill@hawaii.edu.
