Equinox derives from Latin for "equal night," actually referring to equal length of day and night. It is not true that day and night are exactly the same length everywhere around the globe, but they are nearly.
There are two equinoxes each calendar year. The spring or vernal equinox occurs in March, and the autumn equinox occurs in September. This year it is on Monday at 4:29 p.m., but it can occur anywhere between Sept. 21-24.
The equinoxes do not occur on the same day each year because the number of days in a solar year is a fraction greater than 365. This causes solar events such as equinoxes and solstices to creep forward about six hours per year until leap year brings them back into sync.
The equinox occurs the moment the sun crosses the celestial equator, the imaginary line above Earth’s equator. Before the September equinox, the sun rises and sets to the north; afterwards it rises and sets to the south.
On the day of the equinox, sunrise and sunset occur due east and west everywhere in the world.
Yet a glance at sunrise and sunset times reveals that the rising and setting do not occur at 6 a.m. and 6 p.m., and the length of the day is not exactly 12 hours.
There are several reasons for this.
Foremost, the geometric center of the sun is above the horizon for 12 hours. But sunrise is defined as the instant that the upper edge of the sun appears above the horizon, and sunset is the instant that the upper edge disappears below the horizon. This adds approximately four minutes to the day.
Another reason is that Earth’s atmosphere refracts sunlight to bend around the curvature of the earth. We can see the sun for several minutes before it actually appears or disappears below the horizon. This adds about six minutes to the day length.
At higher latitudes the sun approaches the horizon at an angle, so it takes longer for it to disappear once it touches the horizon, which also affects the timing of sunrise and sunset.
If Earth’s orbit around the sun were a perfect circle, the center of the sun would always rise and set at 6 a.m. and 6 p.m. But the orbit is an ellipse — almost circular but elongated enough to affect the times.
When Earth is close to the sun, it moves faster in its orbit so that one rotation does not quite catch up to the sun. The opposite is true when it is farthest from the sun and the rotation gets ahead of the sun.
This is called the Equation of Time, which is also influenced by the angle of the ecliptic — the 23.5-degree inclination of Earth’s axis to its plane of revolution. The Equation of Time causes sun time to differ from clock time by as much as 15 minutes fast or slow.
All of these effects, and more subtle ones as well, affect the time and timing of all solar events.
Richard Brill is a professor of science at Honolulu Community College. His column runs on the first and third Fridays of the month. Email questions and comments to brill@hawaii.edu.
