The December solstice occurs Saturday at 6:20 p.m. Hawaii time. At this instant, the sun will reach its southernmost point on the ecliptic and start northward again.

The ecliptic is the apparent path of the sun as viewed from Earth’s perspective due to the combined effect of the tilt of its axis to the plane of its revolution around the sun.

It is reportedly the shortest day of the year in the northern hemisphere, but the longest day in the southern hemisphere. In reality, the day length is the same within one minute from Dec. 17 to Dec. 24, about 10 hours and 50 minutes in Honolulu.

The multiple “longest” days are due to the meaning of “solstice.” It derives from Latin words meaning ‘sun stands still’. The apparent motion of the sun along the horizon at sunrise and sunset is like a pendulum that slows down and stops instantaneously at the two end points as it swings.

From day to day the sun’s motion along the horizon is faster near the center of the swing of the pendulum, which happens at the two equinoxes in March and September as the sun crosses the equator to begin slowing down in the opposite hemisphere until the two solstice in December and June, when it stops momentarily.

The actual day length (from sunrise to sunset) does change, but only by a matter of seconds. Today the day will be 10 hours, 48 minutes, 51 seconds long. That is 31 seconds longer than Thursday and 29 seconds shorter than Sunday.

One might expect that the December solstice would mark the latest sunrise and earliest sunset. This would be true if Earth’s orbit were perfectly circular, but there are no perfectly circular orbits anywhere in the solar system.

Earth’s orbit is elongated elliptically, but only slightly. If the orbit were circular then Earth would rotate exactly 360/365.25 degrees per day, slightly less than one degree per day, about 0.9856 degrees per day. As Earth moves slower in its orbit near aphelion, it still rotates on its axis at the same rate, so it must turn less to complete a solar day near aphelion than at perihelion.

Eccentricity is the measure of the ellipse, where zero is circular and one is a straight line.

Jupiter’s gravity causes the Earth’s orbit to vary from nearly circular with an eccentricity of 0.005 to quite elliptical with an eccentricity of 0.06. Currently, we enjoy an eccentricity of 0.0174, nearly its minimum. At present, there is only a 3% difference in the distance to the sun at perihelion and aphelion.

Because Earth is closest to the sun (aphelion) in January, the sunrise and sunset times migrate toward a latest sunrise at 7:11 a.m. from Jan. 7 to 25 as the Earth passes through perihelion. By contrast, the earliest sunset occurs at 5:48 p.m. from Dec. 20 to Jan. 1 differing by seconds every day just like this year.

The date and time of the solstice, and all of the seasons, drifts as Earth’s clock gets out of sync with the calendar until leap year brings it back in sync every four years. In 2020 the solstice will be about six hours later at 12:04 a.m. on Dec. 21. This is about one-fourth of a day later.

Richard Brill is a retired 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.