Originally published on June 17, 2023 at Guy Ottewell’s blog. Edits by EarthSky.
The equation of time
The size of our day is 24 hours. Sure, however that’s solely its imply size, the average time between noons, or the sun’s crossings of the meridian. Should you time these crossings with a stopwatch, you discover that the sun typically reaches the meridian barely early. And it typically it crosses the meridian barely late. So, the noticed, or obvious, lengths of the day varies.
That is partly as a result of the Earth travels barely sooner in the course of the internal half of its barely elliptical orbit across the sun. That’s after we’re close to perihelion – its closest level to the sun in its orbit round January 3 – than within the outer half (aphelion). Nonetheless, Earth rotates at a continuing pace.
Earth’s tilt is a significant factor
However the bigger motive is Earth’s obliquity: the tilting of its north pole 23.4° away from the sun across the December solstice and towards it across the June solstices. Across the occasions of the December and June solstices (from early November to early February, and from early Could to early August), the sun travels throughout the traces of right ascension on the map of the sky sooner than it’s touring in longitude.
Then across the March equinox (from early February to early Could), its course slopes north in right ascension. And across the September equinox (from early August to early November) it slopes south, so at these occasions the sun’s right ascension modifications extra slowly than its longitude.
One other approach of placing it’s that, because the sun strikes one day’s journey alongside the ecliptic, it strikes kind of than that angular distance in right ascension, which is the measure of when it’s on the meridian at midday.
The distinction between obvious solar time (the precise time between noons) and imply solar time (the common time between noons) known as the equation of time. This outdated use of equation implies that by subtracting it from the various obvious time you equalize that to the common time.
The outcomes: Minima and maxima
The 2 components above mix to provide a curve exhibiting two minima and two maxima.
The dates can differ barely from yr to yr due to leap days. These are the dates for 2020:
February 11 is at minimal for the yr -14.24 minutes.
April 15 is 0.
Could 13 reaches a shallow most of three.65 minutes.
June 12 is 0.
July 25 reaches a shallow minimal of -6.55 minutes.
September 1 is 0.
November 2 is at most for the yr, 16.49 minutes.
December 24 is 0.
Then, there comes a small most in Could (at imply midday the sun is 4 minutes previous the meridian). Subsequent, it’s adopted by a smaller minimal in July (the sun is 7 minutes late reaching the meridian). And a yr’s most is in November (the sun is 16.5 minutes early crossing the meridian). Lastly, the dates in between – April, June, September, December – are when the equation is zero. So the noon sun is on time!
Now factoring in axial tilt and eccentricity
These are the phrases of good friend Anthony Barreiro. He instructed the addition of the curves for the 2 elements, and I can’t enhance on his clarification.
You may see why the February minimal and November most are excessive, whereas the Could most and July minimal are average, and likewise how the equation of time is generally attributable to obliquity (axial tilt), with eccentricity pulling it first a technique then the opposite.
Backside line: Man Ottewell discusses the distinction between obvious solar time (the precise time between noons) and imply solar time (the common time between noons). And it’s referred to as the equation of time.
