There’s another factor - days where thr earth is orbiting faster, eg on the closer side of the ellipse - are a different length midday to midday from when we are on the far side of the ellipse.
You can convince yourself of this when you consider that the area of the arc we traverse each day is the same (Kepler’s law). On the short side of our eliptical orbit, since the orbital distance is shorter, the arc must have a larger angle that we travel. That means the amount a point on the earth rotates to have the sun come back directly overhead must be different in different parts of the year.
This difference, summed day over day, results in a +/- 20 min movement of actual midday to 12pm. The ‘mean’ in Greenwich Mean Time refers to averaging this difference over the whole orbit.
There’s another factor - days where thr earth is orbiting faster, eg on the closer side of the ellipse - are a different length midday to midday from when we are on the far side of the ellipse.
You can convince yourself of this when you consider that the area of the arc we traverse each day is the same (Kepler’s law). On the short side of our eliptical orbit, since the orbital distance is shorter, the arc must have a larger angle that we travel. That means the amount a point on the earth rotates to have the sun come back directly overhead must be different in different parts of the year.
This difference, summed day over day, results in a +/- 20 min movement of actual midday to 12pm. The ‘mean’ in Greenwich Mean Time refers to averaging this difference over the whole orbit.