What causes the true solar day to lengthen slightly when Earth is near perihelion?

Answer

The Earth's orbital speed speeds up, causing the apparent movement of the Sun to cover a greater angle.

When Earth is at perihelion, its increased orbital speed means the apparent movement of the Sun along the ecliptic covers a larger angle, lengthening the true solar day by about $10$ mean solar seconds.

What causes the true solar day to lengthen slightly when Earth is near perihelion?
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