Here's why:

* Day Length is Determined by Rotation: The length of a day is defined by the time it takes a planet to complete one full rotation on its axis. This is an internal property of the planet itself, not influenced by the sun's distance.

* Orbital Period is Affected by Distance: A planet's distance from the sun *does* affect its orbital period, which is the time it takes to complete one full orbit around the sun. This is what determines the length of a year.

Think of it this way:

Imagine a spinning top. The speed at which the top spins (its rotation) determines how long its "day" is. The distance the top is from you doesn't change how fast it spins.

Example:

* Earth's day is approximately 24 hours long, regardless of whether we're closer to the sun (perihelion) or farther away (aphelion).

* Mars has a day that's slightly longer than Earth's (24.6 hours), despite being farther from the sun.

However, there are some indirect connections:

* Tidal Forces: A planet's distance from the sun can have a slight effect on the rate of its rotation over very long periods due to tidal forces. However, this effect is usually very small compared to other factors.

* Solar Wind and Radiation: The sun's activity (like solar flares and solar wind) can affect a planet's atmosphere, which could, in theory, influence its rotation over time. But this is also a complex and long-term effect.

In conclusion, the primary factor determining a planet's day length is its rotational speed, which is independent of its distance from the sun.