Question

Why the distance between mars and Jupiter is changing?

Answer 1

Planets orbit around their star

Explanation:

Planets orbit at different speeds; the farther out the planet, the slower orbital speed it has. The orbital speed of Earth is around 30km/s while the outermost planet, Neptune, has an orbital speed of around 5km/s. Their orbital period in relation to their distance from the Sun can be calculated with Kepler's 3rd Law.

What this means is that as planets move around the Sun with different speeds, their position relative to each other is always changing, thus their distance does not stay the same. If Mars is aligned with Jupiter, then the distance between them is less than if they are at opposite ends of each other.

tl;dr: Planets are at different distances from the Sun and have different orbital speeds, so the distance between planets are always changing.