A planetary parade, also called a planetary alignment, is a sky event in which several planets of the Solar System become visible together across the same broad arc of the sky at roughly the same time.

All planets travel around the Sun in nearly the same flat plane called the ecliptic, so from the Earth they always appear strung along a single line across the sky rather than scattered randomly.

During the early 2025 event, planets such as Mercury, Venus, Mars, Jupiter and Saturn were seen arranged in the order of their distance from the Sun, that is in the sequence of their orbital positions outward from the centre of the Solar System.

This ordering follows directly from the layout of the Solar System, where each planet occupies a fixed orbit at a set average distance from the Sun.

The arrangement was not based on size, because a small planet like Mercury can lie between larger ones, nor on brightness, since apparent brightness depends on distance and reflectivity, nor on orbital speed, which merely governs how fast each planet moves along its own path.

Hence the correct basis of ordering in the parade was the planets' positions measured outward from the Sun.

Therefore the answer is distance from the Sun.