Complex sentence: Shortly after sunset and before sunrise, artificial satellites often look like stars-similar in brightness and size, but different because they move relatively quickly; those that fly in near-earth orbit cross the sky in a couple of minutes.
Split into simpler sentences: Shortly after sunset and before sunrise, artificial satellites often look like stars-similar in brightness and size, but different because they move quickly. Those that fly in near-earth orbit cross the sky in a couple of minutes.