By johnmcgee | Updated Mar 24, 2022
When we look up at the night sky, it may seem as if the stars, planets, and even the moon are moving across the heavens. This apparent motion is not caused by the celestial bodies themselves but rather by the rotation and orbit of our planet. Below we explain the physics behind this phenomenon and why different objects travel at different speeds.
Our planet spins once every 24 hours. Because of this rotation, any fixed object in space appears to move from east to west. This explains why the sun rises in the east and sets in the west, and why stars that are low on the eastern horizon at dusk climb toward the western sky by dawn.
Stars are effectively stationary relative to Earth’s axis, so their motion across the sky is caused solely by our planet’s spin. The sun, however, also appears to shift because Earth travels around the sun once per year. This combined motion means the sun takes about four minutes longer each day to reach the same position in the sky as the stars. In practical terms, stars reach a given point in the sky roughly four minutes earlier than the sun does.
Each month, the sun lies in front of a different set of constellations that make up the Zodiac. These 12 constellations—Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpio, Sagittarius, Capricorn, Aquarius, and Pisces—represent familiar shapes in the sky. The particular constellation the sun aligns with at the time of your birth determines your zodiac sign.
The moon orbits Earth roughly every 27.3 days, completing a full cycle against the backdrop of stars in about 29.5 days (a synodic month). Because of this faster orbit, the moon reaches the same position in the sky about 52 minutes earlier each day than the sun does. This explains the moon’s rapid drift across the evening and morning skies.
Venus and Mercury are the only planets that can be seen with the naked eye from Earth. Their orbits bring them close to the sun from our perspective, so they appear just after sunset or before sunrise, depending on the season. Their swift movements across the sky are due to their relatively short orbital periods—about 225 days for Mercury and 224 days for Venus.
Mars, Jupiter, and Saturn are visible without a telescope and display a fascinating phenomenon known as retrograde motion. When Earth overtakes one of these planets in its orbit, the planet appears to move backward against the background stars for several weeks. This apparent reversal is purely an observational effect; the planets continue to move forward in their own orbits.
For a deeper dive into planetary motion and the science of astronomy, visit the NASA website or consult the Wikipedia entry on planetary motion.
