The Moving Sky: How Star Charts Map a Curved Sky

The moving sky above our head

Above our heads, the dark skies slowly change in appearance. We might think they are eternal and still because the cycle is not apparent to the eye, but in truth they move in an unending rhythm. What we see is not what our great-grandparents saw, and it is definitely not what the ancients saw. Like a top, Earth wobbles on its axis, completing an entire great cycle every 26,000 years. That wobble affects our view of the sky. The wobble is so slow we cannot feel it, but it is wide enough that the pole star changes. Polaris is our pole star now, but for the Egyptians who built the pyramids it was Thuban, and Vega will come after Polaris.

This is why there are two addresses for the stars. Fix your coordinates to the seasons and you get one set. Fix them to the stars themselves and you get another. The gap between them grows by a degree every 72 years. On this page we look at how the poles have wandered, why the zodiac no longer matches the constellations behind it, and how the planets wander against a background that is itself in motion.

The Wandering North Star

Watch the wanderings of the North Star through epochs of time.

Pole Clock

The North Star

Why the stars aren’t where your ancestors saw them, the North Star won’t always be north. Earth wobbles, and skies turn.

Wandering Skies

Sidereal vs. Tropical

Two valid ways to fix a star’s position, one tied to the seasons and one to the stars, and the slowly widening gap between them.

Place the Stars

Lahiri Ayanamsa

Modern Indian astronomy needed one exact number to separate the two zodiacs. This is how it was selected

Vedic Wisdom

Wandering Planets

The five wandering lights the ancients tracked by eye and how to find tonight’s planets in your own sky.

Five Planets
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Earth's moon is shown in full phase. At the top of the frame, appearing much smaller, is the more distant planet Mars. Please see the explanation for more detailed information.

Wolf Moon Engulfs Mars

January 15, 2025
Credit: Imran Sultan
See this entry on NASA's APOD →
Does the Moon ever engulf Mars? Yes, but only in the sense that it moves in front, which happens on rare occasions . This happened just yesterday, though, as seen from some locations in North America and western Africa . This occultation was notable not only because the Moon was a fully lit Wolf Moon , but because Mars was near its largest and brightest, moving to opposition -- the closest to the Earth in its orbit -- only tomorrow. The engulfing , more formally called an occultation , typically lasts about an hour. The featured image was taken from near Chicago , Illinois , USA just as Earth's largest satellite was angularly moving away from the much more distant red planet . Our Moon occasionally moves in front of all of the Solar System 's planets. Given the temporary alignment of orbital planes , the next time our Moon eclipses Mars will be a relatively soon February 9.

Growing Gallery: Moon-Mars Occultation in January 2025