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.

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

Wandering Planets

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

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The Radio Sky: Tuned to 408MHz

The Radio Sky: Tuned to 408MHz

February 5, 2005

Tune your radio telescope to 408MHz (408 million cycles per second) and check out the Radio Sky! In the 1970s large dish antennas at three radio observatories, Jodrell Bank, MPIfR, and Parkes Observatory, were used to do just that - the data were combined to map the entire sky. Near this frequency, cosmic radio waves are generated by high energy electrons spiraling along magnetic fields. In the resulting false color image, the galactic plane runs horizontally through the center, but no stars are visible. Instead, many of the bright sources near the plane are distant pulsars, star forming regions, and supernova remnants, while the grand looping structures are pieces of bubbles blown by local stellar activity. External galaxies like Centaurus A, located above the plane to the right of center, and the LMC (below and right) also shine in the Radio Sky.