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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An artist's illustration of what the surroundings of the supermassive black hole at the center of BL Lac is shown. A white jet protrudes horizontally toward the bottom of the image, emanating from a orange accretion disk surrounding a black hole. Please see the explanation for more detailed information.

IXPE Explores a Black Hole Jet

May 9, 2025
Credit: Illustration Credit: NASA , Pablo Garcia
See this entry on NASA's APOD →
How do black holes create X-rays? Answering this long-standing question was significantly advanced recently with data taken by NASA's IXPE satellite. X-rays cannot exit a black hole , but they can be created in the energetic environment nearby , in particular by a jet of particles moving outward. By observing X-ray light arriving from near the supermassive black hole at the center of galaxy BL Lac , called a blazar , it was discovered that these X-rays lacked significant polarization , which is expected when created more by energetic electrons than protons . In the featured artistic illustration , a powerful jet is depicted emanating from an orange-colored accretion disk circling the black hole. Understanding highly energetic processes across the universe helps humanity to understand similar processes that occur on or near our Earth .

Put it All Together: Astronomy Puzzle of the Day