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
✦  Astronomy Picture of the Day  ✦
◄ Prev
Next ► ✦ Surprise me
An artistic illustration of a black hole is shown. The black spot in the center is the black hole, while the accretion disk of gas surrounding it is shown in orange. Stars and the darkness of space is shown near the top in the background. Please see the explanation for more detailed information.

Spin up of a Supermassive Black Hole

May 4, 2025
Credit: Illustration Credit: Robert Hurt , NASA / JPL-Caltech
See this entry on NASA's APOD →
How fast can a black hole spin? If any object made of regular matter spins too fast -- it breaks apart. But a black hole might not be able to break apart -- and its maximum spin rate is really unknown. Theorists usually model rapidly rotating black holes with the Kerr solution to Einstein's General Theory of Relativity , which predicts several amazing and unusual things. Perhaps its most easily testable prediction, though, is that matter entering a maximally rotating black hole should be last seen orbiting at near the speed of light, as seen from far away. This prediction was tested by NASA's NuSTAR and ESA's XMM satellites by observing the supermassive black hole at the center of spiral galaxy NGC 1365 . The near light-speed limit was confirmed by measuring the heating and spectral line broadening of nuclear emissions at the inner edge of the surrounding accretion disk . Pictured here is an artist's illustration depicting an accretion disk of normal matter swirling around a black hole, with a jet emanating from the top. Since matter randomly falling into the black hol <a e should not spin up a black hole this much, the NuSTAR and XMM measurements also validate the existence of the surrounding accretion disk .

Hole New Worlds: It's Black Hole Week at NASA!