The Moving Sky: How Star Charts Map a Curved Sky

The Moving Sky Above Our Head

Every star chart begins with the impossible task of fitting the curved dome of the sky onto flat paper. Our night sky has no edges, no corners, no single place to be cut open, so there is no solution that does not distort one part or another. The trouble isn’t the sky’s size, it’s its curvature: a curved surface and a flat sheet simply cannot be matched without something giving way.

However, for thousands of years, people have found ways around this issue. This hub is about those ways. Our resources examine the star chart projection that flattens the dome with the least possible distortion, explain stellar coordinate systems, and explore how early cultures used the night sky as a calendar. To see the trade-off for yourself, drag the sphere in the projection explorer and watch how the math decides where each star lands.

The Wandering North Star

Watch the wanders 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 chosen.

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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A Multiple Green Flash Sunset

March 28, 2023

Yes, but can your green flash do this? A green flash at sunset is a rare event that many Sun watchers pride themselves on having seen. Once thought to be a myth, a green flash is now understood to occur when the Earth's atmosphere acts like both a prism and a lens. Different atmospheric layers create altitude-variable refraction that takes light from the top of the Sun and disperses its colors, creates two images, and magnifies it in just the right way to make a thin sliver appear green just before it disappears. Pictured, though, is an even more unusual sunset. From the high-altitude Cerro Tololo Inter-American Observatory in Chile one day last April, the Sun was captured setting beyond an atmosphere with multiple distinct thermal layers, creating several mock images of the Sun. This time and from this location, many of those layers produced a green flash simultaneously. Just seconds after this multiple-green-flash event was caught by two well-surprised astrophotographers, the Sun set below the clouds.