Interactive astronomy lab

If Earth keeps moving, how can we point to the same galaxy?

Move through time and watch how RA / Dec stays fixed while the direction and altitude in your local sky change.

How to explore

Press play or drag the timeline, then orbit the 3D dome to see how direction and altitude work together.

View from Bangkok

3D sky dome · M31

Above the horizon
Drag to orbit · Arrow keys work too
Gold arc = azimuth on the groundCyan arc = altitude above the horizonViolet ray = line of sight

Azimuth · Direction

50.6°

North-east

Altitude · Height

25.2°

Above the horizon

Simulated time

28 Aug 2026, 15:00 UTC

Faint points show the galaxy's path before and after the selected time.

Space coordinates · RA / Dec

Stays fixed

Right Ascension

00h 42m 44s

Declination

+41° 16′

Think of this as an address on the sky map. It uses a reference frame based on very distant objects, so it does not rotate with the ground beneath us.

Local sky coordinates · Alt / Az

Keeps changing

Azimuth

50.6°

Altitude

25.2°

These coordinates tell a telescope which direction to face and how high to tilt right now, so they must be recalculated for the observer's location and time.

A simple way to remember it

1 · Start with a fixed address

Choose a galaxy by its RA / Dec.

2 · Add Earth context

Use the date, time, and observer location.

3 · Point the telescope

Convert the result to Azimuth / Altitude.

One more thing: The light we see comes from the past. M31 is about 2.5 million light-years away, so we see it as it was roughly 2.5 million years ago—not as a live image of this moment.