Angular Size: The Sky’s Ruler
draft readiness: experimental EarthSky Both 10 min
Core demo behavior is implemented, but parity and launch-gate signoff are still pending.
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Predict
Predict
If two objects have the same physical size, which looks bigger in the sky: the nearer one or the farther one?
Play
Play
- Compare the apparent size of two objects at different distances.
- Hold distance fixed and change physical size; note what changes.
- Try to find two different (size, distance) pairs that look the same.
Explain
Explain
Use your observations to explain how apparent size depends on both size and distance.
Learning goals
- Relate an object’s apparent size to its physical size and distance.
- Interpret small angles as a practical measurement tool in astronomy.
- Explain why nearby objects can look larger than distant ones.
Misconceptions targeted
- If something looks bigger, it must be physically bigger.
Model notes
- Angular diameter uses the exact geometric relation $\theta = 2\arctan\!\left(\frac{D}{2d}\right)$.
- Units: diameter $D$ and distance $d$ are always in km; angles are reported in $^\circ$ / $^\prime$ / $^{\prime\prime}$.
- Moon time modes: (1) Orbit (perigee ↔ apogee) and (2) Recession (Myr from today; a toy linear extrapolation using a constant recession rate).
About this demo
This demo explores why objects can look big or small in the sky depending on both their physical size and their distance.
Use presets (astronomical + everyday) to build intuition, then use Station Mode to record (diameter, distance, angular size) cases you can compare and explain.