Key Takeaways:
- Angular measurements (degrees) do not directly correlate with the actual physical distances between celestial bodies.
- Apparent size, as measured in degrees, is dependent on both the object's physical size and its distance from the observer.
- Changing the distance of a celestial object alters its apparent size (angular diameter) but not its true physical diameter.
- The example of the Moon illustrates this: Doubling its distance from Earth would halve its apparent diameter while leaving its physical diameter unchanged.
Degrees of sky are irrelevant to true distance between stars. If the Moon’s distance from Earth doubled, its apparent size would shrink by half, even though its physical diameter would remain unchanged.
Astronomy: Roen Kelly
A measurement in degrees has nothing to do with the true distances between celestial objects. When we talk about apparent distances or sizes, how- ever, degrees do come into play.
Here’s an example: The Moon’s true diameter is 2,159 miles (3,474 kilometers). But the width that we see of the Full Moon — its apparent diameter — is ½°. That’s how much of an angle it subtends at its distance from us. To prove there’s no connection between true and apparent diameter, let’s imagine that we can double the Moon’s distance from Earth. Its apparent size would shrink to ¼° from our vantage point, but its true diameter would remain unchanged.
Michael E. Bakich
Senior Editor
Senior Editor
