In NASA fact sheets, such as this one for Venus, a radius of the planet is given.

Is that radius the radius of the surface of the planet, or of the envelope of atmosphere?

Put another way, if calculating the angular diameter of a planet/moon from a location, do I only have to look up this radius, or do I have to look up this radius along with the atmosphere height to determine the angular diameter?


1 Answer 1


I looked for a source without luck, so I'll just post cause I've read this answer before. The diameter is the solid surface, not the atmosphere for rocky planets. For gas giants it's the atmosphere up to 1 bar.

To determine Venus' angular diameter, you'd need to add something for it's atmosphere. It's difficult to say how high it's atmosphere stops being reflective (and if anyone knows the details, please answer), but I would guess about 50 km where it's clouds reach to on it's sunny side. You'd need to add more if you look at it through infra-red. But I think it would be an error to add the full 250 km listed as the thickness of it's atmosphere and it's reflectivity probably trails off well before then.

A curious sidebar, Ganymede is listed as the largest moon by diameter, but because of it's atmosphere, Titan appears larger. It's visibly larger on accurate moon charts. Because of ice in it's upper atmosphere and low gravity, Titan's atmosphere remains reflective for a significant distance.


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