Cross-sectional area and angular size are often used in astronomy. For example, the angular size of the Moon and the Sun (as seen from Earth) are essentially the same (which is why we can have total solar eclipses). This may also be why when you ask a "person on the street" whether the Moon or the Sun is bigger, lots of them don't know or give the wrong answer. I'm sure you already know, but in case you forgot or someone else is reading this, for angular size we mean how big something looks in two (angular) dimensions, it matters where you're observing from, and the SI units are steradians; for cross-sectional area we're talking more about something impacting upon something else (stellar wind impacting on dust grains, radiation impacting on a planet), we're talking about two physical size measurements (as if you cut an object down the middle), and SI units are m^2. Oh yeah, and surface area (also in units of m^2) is important when an object is radiating energy to space, like a star or planet radiating light.
Yeah, the ambiguity/vagueness (what's the difference anyway?) of terms relating to size bothers me. In my teaching, I try to be clear about size-related words, so students know how many dimensions I'm referring to. Even worse, there *are* circumstances when an astronomer actually means mass when ze says something related to "size" - i.e., "a blue giant star is bigger than a red dwarf". While it's true that a blue giant star has a larger radius, the mass is what's more important when comparing stars so in saying a blue giant is bigger, what we really care about is that it's got more mass.
I brought this up b/c the thesis I'm reading had a worksheet having to do with how planet size affects the surface appearance.
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timeasmymeasure
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Date: 2013-08-16 05:57 am (UTC)Yeah, the ambiguity/vagueness (what's the difference anyway?) of terms relating to size bothers me. In my teaching, I try to be clear about size-related words, so students know how many dimensions I'm referring to. Even worse, there *are* circumstances when an astronomer actually means mass when ze says something related to "size" - i.e., "a blue giant star is bigger than a red dwarf". While it's true that a blue giant star has a larger radius, the mass is what's more important when comparing stars so in saying a blue giant is bigger, what we really care about is that it's got more mass.
I brought this up b/c the thesis I'm reading had a worksheet having to do with how planet size affects the surface appearance.