Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

Re: Astronomical Unit (AU)



Regarding Leigh's confirmation of my 1 AU distance quote from JPL:

According to the JPL/NASA web site
http://ssd.jpl.nasa.gov/astro_constants.html the numerical value of the
AU is 149597870691 +/- 3 m.

which translates to 499.004783806(10) light seconds. Pretty good.

Remember, though, this nice 499.00 s value (to 5 sig. figs.) is *not* the
light travel time for sunlight to reach the Earth when it is 1 AU from
its orbital focus, because the visual radius of the Sun is about
2.32 light sec, and this gives the light about a 2.32 s head start. The
radius of the Earth is a more negligible 0.02 light sec.

David Bowman
David_Bowman@georgetowncollege.edu

Ah!!! I was hoping someone would grab that bait. I set that that trap
intentionally. This question arose for me when I first considered the
problem of observing the apparent position of the center of the Sun.
Should I take the light time from the nearest point on the Sun as the
time required to produce the circular image of the Sun that I see?
The answer turns out to be "No", and the correct time is just the time
required to come from the Sun's limb. Even Terrell rotation leaves
that result unchanged. While David is correct about the radius of the
Earth being significant (as is the Moon wobble), the radius of the
Sun is not important for correcting the light travel time for the
purpose of locating the center of the image of the Sun's disc.
Alternatively, one must consider that the light from the Sun's surface
takes a range of times to reach Earth. In the problem I was considering
I wanted to know if stellar aberration had a different value for the
Sun than it has for stars.

499 s was merely a good number to remember (together with the nine
digit speed of light) for the purpose of reconstructing the Astronomical
Unit. That "eight minute" number we learned in elementary school just
annoys me with its imprecision and needless inaccuracy. Remember also
that there is a +/- nine second variation due to the eccentricity of
Earth's orbit in the actual transit time.

Leigh