Re: [Hampshire] [OT] Lunar anomaly

On Sat, Mar 19, 2011 at 12:28:15AM +0000, Chris Dennis wrote:
> Whom to believe?
>
> The Chicago Sun-Times[1] says that the full moon today will be "14
> percent larger than normal", whereas The Guardian[2] says "people
> will need to look very closely to spot the 0.3% difference".

Let's be generous to the CST, and assume they're talking area, not
linear size. This means that the moon would have to be 6% larger,
linearly(*). The moon's semi-major axis is 384000km(**), and its mean
radius is 1700km(**). With those figures, its visible disc will be
something on the order of 0.00885 radians across, or about 0.51°
(after a little trigonometry).

In order to be 6% larger, it would need to be approximately 0.538°
across, which, assuming it's not changing its physical size, would
give an orbital distance of 362000km. This tallies closely with the
perigee figure for the moon's orbit, so it looks like the CST has
*some* degree of justification involved.

*However*

This particular event (being 14% larger than normal, by area) is
going to happen once every orbit of the moon (or "month" as we call
it), so it's not exactly "special" to see the moon vary in size by
14%. In fact, it will vary by rather more than that, between its
perigee size and apogee size, because the semi-major axis I used above
is halfway between perigee and apogee. Hardly worth a mention in the
press.

I don't know the details of the orbital mechanics behind this
particular special event, but I suspect that indeed the moon _is_
coming closer than normal at its perigee(***), and that the Guardian
has calculated the difference between the "normal" or mean perigee and
the current one, and that _that_ difference results in a 0.3% variance
in area (equivalent to a 0.15% change in radius).

Hugo.

(*) sqrt(1.14) - 1

(**) This is about halfway between the minimum and maximum orbital
radius, and thus constitutes a reasonable approximation to "normal".

(***) Probably due to the gravity of the Sun, and the particular
alignment of the Earth-Moon-Sun system right now.

-- 
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