Re[2]: [tied] Re: Why borrow 'seven'? (was: IE right & 10)

At 10:08:25 PM on Monday, September 27, 2004,
enlil@... wrote:

> Me:
>> ?? Let's get this right. The time it takes for the moon
>> to go around the earth is said to take 27.321661 days,
>> no?

> Richard:
>> No! That's the sidereal period, i.e. with reference to
>> the fixed stars. Relative to the Earth-Sun axis (which
>> moves with respect to the fixed stars), it's a bit over
>> 29 and a half days [...]

> Yes, but isn't that really irrelevant?

No, because it means that the nearest whole numbers to the
observed period are 29 and 30, not 27. (And indeed we find
29- and 30-day months in lunar calendars.)

> Look again. The criteria here is that we use whole
> numbers, not fractions, and that we want a _multi-cycle_
> coordination of the sun, moon and Venus (the three
> brightest objects in the sky, coincidentally). It looks to
> me that using a common multiple 7 is the most optimal
> solution by far under these stricter rules.

> lunar cycle: 7 * 4 = 28 (off by one day, 27.3 or 29.5 days)

Off by a bit over 1.5 days.

> Venus cycle: 7 * 32 = 224 (off by one day; 225 days)

That's the siderial period. It would be obvious if one were
on Venus, but the synodic period of 583.92 (or about 584)
days is *far* more obvious to a terrestrial observer. (The
synodic period is the time interval between two successive
alignments of a celestial object with respect to the sun as
seen from the earth, i.e., the time it takes for the object
to return to a given location in the sky -- on the meridian
at midnight, say -- as seen from the Earth.)

Brian