Expert: Tom Whiting - 9/16/2009

Question
When do U think the new moon will be visible? 19th or 20th of September.

Answer
Hi Zia,
By definition, the new moon is never visible from the Earth's surface, being too close to the sun.  But if you're referring to a very young crescent moon in the west right after sunset, it is highly dependent on where on the Earth's surface you are observing from. (not to mention naked eye? optical aid with binoculars? clouds in the way? your elevation?  etc etc.)
AND the ecliptic angle that the sky makes with the sun as seen from the Earth.  Turns out for the Sept 18 new moon for the USA observers, it doesn't matter as the new moon is at exactly 18:43 GMT or 11:43 am PDT. Since the naked eye record is about 15 hours after new moon, no one from the continental USA will see the evening crescent on September 18th.  On September 19th, since the ecliptic is at such a low angle for USA, it's going to be a tough sighting even 30 hours after new moon for any USA observer, including Hawaii. In fact, for any observer in the Northern Hemisphere....(But just how far west do U want to go....one high island rock in the middle of the Pacific just east of the IDL (International Date Line)?

BUT for a Southern Hemisphere observer, my skyglobe program shows that in Santiago, Chile, local sunset occurs at 6:30 pm their time on September 19, and moonset is an easy 75 minutes after that. See, for those observers in the S. Hemisphere, the moon is almost straight up from the sun's position, so it makes the September 19th young moon sighting quite easy for them. (assuming no clouds in the way).  So any point in the S. Hemisphere west of Santiago Chile, can (or may) see with naked-eye, a young crescent moon on Saturday, September 19th.
(Does this 'count' in your...calculations?)

Realize a lot of this stuff is man made by definition...our definition of GMT (choosing Greenwich
as our 'standard' hour), the International Date line is by convention, and even our Gregorian
date is by humans, who came up with the idea when it was still thought that the Earth was
stationary and in the center of the Solar System by most people.  (There is no theology involved
here).
Anyway, to answer the question, yes, there are some portions of the Earth in the S. Hemisphere, that the young crescent moon may be visible to the naked eye on the Gregorian date of September 19th, 2009 AD.
And everyone gets to see it on the evening of Sept 20.
Hope this helps,
Clear Skies,
Tom Whiting
Erie, PA