[Findegil]

Here we go with the asked for quotation. I take it from The History of Middle-Earth; volume 10: The War of the Jewels; part I:The Gary Annals (in may HarperCollins hardback-edition it is on page 30). I will start in the middle of paragraph 52 that described the march of Fingolfins host over the ice. I will also shorten it, only to spare me some typing. The Text it self was written in the very early 50th and a typescript was made of it may be in 1958:

... And even as they set foot upon Middle-earth, the ages of the Stars were ended, and the time of the Sun and Moon was begun, as is told in the Chronicle of Aman.

YS 1 [Year of the Sun]
§52 Here the Moon and the Sun, wrought by the Valar after the death of the Trees, rose new in the heaven. First the Moon came forth, and even as it rose above the darkness in the West Fingolfin let blow his silver trumpets, and began his march into Middle-earth; and the shadows of his host went long and black before them.
... But soon after there came the first Dawn of the Sun, ...
...

§56 From this time are reckoned the Years of the Sun. Swifter and briefer are they than the long Years of the Trees in Valinor. Lo! in that time the growth and the changing and ageing of all things was hastened exceedingly; and all living things spread and multiplied in the Second Spring of Arda, ...

Okay to be fair it was the first rising of the Moon that started the First Age as said in the quotation above. But that makes only a very small deferens. (I hope, you are content, Tar-Elenion. [img]smilies/wink.gif[/img] )

quote from Tar-Elenion:

The Letter noting the "long First Age" (#131) is from 1951, and the cited passage from PoME noting for exampe that the First Age was the longest are likely from about 1954 or perhaps a little earlier. This is before the Myths Transformed writings which are from the late 1950's.

Things are often not that simple as they seem to be, when we are looking for dates of changes in Tolkiens fantasy-universe. You are certainly right, Tar-Elenion, that Tolkien worked during the early 50's with the flat-earth version of his myth. But when he started that work he, for the first time, thought about a transformation for which he laid out the plans a couple of years later in the text given as Myths Transformed.
I will give a short quote of the preference to that part of Morgoth's Ring (volume 10 of The History of Middle-Earth) from Christopher Tolkien:

In these writings can be read the record of a prolonged interior debate. Years before this time, the first signs have been seen of emerging ideas that if pursued would cause massive disturbance in “The Silmarillion”: I have shown, as I believe, that when my father first began to revise and rewrite the existing narratives of die Elder Days, before “The Lord of the Rings” was completed, he wrote a version of the Ainulindalë that introduced a radical transformation of the astronomical myth, but that for that time he stayed his hand (pp. 3-6, 43). But now, as will be seen in many of the essays and notes that follow, he had come to believe that such a vast upheaval was a necessity, that the cosmos of the old myth was no longer valid; and at the same time he was impelled to try to construct a more secure 'theoretical' or 'systematic' basis for elements in the 'legendarium' that were not to be dislodged. With their questionings, their certainties giving way to doubt, their contradictory resolutions, these writings are to be read with a sense of intellectual and imaginative stress in the face of such a dismantling and reconstitution, believed to be an inescapable necessity, but never to be achieved.

Especially when you think The Lord of the Rings featured a round-earth myth (which is in it self debatable), you can not deny that JRR Tolkien was not sure what to do with his myth during the complete 50’s until he settled down for the change in the late 50's with the texts published in Morgoth's Ring part 4 and 5.

Respectfully
Findegil