Thierry wrote:

Nov 1610 - began observing Venus

Sure of november ?

No. October or even September might be a better answer, but it depends
on the exact question. It's not likely that Galileo saw any change in
phase before November, nor is it clear that he was specifically looking
for phase change prior to that.

He probably observed Venus as a morning star in the spring of 1610, so
we could also say that he "began observing Venus" almost a year before
he announced the observation of phases.

He wrote to Castelli at the end of December 1610 or the beginning of
January 1611,

Know, therefore, that about 3 months ago I began to observe Venus
with the instrument, and I saw her in a round shape and very small.

which would put the start of his observation at the end of September or
the beginning of October.

Day by day she increased in size and maintained that round shape
until finally, attaining a very great distance from the Sun, the
roundness of her eastern part began to diminish, and in a few days
she was reduced to a semicircle.

Venus was very near the horizon then, and remained low for another month
or so. It's also likely that in his instrument, he wouldn't have been
able to distinguish its phase from a disk until early to mid-November,
which is why he described its change to a semicircle as taking only a
few days.

A quick check in SkyMap produces the following for Venus.

date alt phase diam mag
---------------------------------
1 Sep 6 .86 12 -3.9
1 Oct 7 .78 14 -4.0
1 Nov 11 .68 17 -4.1
1 Dec 18 .58 21 -4.3
1 Jan 28 .41 30 -4.5

Check these numbers if you need them to be accurate. The altitude is in
degrees at half an hour after sunset at a latitude of 45N. The diameter
is in arcseconds.

- Ernie http://home.comcast.net/~erniew