From: David Carter (UK) - PM
Date: 31 Aug 18 - 04:35 PM

When its night in England its also night over almost half the planet. When Mars is visible from England its also visible from almost half the planet. Because Mars is not necessarily in direct opposition to the sun, these halves are not the same exactly. But in principle up to half of all of the rockets launched would be expected to be visible from England.

Except for the English weather. Astronomers know that its very often cloudy in England. Very very often. Which is why the modern day equivalents of Ogilvie put their telescopes in foreign countries. Does anybody who has read the book recently know whether Ogilvie complains about the weather? He would not be much of an astronomer if he did not.

Thanks to Little Hawk for providing a link to the text.
It appears that Ogilvy did not see the first launch (Midnight day 1) but viewed the second (midnight day 2) and the third (midnight day 3)
Page 11 of 293 makes clear that Ogilvy saw a launch at midnight, but stopped watching at 1 am (It doesn't say what time he started) so the fact that "Almost half the planet" (at certain times of the year) can view Mars at the same time means nothing if the astronomers are viewing for a very small portion of the hours of darkness.